Gravity Anomaly Research Papers - Academia.edu (original) (raw)

1] Teleseismic travel times recorded along a 1000 kmlong, $N-S transect across central Mongolia are used together with topography and gravity data to constrain the deep lithospheric structure of this region. Time residuals appear... more

1] Teleseismic travel times recorded along a 1000 kmlong, N−StransectacrosscentralMongoliaareusedtogetherwithtopographyandgravitydatatoconstrainthedeeplithosphericstructureofthisregion.Timeresidualsappearpositivelycorrelatedwiththetopography,suggestingthatP−wavevelocitychangescorrespondtodensityvariationswhichinturncauseanisostaticupliftofthetopography.UsingasimplelocalisostasymodelandaMonte−Carloinversionforthecrustandasthenospherevelocitiesanddensity/velocityconversionfactors,wedeterminebest−fittingMohoandlithosphere−asthenosphereboundary(LAB)geometrieswhichsatisfyinglyreproducetheobservedtopographyandgravitydata.Themodelisvalidatedusingpreviouslypublished1DS−wavevelocitymodelsobtainedfromreceiverfunctionanalyses.Ourresultsindicatethatmostofthelong−wavelengthtopographyissupportedbytheasthenospherebuoyancy.TheHangaidome,inthesouthernhalfofthetransect,appearsmainlysupportedbyarathershallow(60−70km)asthenosphericuplift,whereastheSiberianplatformattheextremenorthoftheprofileisunderlainbyaverythick(N-S transect across central Mongolia are used together with topography and gravity data to constrain the deep lithospheric structure of this region. Time residuals appear positively correlated with the topography, suggesting that P-wave velocity changes correspond to density variations which in turn cause an isostatic uplift of the topography. Using a simple local isostasy model and a Monte-Carlo inversion for the crust and asthenosphere velocities and density/velocity conversion factors, we determine best-fitting Moho and lithosphere-asthenosphere boundary (LAB) geometries which satisfyingly reproduce the observed topography and gravity data. The model is validated using previously published 1D S-wave velocity models obtained from receiver function analyses. Our results indicate that most of the long-wavelength topography is supported by the asthenosphere buoyancy. The Hangai dome, in the southern half of the transect, appears mainly supported by a rather shallow (60 -70 km) asthenospheric uplift, whereas the Siberian platform at the extreme north of the profile is underlain by a very thick (N−StransectacrosscentralMongoliaareusedtogetherwithtopographyandgravitydatatoconstrainthedeeplithosphericstructureofthisregion.Timeresidualsappearpositivelycorrelatedwiththetopography,suggestingthatP−wavevelocitychangescorrespondtodensityvariationswhichinturncauseanisostaticupliftofthetopography.UsingasimplelocalisostasymodelandaMonte−Carloinversionforthecrustandasthenospherevelocitiesanddensity/velocityconversionfactors,wedeterminebest−fittingMohoandlithosphere−asthenosphereboundary(LAB)geometrieswhichsatisfyinglyreproducetheobservedtopographyandgravitydata.Themodelisvalidatedusingpreviouslypublished1DS−wavevelocitymodelsobtainedfromreceiverfunctionanalyses.Ourresultsindicatethatmostofthelong−wavelengthtopographyissupportedbytheasthenospherebuoyancy.TheHangaidome,inthesouthernhalfofthetransect,appearsmainlysupportedbyarathershallow(60−70km)asthenosphericuplift,whereastheSiberianplatformattheextremenorthoftheprofileisunderlainbyaverythick(200 km) lithosphere. Misfits to the observed Bouguer gravity anomaly occur near major faults, probably because of lithospheric flexure. Citation: Petit, C., C. Tiberi, A. Deschamps, and J. Déverchère (2008), Teleseismic traveltimes, topography and the lithospheric structure across central Mongolia, Geophys. Res. Lett., 35, L11301,

Along the two volcanic off-rift zones in Iceland, the Snaefellsnes volcanic zone (SNVZ) and the South Iceland volcanic zone (SIVZ), geochemical parameters vary regularly along the strike towards the centre of the island. Recent basalts... more

Along the two volcanic off-rift zones in Iceland, the Snaefellsnes volcanic zone (SNVZ) and the South Iceland volcanic zone (SIVZ), geochemical parameters vary regularly along the strike towards the centre of the island. Recent basalts from the SNVZ change from alkali basalts to tholeiites where the volcanic zone reaches the active rift axis, and their STSr/86Sr and Th/U ratios decrease in the same direction. These variations are interpreted as the result of mixing between mantle melts from two distinct reservoirs below Snzefellsnes. The mantle melt would be more depleted in incompatible elements, but with a higher 3He/nile ratio (R/Ra = 20) beneath the centre of Iceland than at the tip of the Snaefellsnes volcanic zone (R/Ra = 7.5).

The new gravity and magnetic data recorded along a profile in the Sikkim, NE Himalaya are combined with the existing data from Tibet, Bangladesh and India, to delineate the crustal structure in this part of Himalaya. Modelling of gravity... more

The new gravity and magnetic data recorded along a profile in the Sikkim, NE Himalaya are combined with the existing data from Tibet, Bangladesh and India, to delineate the crustal structure in this part of Himalaya. Modelling of gravity data, constrained from seismic results suggests that long wavelength gravity anomalies arise due to variations in the depth of Moho (36 to 74 km), which are caused by flexed lithosphere of effective elastic thickness of ∼ 50 ± 10 km. Simultaneous modelling of magnetic anomalies and short wave-length gravity anomalies reveals that (a) the magnetic anomalies observed over the Lesser Himalaya and the Higher Himalaya Crystalline rocks might be caused by remnant magnetisation with inclination I = − 18°± 8°and declination D = 147°± 10°, which is in conformity with the palaeomagnetic results. These magnetic parameters correspond to~35 ± 10 Ma age of magnetic direction and suggest that the rocks might have acquired magnetisation during cooling period of metamorphism, (b) low grade meta-sediments of the Lesser Himalaya extend up to 12 km depth and thins on either sides forming a bowl shaped geometry and (c) relative gravity high in the Bengal basin might be caused by intrusion of the Rajmahal volcanics. Modelling has also provided constraint on the geometry of the north dipping thrusts.

Potential field methods are used extensively in mineral exploration. These methods also are used as reconnaissance method in oil and gas exploration. In Contrast with gravity anomaly the magnetic surveying produces dipolar anomaly which... more

Potential field methods are used extensively in mineral exploration. These methods also are used as reconnaissance method in oil and gas exploration. In Contrast with gravity anomaly the magnetic surveying produces dipolar anomaly which is caused complicated interpretation rather than gravity anomaly. The observation magnetic anomaly in each location other than magnetic poles has displacement rather than causative body. Several methods are used to overcome to this problem such as reduction to the pole (RTP) that an asymmetric anomaly is converted to symmetrical anomaly. Boundary analysis is another method to distinguish causative magnetic body from observed magnetic data directly. One of the applicable methods in boundary detection of local scale magnetic anomaly is total gradient of pseudogravity anomaly. In this method, pseudogravity anomaly is calculated in the first step. Pseudogravity converts the magnetic field into gravity field that would be observed if the magnetization dis...

Gravimetry was the main exploration method used in this high risk area to the north of the Graà ndola fault, in the Iberian Pyrite Belt (IPB). It showed several targets, among which the Valverde-Lagoa Salgada area stood out. Here, in... more

Gravimetry was the main exploration method used in this high risk area to the north of the Graà ndola fault, in the Iberian Pyrite Belt (IPB). It showed several targets, among which the Valverde-Lagoa Salgada area stood out. Here, in August 1992, a polymetallic massive sulphide orebody was found, named Lagoa Salgada. According to the geological characteristics of the area and to the nature of the surveyed targets, other exploration techniques were used, namely: geology, magnetometry, resistivity, electrical sounding, magneto-telluric, seismic and mechanical drilling. The deposit was found under a Tertiary cover, 128 m in depth, complete with a 15 m thick gossan, caused by paleoalteration, with a supergene enrichment zone. This gossan gave way to massive sulphides, with intersections, at times, exceeding 60 m. They are limited towards the footwall by an important fault with strong associated kaolinization, which is interpreted as a reverse fault, placed on the inverse limb of an anticlinal structure. Throughout the deposit, the chemical composition of the mineralization showed great variation reaching signi®cant values of Zn, Pb, Sn, Cu, As, Hg, Sb, Cd, Au and Ag. In the initial phase the deposit has an estimated tonnage of 5 Mt. It is related to a large gravity anomaly with the general orientation NW-SE, which was investigated by 15 boreholes made by the Instituto Geolo gico e Mineiro. The drilling cut an important volcanic centre of an acid to intermediate nature, around which intense hydrothermal activity was associated with contemporaneous sulphide mineralization. The gravity anomaly has two nuclei, a Mineralium Deposita (1998) 33: 170±187 Ó Springer-Verlag 1998

A major tool in the initial recognition and study of terrestrial impact craters, -20% of which are buried beneath postimpact sediments, is geophysics. The general geophysical character of terrestrial impact craters is compiled and... more

A major tool in the initial recognition and study of terrestrial impact craters, -20% of which are buried beneath postimpact sediments, is geophysics. The general geophysical character of terrestrial impact craters is compiled and outlined with emphasis on its relation to the impact process and as an aid to the recognition of additional impact craters. The most common and conspicuous geophysical signature is a circular gravity low. For simple bowl-shaped craters, gravity models indicate that the anomaly is largely due to the presence of an interior allochthonous breccia lens. In complex craters, modeling indicates that the main contribution to the gravity anomaly is from fractured parautochthonous target rocks in the floor of the crater. The gravity signature of both simple and complex crater forms can be modeled well, using known morphometric parameters of impact structures. The size of the gravity anomaly generally increases with increasing crater diameter reaching a maximum of--20-30 mGal at diameters D of -20-30 km. Further increases in D have a negligible effect on the magnitude of the gravity anomaly due to lithostatic effects on deep fractures. The general gravity signature of a simple low can be modified by target rock and erosional effects, and there is a tendency for larger complex structures (D > 30 km) to exhibit a relative gravity high restricted to the crater center and extending out to <0.5D. The magnetic signature of craters is more varied. The dominant effect is a magnetic low due to a reduction in susceptibility. Large structures (D > 40 km) tend to exhibit central high-amplitude anomalies, with dimensions of <0.5D, due to remanently magnetized bodies in the target rocks. The sources of these bodies are wide ranging and include the effects of shock, heat, and chemical alteration. The few studies over craters involving electrical methods indicate resistivity lows coinciding with the extent of the potential field anomalies and related to fracturing. Seismic techniques, particularly reflection surveys, have provided details of the subsurface structure of craters. Incoherent reflections and reduced seismic velocities due to brecciation and fracturing are expected, the degree of coherency of reflections increasing away from and below the center of the structure. From the various geophysical techniques a set of general criteria can be established that correspond to the geophysical signature of impact craters. These criteria can be used to evaluate the hypothesis that any particular set of geophysical anomalies is due to impact. Confirmation of an impact origin, however, is based on geologic evidence. 20% of the known terrestrial craters are buffed beneath postimpact sediments. Thus the recognition, which is based on surface morphological and morphometric observations of terrestrial impact craters, differs from planetary impact craters. Terrestrial impact craters are recognized largely by lithological and structural evidence, with a major tool in the recognition of a lithostmctural anomaly being geophysics. Many terrestrial impact structures were identified initially as geophysical anomalies and their impact origin established later through geologic studies, for example, Eagle Butte, Canada. Terrestrial data are currently the main source of information on the nature of the third dimension of impact craters ranging up to over 100 km in diameter. This pages 161-181 Paper number 92RG00192 162 ß Pilkington and Grieve: SIGNATURE OF TERRESTRIAL IMPACT CRATERS 29, 1 /REVIEWS OF GEOPHYSICS

The use of GPS for the estimation of orthometric heights in a given region, with the help of existing levelling data requires the determination of a local geoid and the bias between the local levelling and the one implicitly defined when... more

The use of GPS for the estimation of orthometric heights in a given region, with the help of existing levelling data requires the determination of a local geoid and the bias between the local levelling and the one implicitly defined when the geoid is calculated which is generally based on the gravity anomalies data. The heights of new data can

A 3-m-long, non-propelled cryostat released from a 40-kin-high balloon is used to shield a detector free falling inside for 30 s. This facility provides opportunities for accurate experiments in gravitational physics. Among others, two... more

A 3-m-long, non-propelled cryostat released from a 40-kin-high balloon is used to shield a detector free falling inside for 30 s. This facility provides opportunities for accurate experiments in gravitational physics. Among others, two experiments of particular interest are: (a) a free-fall test of the Equivalence Principle; and Co) the local mapping of the gravity field along vertical lines by means of a gravity gradiometer. In experiment (a), the differential acxelerations between two test masses of different materials (e.g. aluminum and gold) is measured during free fall. The estimated accuracy in testing the Equivalence Principle in a 30-s free fall is 4 parts in 1014 with 95% confidence level. In experiment Co), the knowledge of surface gravity anomalies can be improved with a 10"2-EU gradiometer and the upward continuation gravity models validated.

The GEM-T2 potential coefficient model (incomplete to degree 50) has been combined, in a least squares sense, with 30 arc min mean anomalies, to obtain an adjusted set of coefficients and gravity anomalies. The adjusted anomalies were... more

The GEM-T2 potential coefficient model (incomplete to degree 50) has been combined, in a least squares sense, with 30 arc min mean anomalies, to obtain an adjusted set of coefficients and gravity anomalies. The adjusted anomalies were then harmonically analyzed to yield a set of potential coefficients to degree 360. The 30 arc min mean anomalies were estimated from terrestrial gravity data, from altimeter-derived anomalies, and from 1 ø x 1 ø terrestrial anomalies where such data were available. For areas devoid of gravity information, the anomalies were computed in two ways: (1) from the GEM-T2 coefficients and (2) from the GEM-T2 coefficients to degree 36 plus coefficients implied by a topographic/isostatic model. These "fill-in" anomalies led to two potential coefficient models: OSU89A and OSU89B. The new models were checked in several ways including satellite orbit residual analysis, Geosat undulation comparisons, and Global Positioning System (GPS)/leveling undulation differences. The orbit fits (carried out by NASA) showed improvement over GEM-T2. After correction for sea surface topography, orbit error, and permanent tidal effects, the geoid undulations from the OSU89B model have an RMS discrepancy with the Geosat-implied undulation of +59 cm over a complete 17-day exact repeat cycle. The comparisons with GPS information indicate the accuracy of the computation of a relative undulation is of the order of 3-4 ppm of the distance between stations. The new models represent a substantial improvement over previous high-degree expansions.

Exploration for orogenic Stawell-type deposits in the northern extension of the Stawell zone under the Tertiary Murray Basin sediments, in southeastern Australia, has produced the discovery of at least two new deposits at Wildwood and... more

Exploration for orogenic Stawell-type deposits in the northern extension of the Stawell zone under the Tertiary Murray Basin sediments, in southeastern Australia, has produced the discovery of at least two new deposits at Wildwood and Kewell. Previous research on the Stawell gold deposit has highlighted the importance of litho-facies, structural evolution and hydrothermal alteration in the localization of fluid flow at the time of mineralization. Application of geophysical tools including magnetic and gravity surveys at both regional and more detailed scales delineated a number of significant coincident magnetic and gravity anomalies, which upon further investigation through aircore drilling proved to be basalt. Subsequent drilling on the margins of the Wildwood and Kewell basaltic domes delineated areas of prospective litho-facies, hydrothermal alteration, quartz veins and mineralization akin to that found in the Stawell deposit. Analysis of multi-element geochemistry revealed significant gains in Fe in the transition from unaltered to altered unmineralized (<1 ppb Au) host rock, and gains in As, Sb and Ag with subsequent transition to mineralized (>1 ppm Au) altered host rock. Magnetic remanence properties of the various lithologies within the Stawell Zone showed that high fluid flow can result in the precipitation of sulfides with fixed stable remanent magnetization the orientation of which may not be parallel to the present day magnetic field, whereas zones of low fluid flow have soft magnetization that can be easily modified. The implication of variation in remanence orientation for exploration is that regional aeromagnetic anomalies, which have opposite polarities, may coincide with zones of high fluid flow and are potential target areas for mineralization.

We analyse tectonic and sedimentary field and subsurface data for the Angola onshore margin together with free-air gravity anomaly data for the offshore margin. This enables us to characterize the mode of synrift tectonism inherited from... more

We analyse tectonic and sedimentary field and subsurface data for the Angola onshore margin together with free-air gravity anomaly data for the offshore margin. This enables us to characterize the mode of synrift tectonism inherited from the Precambrian and its impact on the segmentation of the Angola margin. We illustrate that segmentation by the progressive transition from the Benguela transform-rifted margin segment to the oblique-rifted South Kwanza and orthogonal-rifted North Kwanza margin segments. The spatial variation in the intensity of post-rift uplift is demonstrated by the study of a set of geomorphic markers detected in the post-rift succession of the coastal Benguela and Kwanza Basins: Upper Cretaceous to Cenozoic uplifted palaeodeltas, erosional unconformities, palaeovalleys, Quaternary marine terraces and perched Gilbert deltas. The onshore Benguela transform margin has a distinctive, mainly progradational stratigraphic architecture with long-term sedimentary gaps and high-elevation marine terraces resulting from moderate Upper Cretaceous-Cenozoic to major Quaternary uplifting (i.e. 775-1775 mm/ky or m/Ma). By contrast, repeated synchronous episodes of minor Cenozoic to Quaternary uplift occurred along the orthogonal-rifted North Kwanza segment with its Cenozoic aggradational architecture, shortterm sedimentary gaps and low-elevation Pleistocene terraces. Margin style likewise governs spatial variations in the volume of offshore sediment dispersed in the associated deep-sea fans. Along the lowlying North Kwanza margin, sedimentation of the broad Cenozoic to Pleistocene Kwanza submarine fan was probably governed by the width of the Kwanza interior palaeodrainage basin combined with the wet tropical Neogene climate. Along the high-rising Benguela margin, the small size of the Benguela deep-sea fan is related to the interplay between moderate continental sediment dispersal from long-lived small catchments and a warm, very arid Neogene climate. However, the driving forces behind the epeirogenic post-rift uplift of the Angola coastal bulge remain a matter of speculation.

3-D images of P velocity and P-to S-velocity ratio have been produced for the upper crust of the Friuli area (northeastern Italy) using local earthquake tomography. The data consist of 2565 P and 930 S arrival times of high quality. The... more

3-D images of P velocity and P-to S-velocity ratio have been produced for the upper crust of the Friuli area (northeastern Italy) using local earthquake tomography. The data consist of 2565 P and 930 S arrival times of high quality. The best-fitting V P and V P /V S 1-D models were computed before the 3-D inversion. V P was measured on two rock samples representative of the investigated upper layers of the Friuli crust. The tomographic V P model was used for modelling the gravity anomalies, by converting the velocity values into densities along three vertical cross-sections. The computed gravity anomalies were optimized with respect to the observed gravity anomalies. The crust investigated is characterized by sharp lateral and deep V P and V P /V S anomalies that are associated with the complex geological structure. High V P /V S values are associated with highly fractured zones related to the main faulting pattern. The relocated seismicity is generally associated with sharp variations in the V P /V S anomalies. The V P images show a high-velocity body below 6 km depth in the central part of the Friuli area, marked also by strong V P /V S heterogeneities, and this is interpreted as a tectonic wedge. Comparison with the distribution of earthquakes supports the hypothesis that the tectonic wedge controls most of the seismicity and can be considered to be the main seismogenic zone in the Friuli area.

We present a revised model for the opening of the South Atlantic Ocean founded on a remapping of the continent-ocean boundaries and Aptian salt basins, the chronology of magmatic activity in and around the ocean basin and on the timing... more

We present a revised model for the opening of the South Atlantic Ocean founded on a remapping of the continent-ocean boundaries and Aptian salt basins, the chronology of magmatic activity in and around the ocean basin and on the timing and character of associated intraplate deformation in Africa and South America. The new plate tectonic model is internally consistent and consistent with globally balanced plate motion solutions. The model includes realistic scenarios for intraplate deformation, pre-drift extension and seafloor spreading. Within the model, Aptian salt basins preserved in the South American (Brazilian) and African (Angola, Congo, Gabon) continental shelves are reunited in their original positions as parts of a single syn-rift basin in near subtropical latitudes (10 • S-27 • S). The basin was dissected at around 112 Ma (Aptian-Albian boundary) when the model suggests that seafloor spreading commenced north of the Walvis Ridge-Rio Grande Rise.

Recent Earth Geopotential Models (EGMs) represent a great improvement to several applications related to gravity field modeling. These models offer higher accuracy and better resolution of the gravity field. In this research, the most... more

Recent Earth Geopotential Models (EGMs) represent a great improvement to several applications related to gravity field modeling. These models offer higher accuracy and better resolution of the gravity field. In this research, the most recent models’ behavior like EIGENs, EGM2008 as well as the classic EGM96, are analyzed and compared in Argentina, aiming to know which one suits better in the country. This investigation also studies the differences between EGMs and geoid undulation values (calculated with height data from GPS and spirit leveling) and gravity anomalies (obtained from terrestrial gravimetric campaigns). In principle, a great difference is detected with the satellite-only geopotential models. The EGMs that perform better results are those that include satellite altimetry and terrestrial information in their calculation. The comparison between local data and the results of seven global models is shown with the residual statistic.

Credit card fraud occurs when user provides their information to the unknown persons or stolen by the unknown persons, that information can be used for unauthorized online purchase and some other situation. Data mining techniques are... more

Credit card fraud occurs when user provides their information to the unknown persons or stolen by the unknown persons, that information can be used for unauthorized online purchase and some other situation. Data mining techniques are employed to study the patterns and characteristics of normal and abnormal transactions based on normalized and anomalies data. In order to detect fraud activities and feature patterns associated with financial activities, we are proposing novel fraud detection framework, Codetect. Wide variety of applications is associated with anomaly detection such as fraud detection and network intrusion detection which referees to pattern finding problems in data.

A major tool in the initial recognition and study of terrestrial impact craters, -20% of which are buried beneath postimpact sediments, is geophysics. The general geophysical character of terrestrial impact craters is compiled and... more

A major tool in the initial recognition and study of terrestrial impact craters, -20% of which are buried beneath postimpact sediments, is geophysics. The general geophysical character of terrestrial impact craters is compiled and outlined with emphasis on its relation to the impact process and as an aid to the recognition of additional impact craters. The most common and conspicuous geophysical signature is a circular gravity low. For simple bowl-shaped craters, gravity models indicate that the anomaly is largely due to the presence of an interior allochthonous breccia lens. In complex craters, modeling indicates that the main contribution to the gravity anomaly is from fractured parautochthonous target rocks in the floor of the crater. The gravity signature of both simple and complex crater forms can be modeled well, using known morphometric parameters of impact structures. The size of the gravity anomaly generally increases with increasing crater diameter reaching a maximum of--20-30 mGal at diameters D of -20-30 km. Further increases in D have a negligible effect on the magnitude of the gravity anomaly due to lithostatic effects on deep fractures. The general gravity signature of a simple low can be modified by target rock and erosional effects, and there is a tendency for larger complex structures (D > 30 km) to exhibit a relative gravity high restricted to the crater center and extending out to <0.5D. The magnetic signature of craters is more varied. The dominant effect is a magnetic low due to a reduction in susceptibility. Large structures (D > 40 km) tend to exhibit central high-amplitude anomalies, with dimensions of <0.5D, due to remanently magnetized bodies in the target rocks. The sources of these bodies are wide ranging and include the effects of shock, heat, and chemical alteration. The few studies over craters involving electrical methods indicate resistivity lows coinciding with the extent of the potential field anomalies and related to fracturing. Seismic techniques, particularly reflection surveys, have provided details of the subsurface structure of craters. Incoherent reflections and reduced seismic velocities due to brecciation and fracturing are expected, the degree of coherency of reflections increasing away from and below the center of the structure. From the various geophysical techniques a set of general criteria can be established that correspond to the geophysical signature of impact craters. These criteria can be used to evaluate the hypothesis that any particular set of geophysical anomalies is due to impact. Confirmation of an impact origin, however, is based on geologic evidence. 20% of the known terrestrial craters are buffed beneath postimpact sediments. Thus the recognition, which is based on surface morphological and morphometric observations of terrestrial impact craters, differs from planetary impact craters. Terrestrial impact craters are recognized largely by lithological and structural evidence, with a major tool in the recognition of a lithostmctural anomaly being geophysics. Many terrestrial impact structures were identified initially as geophysical anomalies and their impact origin established later through geologic studies, for example, Eagle Butte, Canada. Terrestrial data are currently the main source of information on the nature of the third dimension of impact craters ranging up to over 100 km in diameter. This pages 161-181 Paper number 92RG00192 162 ß Pilkington and Grieve: SIGNATURE OF TERRESTRIAL IMPACT CRATERS 29, 1 /REVIEWS OF GEOPHYSICS

Sivas Havzası, Orta Anadolu havzaları arasında üçüncü büyük ve konum itibariyle de en doğuda olan havzadır. Bu çalışmada, Sivas Havzası’nda elde edilen gravite ve havadan manyetik verileri ile sismik yansıma verileri, Celalli-1 kuyusundan... more

Dalam metode gayaberat secara umum nilai anomali gayaberat atau percepatan gravitasi menjadi bagian yang sangat penting untuk memprediksi lapisan bawah permukaan berdasarkan perbedaan rapat massa (density). Nilai anomali dihitung... more

Dalam metode gayaberat secara umum nilai anomali gayaberat atau percepatan gravitasi menjadi bagian yang sangat penting untuk memprediksi lapisan bawah permukaan berdasarkan perbedaan rapat massa (density). Nilai anomali dihitung berdasarkan hasil pengkoreksian yang dilakukan untuk menghilangkan noise yang terjadi dalam pengukuran di lapangan. Selain itu, pemisahan anomali residual dan regional dari anomali gayaberat total dilakukan dengan menggunakan metode Polynomial Least Square. Salah satu cara untuk menggambarkan sebaran anomali gayaberat adalah dengan membuat peta kontur. Peta kontur dapat dibuat dengan menggunakan perangkat lunak baik dalam bentuk 2D maupun 3D. Program pembuatan peta kontur dapat dilakukan dengan metode Mesh Polygon menggunakan Matlab. Pembuatan program perhitungan dan pemisahan anomali gayaberat dapat memeberikan kemudahan yang lebih efisien dan mengurangi kemungkinan terjadinya human error dalam proses perhitungan, serta memberikan gambaran sebaran anomali gayaberat sama seperti perangkat lunak lainnya.

A gravity anomaly map of Europe was recently compiled that incorporates significant new data, especially from the eastern European countries. The map (Figure 1a), which was developed by a group of geophysicists and geodesists, goes a long... more

A gravity anomaly map of Europe was recently compiled that incorporates significant new data, especially from the eastern European countries. The map (Figure 1a), which was developed by a group of geophysicists and geodesists, goes a long way toward establishing a common and reliable database.Geological information contained in the regional gravity field are highlighted in the map, and all known large-scale geological structures are well coordinated with the anomaly patterns. The map offers basic gravity information for geophysical studies related to subsurface geology and large-scale tectonic features in Europe and reinforces or supports previous interpretations of largescale tectonic features.

Please cite this article as: Catalão, J., Sevilla, M.J., Mapping the geoid for Iberia and the Macaronesian Islands using multisensor gravity data and the GRACE geopotential Abstract A new gravimetric geoid model (ICAGM07) has been... more

Please cite this article as: Catalão, J., Sevilla, M.J., Mapping the geoid for Iberia and the Macaronesian Islands using multisensor gravity data and the GRACE geopotential Abstract A new gravimetric geoid model (ICAGM07) has been determined for the North-East Atlantic Ocean, Iberia, and the Macaronesian Islands using multi-sensor gravity data and a GRACE derived Earth geopotential model. A high resolution gravity model, determined using least squares optimal interpolation of marine, land, and satellite derived gravity anomalies, was used to resolve the medium and short wavelengths of the geoid. Long wavelengths of the geoid were provided by the GRACE derived Earth geopotential model. The topographic effects were computed in the spectral domain using a high resolution (100 m) digital terrain model derived from SRTM mission data and cartographic charts. The remove-restore technique was used to compute the geoid model on a 1.5 arc minute grid, and the residual geoid height was computed using spherical FFT and a modified Stokes' kernel. The effects of different Earth tide models on the geoid were computed and analyzed.

The AUSGeoid98 gravimetric geoid model of Australia has been computed using data from the EGM96 global geopotential model, the 1996 release of the Australian gravity database, a nationwide digital elevation model, and satellite... more

The AUSGeoid98 gravimetric geoid model of Australia has been computed using data from the EGM96 global geopotential model, the 1996 release of the Australian gravity database, a nationwide digital elevation model, and satellite altimeter-derived marine gravity anomalies. The geoid heights are on a 2 by 2 arcminute grid with respect to the GRS80 ellipsoid, and residual geoid heights were computed using the 1-D fast Fourier transform technique. This has been adapted to include a deterministically modi®ed kernel over a spherical cap of limited spatial extent in the generalised Stokes scheme. Comparisons of AUSGeoid98 with GPS and Australian Height Datum (AHD) heights across the continent give an RMS agreement of AE0.364 m, although this apparently large value is attributed partly to distortions in the AHD.

The use of GPS for the estimation of orthometric heights in a given region, with the help of existing levelling data requires the determination of a local geoid and the bias between the local levelling and the one implicitly defined when... more

The use of GPS for the estimation of orthometric heights in a given region, with the help of existing levelling data requires the determination of a local geoid and the bias between the local levelling and the one implicitly defined when the geoid is calculated which is generally based on the gravity anomalies data. The heights of new data can be collected swiftly without using the orthometric heights from levelling; it is what one calls commonly levelling by GPS. In this framework, the Least Squares Collocation method (LSC) has been used to evaluate the quality of the available GPS-Levelling data, to determine a gravimetric geoid in the North region of Algeria and to estimate the constant datum bias. The data used in the setting of this study are: The geopotential model EGM96, a total number of 2534 gravity anomalies, as well as 43 GPS points connected to the geodetic network levelling present on the whole North part of Algerian.

Iran is a mountainous country with large lateral density variations of its crust. Constant density value is commonly used to determine the geoid models as well as topographic corrections. The effect of lateral density variation in the... more

Iran is a mountainous country with large lateral density variations of its crust. Constant density value is commonly used to determine the geoid models as well as topographic corrections. The effect of lateral density variation in the geoid can reach up to 14 cm in Iran which is not negligible in a precise geoid modelling. Also, the current height datum of Iran is based on the orthometric system but the effect of gravity variation was not applied in height parameter. Furthermore, the height systems of most neighbouring countries are defined as normal height. Connection of networks can be useful for the unification of height datum, geodynamics researches and optimal adjustment of levelling network. The new quasi-geoid model based on a recent EGM2008 global geo-potential model was created to solve the mentioned problem. The main purpose of the present study is to discuss the results of a research project in which a gravimetric quasi-geoid model for Iran was computed based on the least-squares modification of Stokes' formula. The evaluation is made using 475 GPS/levelling height anomalies covering the major parts of the country except the mountainous areas to the North and West. After a 7-parameter fit, the most promising attempt achieved a RMS value of 19 cm for the residuals based on the GPS/levelling data.

Aeromagnetic signatures over the Edward VII Peninsula (E7) provide new insight into the largely ice-covered and unexplored eastern flank of the Ross Sea Rift (RSR). Positive anomalies, 10–40 km in wavelength and with amplitudes ranging... more

Aeromagnetic signatures over the Edward VII Peninsula (E7) provide new insight into the largely ice-covered and unexplored eastern flank of the Ross Sea Rift (RSR). Positive anomalies, 10–40 km in wavelength and with amplitudes ranging from 50 to 500 nT could reveal buried Late Devonian(?)–Early Carboniferous Ford Granodiorite plutons. This is suggested by similar magnetic signature over exposed, coeval Admiralty Intrusives of the Transantarctic Mountains (TAM). Geochemical data from mid-Cretaceous Byrd Coast Granite, contact metamorphic effects on Swanson Formation and hornblende-bearing granitoid dredge samples strengthen this magnetic interpretation, making alternative explanations less probable. These magnetic anomalies over formerly adjacent TAM and western Marie Byrd Land (wMBL) terranes resemble signatures typically observed over magnetite-rich magmatic arc plutons. Shorter wavelength (5 km) 150 nT anomalies could speculatively mark mid-Cretaceous mafic dikes of the E7, similar to those exposed over the adjacent Ford Ranges. Anomalies with amplitudes of 100–360 nT over the Sulzberger Bay and at the margin of the Sulzberger Ice Shelf likely reveal mafic Late Cenozoic(?) volcanic rocks emplaced along linear rift fabric trends. Buried volcanic rock at the margin of the interpreted half-graben-like “Sulzberger Ice Shelf Block” is modelled in the Kizer Island area. The volcanic rock is marked by a coincident positive Bouguer gravity anomaly. Late Cenozoic volcanic rocks over the TAM, in the RSR, and beneath the West Antarctic Ice Sheet exhibit comparable magnetic anomaly signature reflecting regional West Antarctic Rift fabric. Interpreted mafic magmatism of the E7 is likely related to mid-Cretaceous and Late Cenozoic regional crustal extension and possible mantle plume activity over wMBL. Magnetic lineaments of the E7 are enhanced in maximum horizontal gradient of pseudo-gravity, vertical derivative and 3D Euler Deconvolution maps. Apparent vertical offsets in magnetic basement at the location of the lineaments and spatially associated mafic dikes and volcanic rocks result from 2.5D magnetic modelling. A rift-related fault origin for the magnetic lineaments, segmenting the E7 region into horst and graben blocks, is proposed by comparison with offshore seismic reflection, marine gravity, on-land gravity, radio-echo sounding, apatite fission track data and structural geology. The NNW magnetic lineament, which we interpret to mark the eastern RSR shoulder, forms the western margin of the “Alexandra Mountains horst”. This fundamental aeromagnetic feature lies on strike with the Colbeck Trough, a prominent NNW half-graben linked to Late Cretaceous(?) and Cenozoic(?) faulting in the eastern RSR. East–west and north–north–east to NE magnetic trends are also imaged. Magnetic trends, if interpreted as reflecting the signature of rift-related normal faults, would imply N–S to NE crustal extension followed by later northwest–southeast directed extension. NW–SE extension would be compatible with Cenozoic(?) oblique RSR rifting. Previous structural data from the Ford Ranges have, however, been interpreted to indicate that both Cretaceous and Cenozoic extensions were N–S to NE–SW directed.

The present-day topography of the Iberian peninsula can be considered as the result of the Mesozoic -Cenozoic tectonic evolution of the Iberian plate (including rifting and basin formation during the Mesozoic and compression and mountain... more

The present-day topography of the Iberian peninsula can be considered as the result of the Mesozoic -Cenozoic tectonic evolution of the Iberian plate (including rifting and basin formation during the Mesozoic and compression and mountain building processes at the borders and inner part of the plate, during the Tertiary, followed by Neogene rifting on the Mediterranean side) and surface processes acting during the Quaternary. The northern-central part of Iberia (corresponding to the geological units of the Duero Basin, the Iberian Chain, and the Central System) shows a mean elevation close to one thousand meters above sea level in average, some hundreds of meters higher than the southern half of the Iberian plate. This elevated area corresponds to (i) the top of sedimentation in Tertiary terrestrial endorheic sedimentary basins (Paleogene and Neogene) and (ii) planation surfaces developed on Paleozoic and Mesozoic rocks of the mountain chains surrounding the Tertiary sedimentary basins. Both types of surfaces can be found in continuity along the margins of some of the Tertiary basins. The Bouguer anomaly map of the Iberian peninsula indicates negative anomalies related to thickening of the continental crust. Correlations of elevation to crustal thickness and elevation to Bouguer anomalies indicate that the di erent landscape units within the Iberian plate can be ascribed to di erent patterns: (1) The negative Bouguer anomaly in the Iberian plate shows a rough correlation with elevation, the most important gravity anomalies being linked to the Iberian Chain. (2) Most part of the so-called Iberian Meseta is linked to intermediate-elevation areas with crustal thickening; this pattern can be applied to the two main intraplate mountain chains (Iberian Chain and Central System) (3) The main mountain chains (Pyrenees and Betics) show a direct correlation between crustal thickness and elevation, with higher elevation/crustal thickness ratio for the Central System vs. the Betics and the Pyrenees. Other features of the Iberian topography, namely the longitudinal pro le of the main rivers in the Iberian peninsula and the distribution of present-day endorheic areas, are consistent with the Tertiary tectonic evolution and the change from an endorheic to an exorheic regime during the Late Neogene and the Quaternary. Some of the problems involving the timing and development of the Iberian Meseta can be analysed considering the youngest reference level, constituted by the shallow marine Upper Cretaceous limestones, that indicates strong di erences induced by (i) the overall Tertiary and recent compression in the Iberian plate, responsible for di erences in elevation of the reference level of more than 6 km between the mountain chains and the endorheic basins and (ii) the e ect of Neogene extension in the Mediterranean margin, responsible for lowering several thousands of meters toward the East and uplift of rift shoulders. A part of the recent uplift within the Iberian plate can be attributed o sostatic uplift in zones of crustal thickening.

Sediment accumulation patterns in the Luni basin have been studied on the basis of tubewell lithologs and Bouguer gravity anomaly profiles. East}West geologic transects using these data reveal several sediment-filled graben depressions,... more

Sediment accumulation patterns in the Luni basin have been studied on the basis of tubewell lithologs and Bouguer gravity anomaly profiles. East}West geologic transects using these data reveal several sediment-filled graben depressions, for example, the Digrana-Bhawal graben in the northern part, the Mangta-Sindari, Sindari-Bhimgoda, and Bhimgoda-Juna Motisara grabens in the central part; and the Dungari-Ratanpura and Ratanpura-Khanpur grabens in the southern part. Maximum sediment accumulation of more than 300 m is located in the southwestern part of the Luni basin, more specifically in the Ratanpura-Khanpur graben. Minor depressions also occur towards west of Jodhpur and east of Bhadrajun.

The integrated geophysical interpretation for the different geophysical tools such as resistivity and gravity is usually used to define the structural elements, stratigraphic units, groundwater potentiality, and depth to the basement... more

The integrated geophysical interpretation for the different geophysical tools such as resistivity and gravity is usually used to define the structural elements, stratigraphic units, groundwater potentiality, and depth to the basement rocks. In the present work, gravity and resistivity data were utilized for detecting the groundwater aquifer and structural elements, as well as the upper and lower surfaces of the subsurface basaltic sheet in an area located at the eastern side of Ismailia Canal, northeastern Greater Cairo, Egypt. Two hundred and ten gravity stations were measured using an Autograv instrument through a grid pattern of 50 × 50 m. The different required corrections were carried out, such as drift, elevation, tide, and latitude corrections. The final corrected data represented by the Bouguer anomaly map were filtered using high- and low-pass filters into regional and residual gravity anomaly maps. The resulting residual gravity anomaly map was used for gravity modeling to calculate the depths to the upper and lower surfaces of the basaltic sheet. The resulting gravity models indicated that the depths to the upper surface of the basaltic sheet are ranged between 26 and 314 m, where the shallower depths were found around the southern and eastern parts. The depths to the lower surface of the basaltic sheet are varied from 86 to 338 m, and the thickness of the basaltic sheet is ranged from 24 to 127 m, where the biggest thicknesses were found around the southern and northern parts of the study area. Forty-two vertical electrical soundings (VES) were carried out using Schlumberger configuration with AB/2 spacings ranged from 1.5 to 500 m. 1D quantitative interpretation was carried out through manual and analytical interpretations. The VES data were also inverted assuming a 3D resistivity distribution. The results from the 3D resistivity inversion indicated that the subsurface section consists of sand, sandstone, and sandy–clays of Miocene deposits overlying the basalts. Such basaltic features (of Oligocene age) are underlain by Gabal Ahmar Formation of Oligocene deposits, which are composed of sand and sandstone. Therefore, two aquifers were deduced in the area. The first is the Miocene aquifer (shallower) and the other is the Oligocene aquifer (deeper). التفسير الجيوفيزيائي التكاملي للطرق الجيوفيزيقية المختلفة مثل المقاومة الكهربية والجاذبية الأرضية عاده تستخدم لتحديد التراكيب الجيولوجية , الوحدات الصخرية , امكاتية تواجد المياه الجوفية وعمق صخور القاعدة. في العمل الحالي تم استخدام طريقة الجاذبية الارضية والمقاومة الكهربية لتحديد الخزان الجوفي , التراكيب الجيولوجية وكذلك عمق السطح العلوي والسفلي لطبقة البازلت لمنطقة الدراسة الواقعة علي الجانب الشرقي لترعه الأسماعيلية , شمال شرق القاهرة الكبري , مصر. مائتان وعشر محطة جاذبية أرضية تم قياسها بأستخدام جهاز اوتوجراف لشبكة من المحطات تمثل 50 x50 متر. تم إجراء التصحيحات المختلفة لبيانات الجاذبية الأرضية مثل تصحيح الجهاز , تصحيح الأرتفاع , تصحيح المد والجزر وتصحيح الوضع المكاني للمحطات بالنسبة لخط العرض. الصوره النهائية للبيانات المصححة تمثل خريطة شاذات البوجير, تم عمل فصل للمركبتين الأ قليمية والمحلية باستخدام الترخيص العالي والمنخفض للبيات المصححة. خريطة الجاذبية الارضية تم استخدامها في عمل نماذج جاذبية في بعدين لتحديد عمق السطح العلوي والسفلي لطبقة البازلت. نتائج نماذج الجاذبية الأرضية اظهرت ان عمق السطح العلوي لطبقة البازلت يتراوح بين 26 -314 متر حيث ان الاعماق الضحلة للسطح العلوي لطبقة البازلت تتواجد في الجانب الشرقي والجانب الجنوبي من منطقة الدراسة, اما عمق السطح السفلي لطبقة البازلت يتراوح بين 86 – 338 متر وسمك طبقة البازلت يتراوح بين 24 – 127 متر واكبر سمك لطبقة البازلت تتواجد في الجانب الجنوبي والشمالي من منطقة الدراسة. خمسه واربعون جسة كهربية عمودية تم اجراؤها بنظام شلمبرجير بمسافة بين الأقطاب أب/2 تتراوح بين 1.5 – 500 متر. التفسير التكاملي احادي البعد لبيانات الكهربية الأرضية تم باستخدام التفسير اليدوي والتحليلي . بيانات الجسات العمودية احادية البعد تم تحويلها الي بيانات مقاومة كهربية ثلاثية البعد . نتائج التحويل لبيانات المقاومة الكهربية ثلاثية البعد اظهرت ان القطاع التحت سطحي لمنطقة الدراسة يتكون من رمل , وحجر رملي وطفلة رملية المنتميين لعصر الميوسن الأوسط متواجدين فوق طبقة البازلت التي تنتمي الي عصر الأوليجوسين . طبقة البازلت تتواجد فوق تكوين الجبل الأحمر الذي يتكون من الرمل والحجر الرملي. يتواجد خزانين للمياه الجوفية بمنطقة الدراسة , الخزان الأول ( الخزان الضحل) ينتمي الي الي عصر الميوسين والخزان العميق ينتمي الي عصر الأوليجوسين.

We have estimated the effective elastic thickness of the continental lithosphere beneath South Africa using the coherence technique. This involves (1) estimating the coherence between Bouguer gravity anomalies and topography in the... more

We have estimated the effective elastic thickness of the continental lithosphere beneath South Africa using the coherence technique. This involves (1) estimating the coherence between Bouguer gravity anomalies and topography in the spectral domain and (2) comparing the coherence with that predicted by an elastic plate model that flexes under loads placed on, within and beneath the lithosphere. The depth to the base of this elastic layer which gives the best root-mean-square fit between estimated and predicted coherence is the effective elastic thickness (Te). Two major tectonic provinces, namely, the Archean Kaapvaal Craton and the Mesoproterozoic Namaqua-Natal Mobile Belt (which together form the Kalahari Craton in southern Africa), are found to have T e values of 72 km and 38 to 48 km, respectively. There is indication from the coherence data that over the Kalahari Craton, topographic features with equivalent wavelengths less than-200-300 km are supported by the rigidity of the lithosphere while features with wavelengths greater than 700 km are compensated. It is implied from the present findings that each of the two South African tectonic provinces can be considered as separate coherent domains. On the basis of geotherms from both provinces, the effective elastic thicknesses obtained point to a lithospheric basal temperature of about 600øC. Geologic and geophysical considerations suggest that the contrast in flexural rigidity of the lithosphere between the two provinces can be attributed to the combined effects of compositional and thickness differences of the lithosphere, as well as variation in present-day asthenospheric heat flow. Our T e estimates of the Namaqua-Natal Mobile Belt are similar to those of the Mesoproterozoic Grenville belt, corroborating tectonic models which suggest that these two belts may once have been part of the same mobile belt. In contrast, the T e estimate of the Kaapvaal Craton fall below average global Archean cratonic values; the reasons for this are not clear. Pilkington, 1991], in Russia [Diament and Kogan, 1990] and in West and Central Africa [Hartley and Allen, 1994]. Gravity studies in South Africa have shown that the lithosphere is, in general, in isostatic equilibrium, although 11,291 11,292 DOUCOUR• ET AL.' ELASTIC THICKNESS OF SOUTH AFRICA several small anomalies cannot be explained entirely by the Airy model of isostatic compensation [Hales and Gough, 1959; De Beer and Meyer, 1983]. The earliest reference to lithospheric flexure in South Africa was made by King [1955] to explain the surface morphology of this country. Following King's [1955] suggestion, ten Brink and Stern [1992] estimated Te in South Africa to be in the range 80-120 km, through forward modeling of rift flank uplifts. These estimates are sensitive, however, to the width of the initial uplift zone, a modeling parameter, which can introduce a variation in the elastic thickness by 10-20 km. The modeling carried out by ten Brink and Stern [1992] also did not discriminate lithospheric domains from distinctly differing geological provinces, while it is known that combining domains with differing flexural rigidities can be source of bias in estimating the elastic thickness [Forsyth, 1985; Bechtel et al., 1990]. In their analysis of variations in the elastic thickness beneath continental thrust belts, McNutt et al. [1988] stress that spectral estimates of elastic plate thickness appear to be much more reliable than theoretical calculations for fractured plates. In this paper, we invert gravity and topography data to determine the effective elastic thickness of the continental lithosphere in South Africa using the coherence technique of Forsyth [1985] which allows for both surface and subsurface loading. No a priori geological model is assumed because of the complexity of the South African lithosphere. This complexity can be seen in the extensive deformation and metamorphism and igneous intrusions which have affected this region episodically over a time span of more than 3.0 Gyr [Tankard et al., 1982]. Geologic Setting Most of the crystaline rocks of South Africa are of Precambrian age. About half this Precambrian basement is hidden beneath a relatively thin cover of Paleozoic sediments, a small remnant of a thick blanket of Jurassic lavas, and a thin veneer of Mesozoic to Recent continental deposits. The Precambrian history may be discussed in terms of four major stages [Tankard et al., 1982; Hartnady et al., 1985; de Wit et al., 1992; Thomas et al., 1993]: (1) formation of the earliest Archean crust and its cratonic stabilization to form the Kaapvaal Craton between circa 3.7 to 2.7 Ga; (2) Late Archean to Paleo-proterozoic sedimentary basins covering the Archean basement with a substantial thickness of sediments and volcanic rocks between 2.7 and 2 Ga; (3) extensive Mesoproterozoic (2-1 Ga) continental crustal growth and orogenic activity flanking the Kaapvaal Craton (the Mesoproterozoic events include early accretion of juvenile Eburnian (2 Ga) arcs and subsequent extensive Kibaran (1.2-1 Ga) deformation, metamorphism, and uplift to form the Namaqua-Natal Mobile Belt (NNMB); together, the Kaapvaal Craton and the NNMB constitute an enlarged stable craton referred to as the Kalahari Craton); and (4) a Neoproterozoic (frequently refered to as "Pan African") Wilson cycle between 1 and 0.5 Ga, terminating in the formation of Gondwana. The Phanerozoic history of South Africa can be

Extending the compact gravity inversion technique by incorporating a priori information about the maximum compactness of the anomalous sources along several axes provides versatility. Thus, the method may also incorporate information... more

Extending the compact gravity inversion technique by incorporating a priori information about the maximum compactness of the anomalous sources along several axes provides versatility. Thus, the method may also incorporate information about limits in the axes lengths or greater concentration of mass along one or more directions. The judicious combination of different constraints on the anomalous mass distribution allows the introduction of several kinds of a priori information about the (arbitrary) shape of the sources. This method is particularly applicable to constant, linear density sources such as mineralizations along faults and intruded sills, dikes, and laccoliths in a sedimentary basin. The correct source density must be known with a maximum uncertainty of 40 percent; otherwise, the inversion produces thicker bodies for densities smaller than the true value and vice-versa. Because of the limitations of the inverse gravity problem, the proposed technique requires an empirical technique to analyze the sensitivity of solutions to uncertainties in the a priori information. The proposed technique is based on a finite number of acceptable solutions, presumably representative of the ambiguity region. By using standard statistical techniques, each parameter is assigned a coefficient measuring its uncertainty. The known hematite and magnetite ore body shape, in the vicinity of Iron Mountain, MO, was reproduced quite well using this inversion technique.

A statistical comparison has been made between gravity field parameters derived from different global geopotential models (GGMs) and observed gravity anomalies, gravimetric geoid and GPS-Leveling data. The motivation behind this study is... more

A statistical comparison has been made between gravity field parameters derived from different global geopotential models (GGMs) and observed gravity anomalies, gravimetric geoid and GPS-Leveling data. The motivation behind this study is the selection of best possible global geopotential model that best matches statistically with the local observed data in Pakistan. This will facilitate in decreasing the load on observed data for the development of regional gravimetric geoid in remove-compute-restore technique when used in the Stokes's integral for computation of the residual part. It is observed that combined geopotential models such as EGM96 and PGM200A, EIGEN-GL04C and EIGEN-CG03C reflect the better match in the total spectral range of gravity and GPS-Leveling data. Results of the precise local geoid model also indicate similar characteristics. A very-high-degree model ''EGM2008" (degree/order 2160) exhibits relatively superior statistical fit with observed ground data in Pakistan region. For satellite-only models an increasing trend in the standard deviation can be seen with maximum of about $4 m in difference between GPS-Leveling and corresponding GGM's geoid with increase in the order from 50 to 120 and then it decreases afterwards. However, for the EIGEN-CHAMP03SP, standard deviation saturates to a value of 3.4 m. This is an indication of contamination in the long to medium wavelength part, i.e. 50-100°for the satellite-only models. Moreover, the models DEOS-CHAMP-01C, GGM02C and then ITG-GRACE03 appear to have better fit for medium to long wavelength and can possibly be recommended for use as long wavelength part with the local observed data. While a hybrid geopotential model selection can be achieved through the selection from either of DEOS-CHAMP-01C, GGM02C, GGM02S, EIGEN-GRACE02S or ITG-GRACE03 in the long wavelength (to degree and order 40) and EGM96, PGM200A, EIGEN-GL04C, EIGEN-CG03C or even EGM2008 in medium to short wavelength, i.e. from degree 41 to maximum degree and order.

The contribution of bathymetry to the estimation of gravity field related quantities is investigated in an extended test area in the Mediterranean Sea. The region is located southwest of the island of Crete, Greece, bounded between 33 ≤ ϕ... more

The contribution of bathymetry to the estimation of gravity field related quantities is investigated in an extended test area in the Mediterranean Sea. The region is located southwest of the island of Crete, Greece, bounded between 33 ≤ ϕ ≤ 35 and 15 ≤ λ ≤ 25. Gravity anomalies from the KMS99 gravity field and shipborne depth soundings are used

Un inclinómetro de agua de base larga (LBT) se construyó en la costa del océano Pacífico de México para realizar observaciones de inclinación de alta precisión en una área de la zona de subducción, la cual aparentemente está en fase de... more

Un inclinómetro de agua de base larga (LBT) se construyó en la costa del océano Pacífico de México para realizar observaciones de inclinación de alta precisión en una área de la zona de subducción, la cual aparentemente está en fase de deformación intersísmicapresísmica. El continuo monitoreo de la inclinación complementará las mediciones de GPS y de la deformación vertical (por nivelación) en esta área con datos continuos de alta precisión. La interfase sismogénica entre las placas tectónicas en la zona de subducción está razonablemente bien determinada a partir de la sismicidad local y de anomalías gravimétricas. La combinación de observaciones geodésicas y de sismicidad permiten monitorear la evolución de la deformación en los límites superior e inferior de la zona sismogénica en la cual es probable la ocurrencia de un sismo de M~7.0. Los datos del LBT se usarán de manera inmediata para aprender mucho acerca del proceso de carga tectónica que excita la ruptura sísmica. Los principios, características técnicas y el procedimiento de instalación del LBT, así como algunas observaciones preliminares y resultados son presentados.

AbstractÐThe currently adopted approach to reduce observed gravity data for geophysical purposes includes several approximations. These were originally used to reduce computational eort, but have remained standard practice, even though... more

AbstractÐThe currently adopted approach to reduce observed gravity data for geophysical purposes includes several approximations. These were originally used to reduce computational eort, but have remained standard practice, even though the required computing power is now readily available. In contrast, more precise gravity reductions are routinely employed in physical geodesy. The dierence between simple Bouguer gravity anomalies derived using the geophysical and geodetic approaches can reach several tens of m mm sec À2 . The geodetic reductions include a more accurate calculation of normal gravity as a function of latitude, and a free air correction that accounts for the non-sphericity of the ®gure of the Earth. Also important, especially given the advent of Global Positioning System coordination of gravity surveys, is the need to ensure that the correct vertical and horizontal coordinate systems are used for the gravity reduction procedure. Errors associated with the use of non-geocentric horizontal coordinates and ellipsoidal heights are signi®cant when compared with the accuracy of an individual gravity measurement. A generalised gravity reduction program and a coordinate transformation program are presented which can be employed to reduce geophysical data in a geodetic manner. # 1998 Elsevier Science Ltd

The North American gravity database as well as databases from Canada, Mexico, and the United States are being revised to improve their coverage, versatility, and accuracy. An important part of this effort is revising procedures for... more

The North American gravity database as well as databases from Canada, Mexico, and the United States are being revised to improve their coverage, versatility, and accuracy. An important part of this effort is revising procedures for calculating gravity anomalies, taking into account our enhanced computational power, improved terrain databases and datums, and increased interest in more accurately defining long-wavelength anomaly components. Users of the databases may note minor differences between previous and revised database values as a result of these procedures. Generally, the differences do not impact the interpretation of local anomalies but do improve regional anomaly studies. The most striking revision is the use of the internationally accepted terrestrial ellipsoid for the height datum of gravity stations rather than the conventionally used geoid or sea level. Principal facts of gravity observations and anomalies based on both revised and previous procedures together with germane metadata will be available on an interactive Web-based data system as well as from national agencies and data centers. The use of the revised procedures is encouraged for gravity data reduction because of the widespread use of the global positioning system in gravity fieldwork and the need for increased accuracy and precision of anomalies and consistency with North American and national databases. Anomalies based on the revised standards should be preceded by the adjective "ellipsoidal" to differentiate anomalies calculated using heights with respect to the ellipsoid from those based on conventional elevations referenced to the geoid.

The Marrakech region is known for its significant mining potential concealed by the Hercynian basement. In order to extend the exploration of this basement beyond its outcrops in the Jebilets and Guemassa massifs, we used magnetic and... more

The Marrakech region is known for its significant mining potential concealed by the Hercynian basement. In order to extend the exploration of this basement beyond its outcrops in the Jebilets and Guemassa massifs, we used magnetic and gravity data from a sub-area in the Haouz basin. Our methodology in this study consists of processing the data, so as to highlight the buried geological contacts and to reveal their dips. The study of the geological contacts is ensured by the upward continuation processing of the residual gravity map at various altitudes, followed by the determination of the horizontal gradient maxima for each level. The progressive migration of these maxima while increasing the upward continuation altitude indicates the dip direction. The structures deduced from gravity are superimposed on the magnetic map in order to emphasize their possible association with magnetic sources.

The Antarctic Peninsula (AP) is a composite magmatic arc terrane formed at the Pacific margin of Gondwana. Through the late Mesozoic and Cenozoic subduction has stopped progressively from southwest to northeast as a result of a series of... more

The Antarctic Peninsula (AP) is a composite magmatic arc terrane formed at the Pacific margin of Gondwana. Through the late Mesozoic and Cenozoic subduction has stopped progressively from southwest to northeast as a result of a series of ridge trench collisions. Subduction may be active today in the northern part of the AP adjacent to the South Shetland Islands. The subduction system is confined by the Shackleton and Hero fracture zones. The magmatic arc of the AP continental margin is marked by high-amplitude gravity and magnetic anomaly belts reaching highest amplitudes in the region of the South Shetland Islands and trench. The sources for these anomalies are highly magnetic and dense batholiths of mafic bulk composition, which were intruded in the Cretaceous, due to partial melting of upper-mantle and lower-crustal rocks. 2-D gravity and magnetic models provide new insights into crustal and upper-mantle structure of the active and passive margin segments of the northern AP. Our models incorporate seismic refraction constraints and physical property data. This enables us to better constrain both Moho geometry and petrological interpretations in the crust and upper mantle. Model along the DSS-12 profile crosses the AP margin near the Anvers Island and shows typical features of a passive continental margin. The second model along the DSS-17 profile extends from the Drake Passage through the South Shetland Trench/Islands system and Bransfield Strait to the AP and indicates an active continental margin linked to slow subduction and on-going continental rifting in the backarc region. Continental rifting beneath the Bransfield Strait is associated with an upward of hot upper mantle rocks and with extensive magmatic underplating.

The Middle-Late Proterozoic Eastern Ghats Mobile Belt (EGMB) exhibits a conspicuous paired gravity anomaly structure with a relative gravity high over the EGMB and low over the adjoining Archaean cratons. 2D gravity modelling across the... more

The Middle-Late Proterozoic Eastern Ghats Mobile Belt (EGMB) exhibits a conspicuous paired gravity anomaly structure with a relative gravity high over the EGMB and low over the adjoining Archaean cratons. 2D gravity modelling across the EGMB, constrained by available seismic information, suggests an eastward dipping crustal column with a thick crust of about 38-40 km under the adjoining older cratons which warped up to 35 km under the younger EGMB. The oblique ridge like structure along with the deep seated intrusive such as anorthosites and carbonatites, and occurrences of ophiolite melange indicate an ancient collision zone along the eastern margin of the adjoining cratons. The westward verging structures together with the exposed charnockite and khondalite rocks along the EGMB may represent lower crustal rock upthrusted from the east to the west. Highly disturbed Middle-Late Proterozoic Purana sedimentary basins abutting the EGMB may represent the peripheral foreland basins over the eastward subducting adjoining Archaean cratons.

Iran is one of the most active regions in the world with respect to earthquakes and tectonic motions in the lithosphere. In order to study the impact of the geoid model in detecting plate tectonic boundaries and in the establishment of an... more

Iran is one of the most active regions in the world with respect to earthquakes and tectonic motions in the lithosphere. In order to study the impact of the geoid model in detecting plate tectonic boundaries and in the establishment of an accurate height datum for future geodynamic observations, a new combined gravimetric geoid model for Iran (IRG04C) was computed by the method of least squares modification of Stokes formula based on the most recent gravity anomaly database, SRTM high resolution Digital Elevation Model (DEM), GRACE GGM02 Global Geopotential Model and GPS/levelling data. The RMS fit of the new geoid model versus GPS/levelling data is 9 cm, which is a 10 times better fit compared to the most recent published gravimetric geoid model in the area. An integrated approach, combining gravity, geoid and seismology data as well as a digital elevation model, was used to find possible correlations between these parameters. Our investigation indicates that all earthquakes with magnitude over 6.0 in the Richter scale are located in areas with a geoid slope exceeding 7.5%. The study shows a significant correlation between the horizontal gradient of the geoid and plate tectonic activities.

The Dead Sea basin is located along the left-lateral transform boundary between the Arabian and Sinai plates. Its structure and history are known from surface geology, drilling, seismic reflection and other geophysical data. The basin... more

The Dead Sea basin is located along the left-lateral transform boundary between the Arabian and Sinai plates. Its structure and history are known from surface geology, drilling, seismic reflection and other geophysical data. The basin comprises a large pull-apart, almost 150 km long and mostly 8-10 km wide, which is flanked by a few kilometres wide zones of normal faulting. The basin formed at about 15 Ma or earlier, close to the beginning of the transform motion, and it reached about half its present length before the end of the Miocene. A strong negative gravity anomaly records a thick sediment basin fill: >5 km under half its length, reaching a maximum of _> 10 km. The fill includes a few km of salt (ca. 6-4 Ma) which forms several diapirs. At any one time large parts of the basin subsided simultaneously, but the site of fastest subsidence seems to have shifted northward. Sedimentation rates reached at least hundreds of metres per million years or more in the Miocene, and > 1 km/Myr in later periods.