Magnetotelluric Research Papers - Academia.edu (original) (raw)

The MT interpretation procedure begins with a set of sounding data in the frequency domain. The overall quality of these data can be variable both as a function of frequency and location. Many simple interpretation procedures, such as the... more

The MT interpretation procedure begins with a set of sounding data in the frequency domain. The overall quality of these data can be variable both as a function of frequency and location. Many simple interpretation procedures, such as the assessment of static distortion, act directly on the sounding data. A number of response characteristics, such as the location (in frequency) and number of turning points, are important to the interpretation. Localised scatter (noise) in the response estimates can produce false gradients which degrade the quality of the inferences made from the data. This study considers how the D + solution can be used to process the raw sounding data to provide a number of interpretational advantages. Although the D + solution has strict formal roots in 1D inverse theory, it is used here simply to enhance those data attributes, particularly that of physical validity, which lead to a more meaningful assessment of data characteristics. The data considered are 84 broadband array soundings from the Paraná basin, Brazil. The advantages provided by the D + processed data set are demonstrated by using the raw and processed data in two main interpretational procedures. The first procedure concerns the ability of the data to provide quantitative assessments of the influence of static distortion. The second procedure concerns the application of transform methods which attempt to recover a resistivity/depth or reflectivity profile directly from the sounding data.

[Abstract]-Magnetotelluric (MT) is a passive geophysical method that utilizing natural electromagnetic wave source by measuring electric field variation in an upright and changing towards time with the result that sub surface resistivity... more

[Abstract]-Magnetotelluric (MT) is a passive geophysical method that utilizing natural electromagnetic wave source by measuring electric field variation in an upright and changing towards time with the result that sub surface resistivity variation can be defined. Estimated time for MT measuring was 12 hours so data was affected by noise and caused decreasing grade of coherence around 1. Increasing grade of coherence can be done by choosing proper robust technique and time series analysis at function transfer time series into frequency series, and then continued by selecting the correct XPR. Increasing grade of coherence analysis was done in 22 points of MT measuring on Sawu Island, East Nusa Tenggara by Pusat Survei Geologi on 2014. The components parameter of coherence include electric field (E) and magnetic field (H) will increase by equal value or coheren value, so that errors in inversion part and interpretation part will decrease. Correct data processing by using parameter of coherence as standard of quality from data changing was proven increased quality of data by 25%. Increasing grade of coherence happened at whole MT measuring with average 90.66% or 0.9 excellent coherence. Impact of this increase is affecting the trend of resistivity value into more representative than before while the data was still affected by noise which means the grade of coherence is capable to be inverted and interpreted.

The general problem of magnetoteluric (MT) data is due to the difference of coordinate system between the measurement strike and the subsurface condition. Whereas, Magnetotelluric is a passive method that used natural electromagnetic (EM)... more

The general problem of magnetoteluric (MT) data is due to the difference of coordinate system between the measurement strike and the subsurface condition. Whereas, Magnetotelluric is a passive method that used natural electromagnetic (EM) field, that affect the MT data ambiguity level. MT data processing refers to coherency parameters. Where the coherence is a real dimensionless number indicating the suitability between the two fields (electric and magnetic). Qualitatively, impedance rotation can be used as a controller of MT data quality in terms of the lack of splitting data that occurs between TE and TM. Rotation process is done with two types of rotation, namely principal axis (strike angle) and user defined (fix angle). We have tested on one MT station by applying a new data processing technique, that is rotating (fix angle & strike angle) data prior to XPR, resulting in five different types of data for comparison is XPR (no rotation), XPR PREfix rotation, XPR POST-fix rotation, XPR PRE-strike rotation, and XPR POST-strike rotation. The result of data analysis of all conventional and non-conventional techniques, with angle correction of 25.5 degrees, we decided that the XPR PREfix rotation data as the best data qualitatively reviewed (lack of splitting between TE and TM to from high frequency to lower frequencies) and quantitative (coherence 89,936%).

Intisari. Pemodelan resistivitas 2-D pada penelitian ini dilakukan dengan 2 data MT yang berbeda. Data pertama adalah data yang dirotasi berdasarkan modus tipper strike dari seluruh frekuensi pada setiap titik pengukuran sehingga tiap... more

Intisari. Pemodelan resistivitas 2-D pada penelitian ini dilakukan dengan 2 data MT yang berbeda. Data pertama adalah data yang dirotasi berdasarkan modus tipper strike dari seluruh frekuensi pada setiap titik pengukuran sehingga tiap titik pengukuran dirotasi pada sudut yang berbeda. Data kedua adalah data yang dirotasi berdasarkan modus tipper strike dari seluruh frekuensi dan seluruh titik pengukuran sehingga seluruh titik pengukuran dirotasi pada sudut yang sama. Data kedua memberikan hasil nilai rms error global yang lebih kecil dibandingkan pendekatan pertama sehingga interpretasi dilakukan menggunakan model kedua. Model kedua menunjukkan adanya benda konduktif vertikal miring pada kedalaman ~8 km di bawah permukaan laut dan diinterpretasikan sebagai intrusi magma yang naik melalui sistem sesar Sumatera. Model dan interpretasi tersebut dapat menjelaskan penyebab anomali negatif gravitasi pada area penelitian serta hasil analisis geokimia yang menunjukkan adanya kandungan air yang berasal dari magma pada manifestasi air panas di sekitar area penelitian. Batas kedalaman zona alterasi smektit diperkirakan berada pada kedalaman 500 m di bawah permukaan laut. Metode magnetotellurik (MT) merupakan salah satu metode geofisika yang banyak diterapkan pada eksplorasi panasbumi serta studi mengenai sesar. Metode tersebut memiliki kelebihan antara lain: dapat memetakan distribusi resistivitas bawah permukaan hingga orde puluhan kilometer tanpa sumber buatan, peralatan yang relatif praktis sehingga dapat diterapkan pada kondisi medan yang cenderung sulit, serta konsep geoelectric strike pada metode MT yang seringkali berhubungan dengan persebaran struktur-struktur geologi di bawah permukaan. Menurut Siripunvaraporn et al (2004), pemodelan resistivitas 2-D metode MT pada umumnya dimulai dari mengumpulkan informasi mengenai struktur geologi regional kemudian mengambil data MT dengan orientasi titik pengukuran yang memotong jurus dari struktur tersebut.

In this paper, some recent topics on the modeling of magnetotelluric data are introduced. The focus is on the handling of real field data for two-dimensional resistivity modeling. First, the removal of the effects of near surface... more

In this paper, some recent topics on the modeling of magnetotelluric data are introduced. The focus is on the handling of real field data for two-dimensional resistivity modeling. First, the removal of the effects of near surface heterogeneity is reviewed. It covers telluric distortions (phase mixing and static shift) and magnetic distortions using conventional Groom-Bailey type 3D/2D model (three-dimensional local anomaly underlain by regional two-dimensional structure). The extension of a 3D/2D distortion model for multi-site, multi-frequency is a new development. Magnetic distortion seems to be less significant for land observations, but significant for sea floor data, where the regional magnetic field is weak due to seawater. In special cases involving for example, distortion due to topography and bathymetry, explicit removal is possible. There are some schemes proposed for a 3D/3D model (three-dimensional local anomaly underlain by regional three-dimensional structure). Along with the removal of the distortion, it is important to recognize the dimensionality of the dataset prior to modeling. A property using strike estimates for each site is an indicator for dimensionality which is unaffected by local distortion. Mapping the local strike or a rose diagram is an effective visualization of the dimensionality. Two-dimensional inversion is becoming routine. For the fast calculation of derivatives, approximate calculation, reciprocity or conjugate gradient methods are used. In order to incorporate a priori information and to overcome the intrinsic ill-posed nature of the inversion problem, imposing constraints on the model structure is important. A proper tradeoff between the data fit and constraints should be optimized to obtain minimum structures that are required by the field data. However, the choice of constraints is rather subjective and depends on the geological situations. For field data, two-dimensional inversion has limits on modes, area, and period range. Special care must be taken for the structure outside the profile. Two-dimensional inversion incorporating anisotropy is interesting and becoming popular, but the structure may not be unique. Future development in three-dimensional inversion for real datasets should take the above points into consideration.

Due to the big consuming of ground water in agricultural activities, the reservoir water of Kharga Oasis is noticeably decreased. The demand of drilling deeper wells is increased and most of them reached to the basement rocks. The well... more

Due to the big consuming of ground water in agricultural activities, the reservoir water of Kharga Oasis is noticeably decreased. The demand of drilling deeper wells is increased and most of them reached to the basement rocks. The well data have been observed since 40 years. Over-exploitation of reservoir systems has caused a decline in water table in many parts in the area under studied between 60-80 m. Hot springs are found in the southern part of Kharga City of water temperature 45° C. Magnetotelluric (MT) measurements are carried out in the northern part of Kharga oasis, in order to image the deep part of its reservoir water, using available data of hydro-geology and boreholes information. The results of MT analysis in 1D and 2D show that the sediments of Quaternary and Paleocene (less than 1 km.) have low resistivity value, while the high resistivity value belongs to the basement complex of Pre-Cambrian rocks at depths greater than 1 km.
Keywords: Kharga Oasis Reservoir water (KOR), hydro-geology and magnetotellurics

Within the framework of the Southern African MagnetoTelluric EXperiment (SAMTEX) a focused study was undertaken to gain better knowledge of the lithospheric geometries and structures of the westerly extension of the Zimbabwe Craton into... more

Within the framework of the Southern African MagnetoTelluric EXperiment (SAMTEX) a focused study was undertaken to gain better knowledge of the lithospheric geometries and structures of the westerly extension of the Zimbabwe Craton into Botswana, with the overarching aim to increase our understanding of southern African tectonics. The area of interest is located in northeastern Botswana, where Kalahari sands cover

Mt Ruapehu is an active andesite cone volcano, which marks the southern termination of the Kermadec volcanic arc. Results from 40 broad-band magnetotelluric soundings have been analysed using the phase tensor. This approach provides a way... more

Mt Ruapehu is an active andesite cone volcano, which marks the southern termination of the Kermadec volcanic arc. Results from 40 broad-band magnetotelluric soundings have been analysed using the phase tensor. This approach provides a way of determining dimensionality, allowing for distortion removal, and visualizing data in a 3-D situation. The phase tensor analysis suggests that the shallow resistivity structure is largely 1-D in character, but that the deeper structure requires a 3-D interpretation. 1-D inversions show that at sites on Ruapehu a shallow conductive layer lies between a high resistivity layer, of a few hundred metres thickness, and higher resistivity layer corresponding to basement greywacke. The low resistivity layer is contiguous with the waters of the highly acidic Crater Lake, and thus is believed to be the hydraulically controlled upper limit of a zone of acid alteration overlain by dry volcanic rock and ash. To the southwest of the volcano the conductive layer merges with a surface conductor associated with Tertiary sediments. Following initial 2-D inversions, the deep resistivity structure has been derived through 3-D inversion of data from 38 sites. This indicates the existence of a dyke-like low resistivity zone that persists to at least 10 km depth and extends from beneath the summit of Ruapehu to the northeast where it appears to connect to a poorly constrained region of high conductivity, which lies outside the network of measurement sites. The low resistivity dyke-like feature may be identified with a volcanic feeder system, which also supplies the other volcanoes of the Tongariro Volcanic Centre and marks the conduit by which hot gases and (occasionally) magma reach the surface.

The Deccan Volcanic Province in the western part of the peninsular India consists of a thick pile of Cood basalts resting mainly on the Archaean and Proterozoic rocks forming the basement. This intraplate region experiences moderate... more

The Deccan Volcanic Province in the western part of the peninsular India consists of a thick pile of Cood basalts resting mainly on the Archaean and Proterozoic rocks forming the basement. This intraplate region experiences moderate seismic activity, the most recent one being a swarm-type activity in the Palghar region, about 120 km north of Mumbai, that started in November 2018 and has produced a few thousand micro-earthquakes and a 4.3 magnitude earthquake since then. We have carried out a mag-netotelluric (MT) study along a 35-km long proBle across the seismic zone to delineate the subsurface structure to understand the possible cause for the seismic activity. Broadband MT data were acquired at 18 sites with average station spacing of 2 km. Impedance tensors were analyzed for distortion and dimensionality, decomposed into TE-and TM-mode, and inverted by a 2D inversion algorithm. The geoelectric structure yields an assemblage of highly resistive and moderately conductive blocks in the uppermost crust resting on a major listric-type fault, that possibly reaches the surface at the West Coast Fault from a depth of about 15 km beneath the Panvel Cexure zone. In conjunction with the regional Bouguer gravity anomaly and the seismicity distribution, we infer that the upper crustal heterogeneities coupled with the basement fault and low rheological strength of the fractured upper-to-middle crust might be leading to triggering of the seismicity in the region.

Abstract: The frequency-normalised impedance (FNI) function is introduced to give a physical significance to the magnetotelluric data. The FNI function is separated into its real and imaginary parts that have the same denominator to... more

Abstract: The frequency-normalised impedance (FNI) function is introduced to give a physical significance to the magnetotelluric data. The FNI function is separated into its real and imaginary parts that have the same denominator to estimate their values in the high- and low-frequency limits. The behaviour of the apparent resistivity definitions and the phase is explained theoretically by considering the real and the imaginary parts of the FNI function. The oscillations on the apparent resistivity curves are explained by using ascending and descending types two-layer curves of the FNI function. The concept of the reciprocal geoelectric section is developed by using the properties of the FNI function. It has been shown that the apparent resistivity curves of a geoelectric section and those of corresponding to the reciprocal geoelectric section are symmetric. The axis of symmetry is the horizontal axis that intersects the vertical axis at unity. The FNI function assists the interprete...

The Gibraltar Arc, formed by the alpine Betic and Rif Cordillera, surrounds the western Alborán Sea and is located on the Eurasian-African plate boundary. In a region with a complex geological setting, several tectonic models have been... more

The Gibraltar Arc, formed by the alpine Betic and Rif Cordillera, surrounds the western Alborán Sea and is located on the Eurasian-African plate boundary. In a region with a complex geological setting, several tectonic models have been proposed up to present, but they are not fully supported due to the absence of detailed geophysical data on the deep structure. In

Topographic effects due to irregular surface terrain may prevent accurate interpretation of magnetotelluric (MT) data. Three-dimensional (3-D) topographic effects have been investigated for a trapezoidal hill model using an edge... more

Topographic effects due to irregular surface terrain may prevent accurate interpretation of magnetotelluric (MT) data. Three-dimensional (3-D) topographic effects have been investigated for a trapezoidal hill model using an edge finite-element method. The 3-D topography generates significant MT anomalies, and has both galvanic and inductive effects in any polarization. This paper presents two different correction algorithms, which are applied to the impedance tensor and to both electric and magnetic fields, respectively, to reduce topographic effects on MT data. The correction procedures using a homogeneous background resistivity derived from a simple averaging method effectively decrease distortions caused by surface topography, and improve the quality of subsurface interpretation. Nonlinear least-squares inversion of topography-corrected data successfully recovers most of structures including a conductive or resistive dyke.

The Dead Sea Transform (DST) is a prominent shear zone in the Middle East. It separates the Arabian plate from the Sinai microplate and stretches from the Red Sea rift in the south via the Dead Sea to the Taurus-Zagros collision zone in... more

The Dead Sea Transform (DST) is a prominent shear zone in the Middle East. It separates the Arabian plate from the Sinai microplate and stretches from the Red Sea rift in the south via the Dead Sea to the Taurus-Zagros collision zone in the north. Formed in the Miocene 17 Ma ago and related to the breakup of the Afro-Arabian continent, the DST accommodates the left-lateral movement between the two plates. The study area is located in the Arava Valley between the Dead Sea and the Red Sea, centered across the Arava Fault (AF), which constitutes the major branch of the transform in this region.
A set of seismic experiments comprising controlled sources, linear profiles across the fault, and specifically designed receiver arrays reveals the subsurface structure in the vicinity of the AF and of the fault zone itself down to about 3-4 km depth. A tomographically determined seismic P velocity model shows a pronounced velocity contrast near the fault with lower velocities on the western side than east of it. Additionally, S waves from local earthquakes provide an average P-to-S velocity ratio in the study area, and there are indications for a variations across the fault. High-resolution tomographic velocity sections and seismic reflection profiles confirm the surface trace of the AF, and observed features correlate well with fault-related geological observations.
Coincident electrical resistivity sections from magnetotelluric measurements across the AF show a conductive layer west of the fault, resistive regions east of it, and a marked contrast near the trace of the AF, which seems to act as an impermeable barrier for fluid flow. The correlation of seismic velocities and electrical resistivities lead to a characterisation of subsurface lithologies from their physical properties. Whereas the western side of the fault is characterised by a layered structure, the eastern side is rather uniform. The vertical boundary between the western and the eastern units seems to be offset to the east of the AF surface trace.
A modelling of fault-zone reflected waves indicates that the boundary between low and high velocities is possibly rather sharp but exhibits a rough surface on the length scale a few hundreds of metres. This gives rise to scattering of seismic waves at this boundary. The imaging (migration) method used is based on array beamforming and coherency analysis of P-to-P scattered seismic phases. Careful assessment of the resolution ensures reliable imaging results.
The western low velocities correspond to the young sedimentary fill in the Arava Valley, and the high velocities in the east reflect mainly Precambrian igneous rocks. A 7 km long subvertical scattering zone (reflector) is offset about 1 km east of the AF surface trace and can be imaged from 1 km to about 4 km depth. The reflector marks the boundary between two lithological blocks juxtaposed most probably by displacement along the DST. This interpretation as a lithological boundary is supported by the combined seismic and magnetotelluric analysis. The boundary may be a strand of the AF, which is offset from the current, recently active surface trace. The total slip of the DST may be distributed spatially and in time over these two strands and possibly other faults in the area.

There is a clear correlation between the principal areas of current geothermal development and the seismically active boundaries of the moving segments of lithosphere defined by the plate tectonic models of the Earth. The tectonic... more

There is a clear correlation between the principal areas of current geothermal development and the seismically active boundaries of the moving segments of lithosphere defined by the plate tectonic models of the Earth. The tectonic position of Egypt in the northeastern corner of African continent suggests that the most important areas for geothermal exploration are in the region where a cluster of hot springs with varied temperatures was located around the Gulf of Suez. Gravity and magnetotelluric surveys were made in the area of Hammam Faraun hot spring, which represents the most promising area for geothermal development in Egypt. These surveys were carried out for the purpose of eliciting the origin of Hammam Faraun hot spring. The results of the analyses and interpretations of these data show that the heat source of the hot spring is due to uplift of hot basement rock. This uplift may cause deep circulation and heating of the undergroundwater.