Exploring and Using the Magnetic Methods (original) (raw)
Related papers
The historical development of the magnetic method in exploration
GEOPHYSICS, 2005
The magnetic method, perhaps the oldest of geophysical exploration techniques, blossomed after the advent of airborne surveys in World War II. With improvements in instrumentation, navigation, and platform compensation, it is now possible to map the entire crustal section at a variety of scales, from strongly magnetic basement at regional scale to weakly magnetic sedimentary contacts at local scale. Methods of data filtering, display, and interpretation have also advanced, especially with the availability of low-cost, high-performance personal computers and color raster graphics. The magnetic method is the primary exploration tool in the search for minerals. In other arenas, the magnetic method has evolved from its sole use for mapping basement structure to include a wide range of new applications, such as locating intrasedimentary faults, defining subtle lithologic contacts, mapping salt domes in weakly magnetic sediments, and better defining targets through 3D inversion. These new...
Journal of Earth System Science, 1990
The magnetic method is the oldest and one of the most widely used geophysical techniques for exploring the earth’s subsurface. It is a relatively easy and inexpensive tool to employ, being applicable to a wide variety of subsurface exploration problems involving horizontal magnetic property variations occurring from near the base of the crust to within the uppermost meter of soil.
Crustal geologic studies with Magsat and surface magnetic data
Reviews of Geophysics, 1987
to make a basement magnetization map for the San surface. Juan Basin, New Mexico, in a detail which would Grauch and Campbell (1984) showed that draping not have been possible with the original data as an aeromagnetic survey does not necessarily reduce flown. Blakely and Simpson (1986) developed an terrain effects, and in fact exaggerates them automated procedure for delineating magnetization relative to a high-level survey. Nevertheless, (or density) boundaries for machine application of they argue that a draped survey is still desirable the Cordell-Grauch method; this is simply an to keep the spectral content of the data uniform efficient way of finding the loci of maxima in the and to avoid overly attenuating short-wavelength horizontal gradient for a grid of data. Hildenbrand (1985) applied the drape-over-basement technique to the whole of the digital data set (Hildenbrand et al., 1983) for the U.S. midcontinent; the resulting map reveals much more apparent detail under the basin areas, as expected. Filtered, shaded-relief, and magnetization-density ratio maps were also produced via digital processing. In addition to Copyright 1987 by the American Geophysical Union. Paper number 7R0355. 8755-1209/87 /007R-0355 $15.00 anomalies in the data noise over valleys; such data can always be continued to a level surface if desired. Similar arguments were made by Cordell and Grauch (1985). Also see comments on this paper by Reford (1984) and Hansen (1984). Blakely and Grauch (1983) dealt with the effect of terrain by using Parker's Fourier transform method of calculating the magnetic anomaly due to uniformly magnetized terrain. The method is applied to registered 2-D elevation and magnetic data grids. Observed and computed fields can be compared visually and numerically by computing correlation coefficients, and anomalies due to topography can easily be distinguished from those due to buried 971 972 Mayhew and LaBrecque: Magsat and Surface Magnetic Data sources. The method was applied to the Oregon Cascades. On a much smaller scale, terrain anomalies due to arroyos cut into suprisingly magnetic (ca. 1 A/m) alluvial fan sediments were modeled by Mahrer et al. (1984). Keller et al. (1985) applied high and low pass, reduction to the pole, upward continuation, map presentation. This is in fact the purpose of all the techniques for manipulation of digital data sets reviewed above. They are not ends in themselves, but first steps in the process of model construction and hypothesis testing. Several other technique-development papers published during this quadrennium are worthy of strike, and vertical derivative filters to gridded note. Silva and Hohmann (1983) described an aeromagnetic and gravity data for west Texas. A optimization technique for magnetic modeling which major crustal transition associated with the involves a random sampling of an objective Ouachita system is revealed by the data. Yarger function hypersurface within a specified (1985) applied similar filters to the aeromagnetic "feasible" region of the parameter space; the mean data set for Kansas, and delineated Precambrian of the sample is a better estimate than any age boundaries, younger plutons, the southern individual estimate w_hen data noise is present. extension of the Central North American rift Ku and Sharp (1983) discuss the Werner
Chapter 7 Exploring and Using the Magnetic Methods
2017
The Earth is principally made up of three parts: core, mantle and crust (Fig. 1). As understood today, right at the heart of the Earth is a solid inner core composed primarily of iron. At 5, 700°C, this iron is as hot as the Sun’s surface, but the crushing pressure caused by gravity prevents it from becoming liquid. Surrounding this is the outer core, a nearly 2, 000 km thick layer of iron, nickel, and small quantities of other metals. Lower pressure than the inner core means the metal here is fluid. Differences in temperature, pressure and composition within the outer core cause convection currents in the molten metal as cool, dense matter sinks while warm, less dense matter rises. This flow of liquid iron generates electric currents, which in turn produce magnetic fields (Earth’s field). These convection processes in the liquid part of core (outer core) give rise to a dipolar geomagnetic field that resembles that of a large bar magnet aligned approximately along the Earth’s rotati...
Magnetic Logging and In-Situ Magnetostratigraphy: A Field Test
Proceedings of the Ocean Drilling Program, 1994
During Ocean Drilling Program Leg 134 (Vanuatu), geological high sensitivity magnetic tools (GHMT) developed by CEA-LETI and TOTAL were used at two drill sites. GHMT combine two sensors, a proton magnetometer for total magnetic field measurements with an operational accuracy of 0.1 nanoteslas (nT), and a highly sensitive induction tool to measure the magnetic susceptibility with an operational accuracy of a few I0" 6 S1 units. Hole 829 A was drilled through an accretionary prism and the downhole measurements of susceptibility correlate well with other well-log physical properties. Sharp susceptibility contrasts between chalk and volcanic silt sediment provide complementary data that help define the lithostratigraphic units. At Hole 83 IB magnetic susceptibility and total field measurements were performed through a 700-m reef carbonate sequence of a guyot deposited on top of an andesitic volcano. The downhole magnetic susceptibility is very low and the amplitude of peak-to-peak anomalies is less than a few 10~5 S1 units. Based on the repeatability of the measurements, the accuracy of the magnetic logging measurements was demonstrated to be excellent. Total magnetic field data at Hole 83IB reveal low magnetic anomalies of 0.5 to 5 nT and the measurement of a complete repeat section indicates an accuracy of 0.1 to 0.2 nT. Due to the inclination of the earth's magnetic field in this area (~-40°) and the very low magnetic susceptibility of the carbonate, the contribution of the induced magnetization to the total field measured in the hole is negligible. Unfortunately, because the core recovery was extremely poor (<5%) no detailed comparison between the core measurements and the downhole magnetic data could be made. Most samples have a diamagnetic susceptibility and very low intensity of remanent magnetization (< I0" 4 A/m), but a few samples have a stable remanent magnetization up to 0.005 A/m. These variations of the intensity of the remanent magnetization suggest a very heterogeneous distribution of the magnetization in the carbonate sequence that could explain the magnetic field anomalies measured in these weakly magnetized rocks.
Eppelbaum et al Detailed magnetic paleomagnetic data analysis 2003
EAGE Annual Conference, 2003
Sea of Galilee (Lake Kinneret) is one of the main sources of fresh water in Israel. The sea is located in the area of complicated tectonic setting at the northern continuation of the Dead Sea Rift. Practical absence of wells in the Sea of Galilee basin sufficiently complicates geophysical data interpretation. Magnetic map of the total magnetic field of the sea area shows a complex pattern of the magnetic field distribution caused by a combined influence of the basalt flows surrounding this lake and magnetic sources occurring this sea. Positive and negative magnetic anomalies were recognized in the Sea of Galilee basin corresponding to the basalts of normal and reverse magnetization, respectively. These anomalies in the sea were investigated using modern procedures developed specially for complicated geological conditions. Applying these procedures quantitative parameters of the targets were determined and their classification was performed. A 3-D modeling of the magnetic field has been successfully carried out for refining the data obtained at the previous stage and to computing effects due to proposed geological boundaries and bodies. Developed paleomagnetic map of basalt associations framing Sea of Galilee basin was correlated with the paleomagnetic zones revealed in the sea. The recognized paleomagnetic zones in the sea basin basically are in accordance with the western and northern framing of the sea and have some disagreement with the eastern and southern framing. Analysis of radiometric and paleomagnetic data allowed us to conclude that in western part of the sea are developed Early Pliocene basaltic associations.