Airborne resistivity data leveling

Geophysics ◽  
1999 ◽  
Vol 64 (2) ◽  
pp. 378-385 ◽  
Author(s):  
Haoping Huang ◽  
Douglas C. Fraser
Keyword(s):  
Frequency Domain ◽  
Mineral Exploration ◽  
Primary Field ◽  
Geologic Mapping ◽  
Resistivity Data ◽  
High Magnitude ◽  
Geological Features ◽  
Absolute Measure ◽  
Tie Lines ◽  
Automated Technique

Helicopter‐borne frequency‐domain electromagnetic (EM) data are used routinely to produce resistivity maps for geologic mapping, mineral exploration, and environmental investigations. The integrity of the resistivity data depends in large part on the leveling procedures. Poor resistivity leveling procedures may, in fact, generate false features as well as eliminate real ones. Resistivity leveling is performed on gridded data obtained by transformation of the leveled EM channel data. The leveling of EM channel data is often imperfect, which is why the resistivity grids need to be leveled. We present techniques for removing the various types of resistivity leveling errors which may exist. A semi‐automated leveling technique uses pseudo tie‐lines to remove the broad flight‐based leveling errors and any high‐magnitude line‐based errors. An automated leveling technique employs a combination of 1-D and 2-D nonlinear filters to reject the rest of the leveling errors including both long‐and short‐wavelength leveling errors. These methods have proven to be useful for DIGHEM helicopter EM survey data. However, caution needs to be exercised when using the automated technique because it cannot distinguish between geological features parallel to the flight lines and leveling errors of the same wavelength. Resistivity leveling is not totally objective since there are no absolutes to the measured frequency‐domain EM data. The fundamental integrity of the EM data depends on calibration and the estimate of the EM zero levels. Zero level errors can be troublesome because there is no means by which the primary field can be determined absolutely and therefore subtracted to yield an absolute measure of the earth’s response. The transform of incorrectly zero‐leveled EM channels will yield resistivity leveling errors. Although resistivity grids can be leveled empirically to provide an esthetically pleasing map, this is insufficient because the leveling must also be consistent across all frequencies to allow resistivity to be portrayed in section. Generally, when the resistivity looks correct in plan and section, it is assumed to be correct.

Approximate semianalytical solutions for the electromagnetic response of a dipping-sphere interacting with conductive overburden

Geophysics ◽  
2016 ◽  
Vol 81 (4) ◽  
pp. E265-E277 ◽  
Author(s):  
Jacques K. Desmarais ◽  
Richard S. Smith
Keyword(s):  
Mineral Exploration ◽  
Thin Sheet ◽  
Electromagnetic Response ◽  
Uniform Field ◽  
Geologic Mapping ◽  
Sphere Model ◽  
Northern Ontario ◽  
Simple Method ◽  
Convolution Integrals ◽  
Metamorphic Terranes

Electromagnetic exploration methods have important applications for geologic mapping and mineral exploration in igneous and metamorphic terranes. In such cases, the earth is often largely resistive and the most important interaction is between a conductor of interest and a shallow, thin, horizontal sheet representing glacial tills and clays or the conductive weathering products of the basement rocks (both of which are here termed the “conductive overburden”). To this end, we have developed a theory from which the step and impulse responses of a sphere interacting with conductive overburden can be quickly and efficiently approximated. The sphere model can also be extended to restrict the currents to flow in a specific orientation (termed the dipping-sphere model). The resulting expressions are called semianalytical because all relevant relations are developed analytically, with the exception of the time-convolution integrals. The overburden is assumed to not be touching the sphere, so there is no galvanic interactions between the bodies. We make use of the dipole sphere in a uniform field and thin sheet approximations; however, expressions could be obtained for a sphere in a dipolar (or nondipolar) field using a similar methodology. We have found that there is no term related to the first zero of the relevant Bessel function in the response of the sphere alone. However, there are terms for all other zeros. A test on a synthetic model shows that the combined sphere-overburden response can be reasonably approximated using the first-order perturbation of the overburden field. Minor discrepancies between the approximate and more elaborate numerical responses are believed to be the result of numerical errors. This means that in practice, the proposed approach consists of evaluating one convolution integral over a sum of exponentials multiplied by a polynomial function. This results in an extremely simple algorithmic implementation that is simple to program and easy to run. The proposed approach also provides a simple method that can be used to validate more complex algorithms. A test on field data obtained at the Reid Mahaffy site in Northern Ontario shows that our approximate method is useful for interpreting electromagnetic data even when the background is thick. We use our approach to obtain a better estimate of the geometry and physical properties of the conductor and evaluate the conductance of the overburden.

Statistical analysis of airborne gamma‐ray data for geologic mapping purposes: Crixas‐Itapaci area, Goias, Brazil

Geophysics ◽  
1989 ◽  
Vol 54 (10) ◽  
pp. 1326-1332 ◽  
Author(s):  
A. C. B. Pires ◽  
N. Harthill
Keyword(s):  
Statistical Analysis ◽  
Gamma Ray ◽  
Mineral Exploration ◽  
Aerial Survey ◽  
Geologic Mapping ◽  
Good Correspondence ◽  
Gamma Ray Spectrometer ◽  
Data Points ◽  
Spectrometer Data ◽  
Mode Technique

Q‐mode factor analysis, K‐means clustering, and G‐mode clustering were used on digitized gamma‐ray spectrometer data from an aerial survey of the Crixas‐Itapaci area, Goias, Brazil. The data points including seven variables—eU, eTh, K, total count, U/Th, U/K, and Th/K—were digitized for a 2 km square grid. For the northwest corner of the area the data were gridded at 1 km. The Q‐mode classification method supplied results that do not show a good correspondence with the known geology. The K‐means clustering procedure barely identified the main lithologic features of the area. The G‐mode technique produced results that correlate well with the known geology and identified the greenstone belts present in the area by discriminating their ultramafic and mafic components from adjacent felsic rocks. Statistical analysis of aerial gamma‐ray spectrometer data can be very helpful in mapping geologic units in poorly known areas. It can also be used for mineral exploration purposes if mineralization is known to be associated with lithologies that can be identified by the techniques used in this study.

Differential Target Antenna Coupling (DTAC) EM Surveying with Stationary Transmitter Loop and Moving In-Loop Receivers

2020 ◽  
Vol 25 (1) ◽  
pp. 111-127
Author(s):  
Ben K. Sternberg
Keyword(s):  
False Alarm ◽  
Frequency Domain ◽  
Ground Surface ◽  
Primary Field ◽  
Vertical Arrays ◽  
Antenna Coupling ◽  
Operational Tests ◽  
Airborne Vehicle

Following our previous studies of the Differential Target Antenna Coupling (DTAC) method with horizontal and vertical arrays for EM surveys, in this paper we study the application of the DTAC method to a different configuration, where a large, stationary transmitter loop is on the ground surface. We then run profile lines inside this loop. The DTAC method is effective in eliminating errors due to the large variations in the primary field along profile lines within the transmitting loop. Operational tests show that we obtain more diagnostic DTAC anomalies over buried targets than using just the B x and B y data. The DTAC method also produces smaller false-alarm targets due to background geology variations, compared with B z measurements. The DTAC method can be used with either time- or frequency-domain data and the receiver can be moved on the ground or deployed from an airborne vehicle, such as a drone.

CONVERSION OF WIDEBAND EM FREQUENCY RESPONSE TO TRANSIENT RESPONSE USING SEGMENTED TRANSFORMATION

Geophysics ◽  
1978 ◽  
Vol 43 (1) ◽  
pp. 189-193 ◽  
Author(s):  
M. W. Asten ◽  
S. K. Verma
Keyword(s):  
Frequency Domain ◽  
Continuous Wave ◽  
Primary Field ◽  
Analog Model ◽  
Response Curves ◽  
Input System ◽  
Transient Nature ◽  
Field Response ◽  
Different Types ◽  
Linear Ramp

Electromagnetic (EM) methods measure the distortions of a primary field which are caused by a sub‐surface conductor. The resultant field is recorded as a function of frequency or of time, depending upon the harmonic or transient nature of the primary field. The two different types of measurements thus recognized are frequency domain (or continuous‐wave) and time domain (transient or TEM) methods. Interpretation of EM data is possible by comparing field response with the analytic or experimental response of a heuristic model. Most of the interpretive developments have been done for the frequency domain technique, which is mathematically more tractable than the TEM technique when we consider generalized models possessing a conductive halo, over‐burden, or host rock. For such models, TEM response is more easily obtained from analog model experiments (e.g., Velekin and Bulgakov, 1967; Palacky, 1975). Response curves thus obtained, however, are dependent upon the shape of the excitation pulse which varies among the different transient EM systems available; e.g., the Input system (Barringer, 1962) uses a 1.1 msec half‐sine pulse, the Crone pulse EM system (Crone, 1975) uses a linear ramp pulse with 1.4 msec rise time, while the Russian MPP01 system (Velekin and Bulgakov, 1967) uses a 15 msec square step pulse.

Composite color images of aerial gamma‐ray spectrometric data

Geophysics ◽  
1983 ◽  
Vol 48 (6) ◽  
pp. 722-735 ◽  
Author(s):  
Joseph S. Duval
Keyword(s):  
Gamma Ray ◽  
Color Image ◽  
Mineral Exploration ◽  
Color Images ◽  
South Texas ◽  
Geologic Mapping ◽  
Partial Synthesis ◽  
Data Set ◽  
Spectrometric Data ◽  
Sample Data

Aerial gamma‐ray data provide estimates of the apparent surface concentrations of potassium (K), equivalent uranium (eU), and equivalent thorium (eTh). These data can be expressed as nine radiometric parameters: K, eU, eTh, eU/eTh, eU/K, eTh/K, eTh/eU, K/eU, and K/eTh. The U.S. Geological Survey (USGS) has developed a technique which combines any three of these parameters to form a composite color image. The color image provides a partial synthesis of the radiometric data that can be used to aid geologic mapping and mineral exploration. The sample data set, from the Freer area in south Texas, illustrates the use of the color images.

On: “An Automatic Method of Compilation and Mapping of High‐Resolution Aeromagnetic Data,” by B. K. Bhattacharyya (GEOPHYSICS, August 1971, p. 695–716)

Geophysics ◽  
1972 ◽  
Vol 37 (3) ◽  
pp. 544-544
Author(s):  
M. S. Reford
Keyword(s):  
High Resolution ◽  
Mineral Exploration ◽  
Automatic Method ◽  
Aeromagnetic Data ◽  
Geologic Mapping ◽  
Valuable Contribution ◽  
Compilation Techniques

The use of high‐resolution aeromagnetic surveys for detailed geologic mapping and mineral exploration is not yet common, nor are the flying and compilation techniques as standardized as those of conventional or lowsensitivity aeromagnetic surveys. Dr. Bhattacharyya has made a valuable contribution by presenting particularly interesting results and describing the techniques in some detail. But there are some points in his comparisons between high‐resolution and conventional surveys which could be misleading.

Aeromagnetic gradiometer program of the Geological Survey of Canada

Geophysics ◽  
1989 ◽  
Vol 54 (8) ◽  
pp. 1012-1022 ◽  
Author(s):  
Peter J. Hood ◽  
Dennis J. Teskey
Keyword(s):  
Mineral Exploration ◽  
Geological Survey ◽  
Geologic Mapping ◽  
Aeromagnetic Survey ◽  
R And D ◽  
The Past ◽  
Mapping Tool ◽  
The World ◽  
Computer Scientists ◽  
Color Pixel

During the past two decades, the Geological Survey of Canada Aeromagnetic Survey Group, consisting of geophysicists, electronic engineers, technicians, and computer scientists, developed the aeromagnetic gradiometer technique for mineral exploration. The same group ran the aeromagnetic survey program in Canada, perhaps the largest such continuing aeromagnetic survey program in the world. In 1973, fabrication commenced on an inboard vertical gradiometer system on the GSC Queenair aeromagnetic survey aircraft. During the period 1978–1981, a number of experimental gradiometer surveys were carried out by the Geological Survey of Canada to demonstrate the efficacy of the aeromagnetic gradiometry technique as a geologic mapping tool in mineral exploration programs. Because of a need for aeromagnetic gradiometer surveys in the topographically rugged Gaspé Peninsula of Quebec, the GSC began in 1983 to foster the development of helicopter‐borne gradiometer systems through R and D contracts. Four companies responded and built towed‐boom helicopter gradiometer systems which have now been used in surveys in four eastern provinces. It is clear that the aeromagnetic gradiometer technique combined with VLF EM is an excellent geophysical tool to improve the accuracy of detailed geologic mapping for mineral exploration programs. VLF EM is an inexpensive add‐on that materially improves the geologic mapping capability of the airborne system. The product in color pixel form is in essence a pseudogeologic map and it is presently being employed as such.

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