GEOPHYSICS, VOL. 72, NO. 5 共SEPTEMBER-OCTOBER 2007兲; P. B133–B140, 11 FIGS. 10.1190/1.2759921
Case History Modeling and inversion of magnetic and VLF-EM data with an application to basement fractures: A case study from Raigarh, India
V. Ramesh Babu1, Subhash Ram1, and N. Sundararajan2
tion and processing of data in optimized forms for subsequent interpretation. Quantitative interpretation has advanced, particularly with the development of computerized multiparameter inversion methods. Methods such as Euler deconvolution 共Reid et al., 1990; FitzGerald et al., 2004兲 are mainly based on ideas that are reasonably well established. However, these techniques are now being implemented on much faster computing platforms with more useful graphic output. The amplitude of the analytical signal approach 共Nabighian, 1972; Sundararajan, 1983; Hsu et al., 1996; Srinivas, 2001兲 is an important method to delineate boundaries of different geologic structures. The theories of magnetic method and interpretation are readily available in the literature 共e.g., Paterson and Reeves, 1985; Sharma, 1987兲. Conversely, the very low frequency electromagnetic 共VLF-EM兲 prospecting method is suitable for studying structural details and shallow subsurface conductors 共Fisher et al., 1983兲 because it is not only rapid but also cost effective. VLF-EM is a passive method that uses radiation from military navigation radio transmitters; according to ABEM, there are approximately 42 global ground military communication transmitters operating in the VLF band 共15–30 kHz兲 as the primary EM field. These stations located around the world generate signals used for a variety of applications, including groundwater detection and contamination evaluation, soil engineering, archaeology, and mineral exploration for mapping narrow mineralized fault zones 共Philips and Richards, 1975; Wright, 1988兲. The VLF-EM method also has been used in mapping weathered layers in granitic terrain 共Poddar and Rathor, 1983兲 and in the detection of weak conductors probably caused by water-filled fractures or shallow faults in nuclear waste management programs 共Hayles and Sinha, 1986兲. For groundwater exploration, the VLF-EM method has proved to be quite effective in comparison with vertical electrical sounding in hard rock terrains 共Sundararajan et al., 2007兲. Hence, the VLF-EM method is a popular EM tool for quick mapping of near-surface geo-
ABSTRACT We present modeling of magnetic and very low frequency electromagnetic 共VLF-EM兲 data to map the spatial distribution of basement fractures where uranium is reported in Sambalpur granitoids in the Raigarh district, Chhattisgarh, India. Radioactivity in the basement fractures is attributed to brannerite, U-Ti-Fe complex, and uranium adsorbed on ferruginous matter. The amplitude of the 3D analytical signal of the observed magnetic data indicates the trend of fracture zones. Further, the application of Euler 3D deconvolution to magnetic data provides the spatial locations and depth of the source. Fraser-filtered VLF-EM data and current density pseudosections indicate the presence of shallow and deep conductive zones along the fractures. Modeling of VLF-EM data yields the subsurface resistivity distribution of the order of less than 100 ohm-m of the fractures. The interpreted results of both magnetic and VLF-EM data agree well with the geologic section obtained from drilling.
INTRODUCTION Of all the geophysical methods, magnetic mapping is the oldest, simplest, most reliable, and most widely used technique for locating both hidden ores and structures associated with mineral deposits. For the magnetic method, the important petrophysical parameters are the magnetic susceptibility and remanent magnetization, which can exist only at temperatures cooler than the Curie point. Magnetic susceptibility of rocks is primarily dependent on the presence of ferromagnetic minerals, of which magnetite and members of the titanomagnetite series are most common in the earth’s crust 共Grant, 1985; Clark, 1997兲. Parallel advances have been made in digital compila-
Manuscript received by the Editor June 17, 2006; revised manuscript received May 17, 2007; published online August 14, 2007; corrected version published online August 31, 2007. 1 Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Hyderabad, Nagpur, India. E-mail:
[email protected]. 2 Osmania University, Centre for Exploration Geophysics, Hyderabad, India. E-mail:
[email protected]. © 2007 Society of Exploration Geophysicists. All rights reserved.
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logic structures 共Philips and Richards, 1975; Parker, 1980; Saydam, 1981兲. The theory of VLF-EM technique is well described in the literature 共Paterson and Ronka, 1971; Philips and Richards, 1975; Wright, 1988; McNeill and Labson, 1991; Hutchinson and Barta, 2002兲. In the far-field above a uniform earth, the ground wave of the vertically polarized radio wave has three field components: a radial horizontal electric field, a vertical electric field, and a tangential magnetic field. When these three fields encounter electrically conducting ore bodies in the subsurface, eddy currents are induced, causing the secondary fields to radiate outward from these conductors. Although this range is very low for radio transmission, it is higher than that used in standard low-frequency 共1–3 kHz兲 electromagnetic geophysical methods. Paal 共1965兲 observed that radio waves at VLF frequencies could be used to prospect for conductive mineral deposits. Since then, VLF transmitters situated around the world are being used widely as electromagnetic sources for near-surface geologic mapping as an inductive survey technique. The VLF-EM method has been widely used over the last three decades to map shallow subsurface structural features of varying interests. However, the interpretation of observed VLF-EM anomalies is mainly carried out using anomaly curves and nomograms
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共Kaikkonen, 1979; Saydam, 1981兲. Filtering using the approaches of Fraser 共1969兲 and Karous and Hjelt 共1983兲 and subsequent contouring of the observed responses are the most common practice to derive qualitative information about the subsurface. Multidimensional numerical modeling and inversion are needed to quantitatively determine geometrical and physical subsurface parameters from VLFEM anomalies. Kaikkonen and Sharma 共1998兲 carried out an extensive study of the interpretation of VLF-EM data and suggested the forward modeling using finite-element methods. Kaikkonen and Sharma 共2001兲, made a comparative study of linear and global nonlinear inversion and observed that there is a possibility that linearized inversion can yield more accurate and reliable results than global nonlinear inversion. However, these techniques require considerable computing time and do not ensure uniqueness. Although Sundararajan et al. 共2006兲 developed a simple, user-friendly Matlab code for processing and interpreting VLF-EM data, a comprehensive quantitative method free from ambiguity has yet to be developed. In this paper, the measured total magnetic field and the observed in-phase and out-of-phase components of the VLF-EM method were employed in modeling and inversion to decipher the basement fractures associated with uranium mineralization in the Raigarh district, Chhattisgarh, India. The results obtained from both magnetic and, particularly, VLF-EM data define spatial locations and depths of fractures that agree well with drilling data and are presented here as a case history.
GEOLOGY OF THE AREA
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Figure 1. Location of the study area showing the traverses of the magnetic and VLFEM surveys along with the geology, Raigarh district, Chhattisgarh, India.
The area of investigation falls in the southeastern part of the Chhattisgarh basin, Raigarh district, Chhattisgarh, India, in an area where the Sambalpur granitoids are exposed 共Das et al., 1992兲. Basement fractures occur mainly along the southeastern periphery of the Chhattisgarh basin and appear to be genetically related to its evolution 共Chakraborti, 1997兲. Uranium mineralization mainly occurs in the basement fractures along N50E–S50W and N25E–S25W 共Benerjee, 2005兲. The fracture zones are occupied by breccia, cataclasite, or mylonite of acidic and basic compositions representing granite and basic rock parentage, respectively 共Figure 1兲. The mineralized breccia/cataclasite is highly altered and generally associated with ubiquitous silicification. Petrographic studies on the altered breccia/cataclasite have identified them variously as altered basalt, quartzofeldspathic cataclasite, protocataclasite, and ferruginous breccia 共Kamalesh and Mukundan, 1998; Shobita, 1998; Rameshbabu et al., 2004兲.
GEOPHYSICAL INVESTIGATIONS In general, the magnetic method is rapid and reliable for structural delineation; however, the VLFEM method is ideal for locating shallow subsurface conductors. Hence, the magnetic and VLF-EM methods were employed simultaneously for uranium exploration in our study to delineate the basement fractures associated with conductive mineralization.
Modeling of magnetic and VLF-EM data Total magnetic field measurements were recorded using a proton precession magnetometer along 17 traverses 共S1 to N30兲 40 m apart with a station interval of 10 m 共Figure 1兲. In addition to the magnetic measurements, the in-phase, out-of-phase, resistivity, and phase angle of the VLF-EM method were acquired along 11 traverses 共N2 to N22, Figure 1兲. The VLF transmitter 共JJI Japan兲 operating at a frequency of 22.2 kHz was used as the source for the entire VLF survey because it lies along the direction of the geologic strike. Measurements were made both in magnetic and electric field mode. The magnetic and VLF-EM data acquired were processed with the help of available techniques, as discussed in the following sections.
PROCESSING AND INTERPRETATION After correcting for diurnal variations, the magnetic data were gridded using the bicubic spline method with a cell size of 10 m 共because the data were acquired on a regular grid of 10⫻ 40 m兲, then the image map was prepared 共Figure 2兲. Two linear anomalies 共M1 and M2兲 of varying amplitude and wavelength are observed in Figure 2. Anomaly M1 trends NE–SW, and M2 trends NNE–SSW. Between traverses S1 and N8, the anomalies are of long wavelength with varying amplitudes, implying an en-echelon-type of fracture system, which is well known in basement rocks. The fracture system is vertical to subvertical between traverses S1 and N8 and dips southeast between traverses N8 and N30. Further, the amplitude of the analytical signal of the total magnetic field yields a maximum over magnetic contacts, regardless of the
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direction of magnetization 共Figure 3兲. Although the amplitude of the analytic signal is dependent on the magnetization and the direction of geologic strike with respect to the magnetization vector, this dependency is easier to address in the interpretation of amplitude of analytic signal than the original field data 共MacLeod et al., 1993兲. The amplitude and wavelength of the analytical signal 共Figure 3兲 increase from traverse N30 to S1. The fracture system southwest of N14 appears to be relatively wide at a greater depth. Conversely, northeast of N14, the fracture system probably narrows at a shallower depth, as is evident from the width of the anomaly 共Figure 3兲. The isolated linear closures of the magnetic anomalies M1 and M2 suggest that the fractures are en echelon. Application of Euler’s deconvolution 共Thompson, 1982兲 indicates geologically plausible spatial locations of fractures at depths varying from 15 to 40 m 共Figure 4兲. Modeling and inversion of magnetic anomaly AB 共Figure 2兲 was carried out using a Matlab-based graphical user interface 共GUI兲 code 共GRAVMAG-GUI兲 developed by the authors based on the algorithms of Won and Bevis 共1987兲 and Rao et al. 共1995兲. The magnetic profile observed at an interval of 10 m was modeled with a measured susceptibility contrast of 2200⫻ 10−6 CGS units and borehole data
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Figure 3. Amplitude of the analytical signal map with a variable contour interval.
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to constrain the inversion. An rms error of less than 2.55% has been achieved with as many as 20 iterations. The magnetic model 共Figure 5兲 generated was helpful in planning the spatial locations of other boreholes and their directions in the area. The VLF-EM in-phase and out-of-phase components are presented in the form of stacked profiles in Figure 6, and the recorded resistivity 共a兲 is shown in the form of a contour map in Figure 7. The crossovers of in-phase and out-of-phase components are inferred to be the conductor axes 共C1 and C2兲 共Wright, 1988兲, which are caused by pyrite and chalcopyrite associated with the fractures 共Figure 8兲. Further, the asymmetry in the in-phase and out-of-phase components is caused by the dipping nature of conductors, wherein the 600
larger anomaly peak identifies the down-dip side 共Coney, 1977; Baker and Myers, 1979兲. The profiles 共Figure 6兲 indicate that the conductor C1 is dipping southeastward and that C2 is dipping almost due east, which is comparable with the dips obtained from the magnetic method 共Figure 2兲. Resistivities of the order of 100 ohm-m or less observed on the resistivity contour map 共Figure 7兲 correspond to the conductors C1 and C2. Further, the resistivity contour map indicates the trend of fractures within the basement granitoids, which are 100 50 0 –50 –100 –150 –200 –250 –300
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Modeling of magnetic and VLF-EM data similar to the fracture system inferred already from the magnetic data. Based on these results, two boreholes 共namely DPL/2 and DPL/10兲 were drilled as shown in Figure 1. The boreholes not only confirmed the fracture system but also substantiated the geophysical interpretation.
FILTERING PROCEDURE The VLF-EM in-phase component often results in complex patterns and hence requires filtering before interpretation. Simple filtering techniques such as those that we employed transform crossovers into peaks, remove regional gradients, and amplify anomalies from the near surface. Further, application of Fraser 共1981兲 filtering of the in-phase component transforms the noncontourable in-phase component into contourable form, which enables better delineation of the conductors. In-phase contoured data generally peak almost directly over a conductor, as is evident from Figure 8, and are thus an effective complement to the amplitude of the analytical signal of magnetic map 共Figure 3兲. The conductors C1 and C2 demarcated in Figure 8 are similar to the magnetic trends M1 and M2 共Figure 2兲. In the absence of numerical modeling, the Fraser and Karous– Hjelt linear filtering techniques have been proved to be effective because they provide a simple scheme for semiquantitative interpretation 共Ogilvy and Lee, 1991; Sundararajan et al., 2006兲. Based on the Matlab GUI software 共Sundararajan et al., 2006 available at http:// www.iamg.org/CGEditor/index.htm兲, the in-phase components of traverses N6 and N12 were Fraser filtered for various lengths of the filter and are shown in Figures 9c and 10c. The longer the length of the filter is, the better the response of the deeper sources will be. According to Ogilvy and Lee 共1991兲, the current density pseudosection provides good visualization of targets such as mineralized veins, fractures, and vertical stratigraphy that produce vertical to subvertical conductors. In this study, the computed current density
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pseudosections of the in-phase component of traverses N6 and N12 are given in Figures 9d and 10d, respectively. Small as well as localized conductors are well resolved in the pseudosection 共Figure 9d兲 obtained from Karous-Hjelt filtering relative to the ones obtained from Fraser 共Figure 9c兲 filtering. The current density pseudosections were obtained using a Karous-Hjelt filter 共Karous and Hjelt, 1983兲 that is given by following the equation:
⌬Z/2Ia共0兲 = − 0.102H−3 + 0.059H−2 − 0.561H−1 + 0.561H1 − 0.059H2 + 0.102H3 where Ia共0兲 = 0.5关I共⌬X/2兲 + I共− ⌬X/2兲兴 and H−3, H−2, and so forth, are the measured data at consecutive stations, ⌬X is the measurement interval, Ia is the apparent current density, ⌬Z is the assumed thickness of the conductor, and I is the induced current density.
INVERSION OF VLF-EM DATA The quantitative interpretation of single-frequency VLF-EM data was examined by Beamish 共1994兲, Chouteau et al. 共1996兲, Kaikkonen and Sharma 共1998兲, Beamish 共2000兲, and Monteiro et al. 共2006兲. They provided detailed information about how the subsurface resistivity distribution can be obtained from a regularized inversion of the data. Generally, interpretation of VLF-EM data is not as simple as other geophysical methods because the relatively high transmitter frequency gives rise to secondary fields from many geologic features that are electrically conductive 共Philips and Richards, 1975兲. Further, the three dimensionality of geologic structures complicates the 2D inversion of VLF data. Numerical modeling 共Coney, 440
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Figure 7. Resistivity contour map derived from the VLF-EM data with contour intervals of 20 and 40 ohm-m. C1 and C2 indicate the conductors discussed in the text.
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Figure 8. Fraser-filtered in-phase contour map of the VLF-EM data with contour interval of 2%. C1 and C2 indicate the conductors discussed in the text.
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1977兲 is a useful way to interpret VLF-EM data, wherein a realistic model of the earth must incorporate different conductivities of regular or irregular geometry. Because the vertical component of the magnetic field decreases at sites far from conductors, it is possible, at least theoretically, to use the tipper to characterize the subsurface resistivity distribution. The tipper is a complex quantity originated by the time lag between horizontal and vertical components of the magnetic fields resulting from the electromagnetic induction phenomena. The tipper does not exist over a homogeneous earth 共or over a layered earth兲. Over a 2D earth, the tipper varies along the measuring profile showing the strongest variations in the vicinity of resistivity contrasts. In the case of the VLF-EM method, the in-phase and out-of-phase components of the tipper are usually expressed as percentages. Sasaki 共1989, 2001兲 developed a 2D regularized inversion code for EM data; subsequently, Monteiro et al. 共2006兲 modified the code to suit VLF-EM data and discussed its effectiveness. We have used the modified code of Monteiro et al. 共2006兲 in our inversion. The observed in-phase and out-of-phase components of the VLF100 Observed in phase Observed out of phase Computed in phase Computed out of phase Fraser filtered in phase
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RESULTS AND DISCUSSION Qualitative and quantitative interpretation of magnetic data, namely the amplitude of analytical signal and the Euler depth, are equally useful in deciphering the spatial locations of the fractures in
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Figure 9. VLF-EM response along traverse N6. 共a兲 Observed and computed in-phase and out-of- phase components with Fraser-filtered in-phase component, 共b兲 2D resistivity model obtained from the inversion of the VLF-EM data, 共c兲 Karous–Hjelt current density section, and 共d兲 Fraser-filtered current density pseudosection.
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EM data and computed values derived by inversion for traverses N6 and N12 are shown in Figure 9a and b and Figure 10a and b, respectively.An rms error of less than 1% is achieved between the observed and computed response in both the cases based on as many as 30 iterations. In these models 共Figures 9b and 10b兲, resistivities of less than 100 ohm-m extend to a depth of 80 m in a host rock with resistivity of 800 ohm-m. The resistivity distributions in the models agree well with those obtained from the Fraser and Karous–Hjelt filtered data 共Figure 9c and d and Figure 10c and d兲, which in turn agree well with the borehole section 共Figure 11兲.
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Figure 10. VLF-EM response along traverse N12. 共a兲 Observed and computed in-phase and out-of-phase components with Fraser-filtered in-phase component, 共b兲 2D resistivity model obtained from the inversion of the VLF-EM data, 共c兲 Karous–Hjelt current density section and 共d兲 Fraser-filtered current density pseudosection.
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these positive peaks gives the spatial locations of the conductor axis. The Fraser-filtered response obtained as suggested by Wright 共1988兲 gives the dip attitude of the conductors. The computed current density pseudosections of the traverses N6 共Figure 9c and d兲 and N12 共Figure 10c and d兲 using Fraser and Karous–Hjelt filters help in understanding the nature of conductors. The low-resistivity distributions observed in the models for traverses N6 and N12 are caused by fracturing in the granite. Further, this suggests that the interpreted conductors 共namely C1 and C2兲 dip toward southeast in low-resistivity zones.
CONCLUSIONS
VLF-EM surveys are relatively simple and economical to carry out along with magnetic surDistance (m) DPL/2 DPL/10 veys to provide a better understanding of shallow –300 vertical to subvertical subsurface conductors. 50 100 150 200 250 300 350 Transformation of noisy, noncontourable, inReference level 0 S40 E 0 280 DPL/2 DPL/10 (R.L.) N40 W phase data into relatively noise-free, contourable (R. L 262.37 m) (R. L 261.95 m) data by the Fraser filter greatly helped in deciphering the conductors. Both magnetic and VLF250 EM surveys are able to identify the spatial location and depth extent of fractures hosting uranium mineralization. Initially, the magnetic method helped in delineating the trend of the basement 200 fractures associated with ferruginous breccia followed by the VLF-EM method, which clearly demarcated the fracture system associated with meIndex tallic sulphides hosting uranium mineralization. 150 The current density pseudosections provide Overburden soil Gray granite first-hand information regarding the number, Pink granite size, depth, and relative disposition of the discrete Breccia zone conductors. However, both Fraser and Karous– Uranium Mineralization 100 Hjelt filters are capable of providing dip attitudes. D.D. 200.05 m D. D. 200.05 m D.D. (drilled depth) Two boreholes drilled based on the results intercepted the basement fractures in association with conducting mineralization. The salient features Figure 11. The VLF-EM in-phase and out-of-phase components, the resistivity, and magnetic anomaly along traverse N12 with section from boreholes DPL/2 and DPL/10. of the combined effect of these two methods are that the magnetic method clearly demarcated the the basement granitoids. Further, the 2D model of profile AB 共Figure fractures, whereas the VLF-EM method has brought out the conduc2兲 paved the way for planning the borehole locations and their tors associated with these fractures. The overall results paved the directions. way for drilling of many boreholes in the study area for further exThe modeling of VLF-EM data has provided a useful basis for the ploration of uranium. delineation of shallow subsurface vertical to subvertical conductors associated with basement fractures. The combined interpretation of ACKNOWLEDGMENTS magnetic and VLF-EM data has clearly brought out the subsurface fractures associated with the conducting mineralization, which in We record our sincere and profound thanks to the editorial board turn led to drilling. Crossovers of the in-phase and out-of-phase and reviewers P. J. Hutchinson and Kevin Mickus for their construccomponents demarcate the conductors 共Figure 6兲. Drilling data subtive review and useful suggestions to improve the manuscript. G. stantiated the presence of a low-resistivity zone of less than 20 Randy Keller, associate editor, is thanked profusely for his time and ohm-m over a background of 800 ohm-m obtained from VLF-EM efforts in so keenly reviewing the manuscript and editing it in its data. The fractures inferred from VLF-EM 共Figure 7兲 are conductive present form. We are highly thankful to Fernando Monteiro Santos due to pyrite, chalcopyrite, ilmenite, and brecciated matrix in the for providing the 2D code and guidelines for modeling the VLF-EM presence of hematite and magnetite. This combination of minerals data. Further, we extend our heartfelt thanks to R. M. Sinha, former yields strong magnetic and VLF-EM anomalies. Director, and Anjan Chaki, Director, Atomic Minerals Directorate The contour map of the Fraser-filtered, in-phase component obfor Exploration and Research 共AMD兲, for according permission to tained by converting crossovers into peaks in which the line joining publish this work. In addition, thanks are due to Shri D. B. Sen,AddiDepth (m)
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tional Director, for the encouragement. Discussions with and fruitful suggestions from A. K. Bhattacharya and D. Veerabhasker, former Regional Directors, AMD, D. C. Fraser, Geoterrex-Dighem, Canada, and R. L. Narasimha Rao, K. Kamlesh, A. R. Mukundan, and R. Srinivas, AMD have been of immense use at various stages of the work.
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