Controlled-source Audio-frequency Magnetotelluric Data Research Articles

2D joint inversion of CSAMT and magnetic data based on cross-gradient theory

A two-dimensional forward and backward algorithm for the controlled-source audio-frequency magnetotelluric (CSAMT) method is developed to invert data in the entire region (near, transition, and far) and deal with the effects of artificial sources. First, a regularization factor is introduced in the 2D magnetic inversion, and the magnetic susceptibility is updated in logarithmic form so that the inversion magnetic susceptibility is always positive. Second, the joint inversion of the CSAMT and magnetic methods is completed with the introduction of the cross gradient. By searching for the weight of the cross-gradient term in the objective function, the mutual influence between two different physical properties at different locations are avoided. Model tests show that the joint inversion based on cross-gradient theory offers better results than the single-method inversion. The 2D forward and inverse algorithm for CSAMT with source can effectively deal with artificial sources and ensures the reliability of the final joint inversion algorithm.

Improved Pseudo-section Representation for CSAMT Data in Geothermal Exploration

Controlled-Source Audio-frequency Magnetotellurics (CSAMT) is a frequency domain sounding technique employing typically a grounded electric dipole as the primary electromagnetic (EM) source to infer the subsurface resistivity distribution. The use of an artificial source provides coherent signals with higher signal-to-noise ratio and overcomes the problems with randomness and fluctuation of the natural EM fields used in MT. However, being an extension of MT, the CSAMT data still uses apparent resistivity and phase for data representation. The finite transmitter-receiver distance in CSAMT leads to a somewhat “distorted” response of the subsurface compared to MT data. We propose a simple technique to present CSAMT data as an apparent resistivity pseudo-section with more meaningful information for qualitative interpretation. Tests with synthetic and field CSAMT data showed that the simple technique is valid only for sounding curves exhibiting a transition from high – low – high resistivity (i.e. H-type) prevailing in data from a geothermal prospect. For quantitative interpretation, we recommend the use of the full-solution of CSAMT modelling since our technique is not valid for more general cases.

The non-static effect of near-surface inhomogeneity on CSAMT data

Controlled source audio-frequency magnetotelluric (CSAMT) method has several advantages over magnetotelluric (MT) method, which includes the recording of lower noise signal and higher resolution data. However, CSAMT field data can still be distorted by the effect of near surface inhomogeneous body. It may be confused with static effect just like in MT, if three-dimensional subsurface geological body is buried under the receiver. Traditionally, the method used in static correction is adopted similar to that used in MT method for many years. In comparison, MT are the natural electric and magnetic fields in the frequency range of 0.0001Hz to 500Hz, while CSAMT fields are applied at frequencies ranging from 0.1 to 10kHz. Hence, in this paper, the non-static effect of near-surface inhomogeneity in CSAMT was simulated through theoretical modeling and we summarized its characteristics. If the skin depth is much larger than the size of the near-surface inhomogeneous body that is close to the measurement point, the anomalous body causes a static effect which is represented by vertical shift in apparent resistivity curves for all frequencies from their expected values, but when the skin depth is much smaller than the size of the near-surface inhomogeneous body in the vicinity of the measurement point, the apparent resistivity curve at high frequencies remains unchanged, while at lower frequencies shift in value is observed. The near-surface effect may be confused with static effect in data processing; however, it cannot be corrected using previous static correction methods, but by using the two-dimensional inversion method. Hence, for such CSAMT data, both effective processing technique and inversion process is of great significance.

Resistivtiy Structure in Kamojang Geothermal Field Derived from CSAMT Data

Controlled-source audio frequency magnetotelluric (CSAMT) survey consisting of 60 soundings has been carried out in the Kamojang geothermal field to delineate resistivity structure correlated to the present geothermal reservoir. The transmitter site was located about 3 − 5 km in the south-west of the survey area. The sounding sites were distributed irregularly and the distance between each of them varied from 200 to 500 m, covering the Kamojang Geothermal area. The two dimensional (2D) inversion scheme was applied to interpret the CSAMT data. The results show that the subsurface resistivity structures in the Kamojang geothermal field consists of three types of resistivity features correlated to the existing geothermal reservoir in the area, that is, the overburden (first layer) having resistivity value of 30 – 150 ohm.m and thickness of 30 to 100 m overlaying the intermediate (second) layer which has an extremely low resistivity of 3 – 10 ohm.m with 500 – 1200 m thick. This low resistivity layer can be considered as a hydrothermal alteration zone comprising the impermeable layer or sealing cap rock. Finally, the third layer which is relatively more resistive than the second layer with resistivity of 30 − 100 ohm-m. From the 3D view of the low resistivity anomaly (<10 ohm.m) considered as a hydrothermal alteration zone, it is obtained that altered zone is deep in the south-west and becomes shallow in the north-east of the area.

CSAMT Data Processing with Source Effect and Static Corrections, Application of Occam's Inversion, and Its Application in Geothermal System

Controlled source audio-frequency magnetotellurics (CSAMT) is a frequency-domain electromagnetic sounding technique which uses a fixed grounded dipole as an artificial signal source. Measurement of CSAMT with finite distance between transmitter and receiver caused a complex wave. The shifted of the electric field due to the static effect caused elevated resistivity curve up or down and affects the result of measurement. The objective of this study was to obtain data that have been corrected for source and static effects as to have the same characteristic as MT data which are assumed to exhibit plane wave properties. Corrected CSAMT data were inverted to reveal subsurface resistivity model. Source effect correction method was applied to eliminate the effect of the signal source and static effect was corrected by using spatial filtering technique. Inversion method that used in this study is the Occam's 2D Inversion. The results of inversion produces smooth models with a small misfit value, it means the model can describe subsurface conditions well. Based on the result of inversion was predicted measurement area is rock that has high permeability values with rich hot fluid.

Laterally constrained inversion for CSAMT data interpretation

Laterally constrained inversion (LCI) has been successfully applied to the inversion of dc resistivity, TEM and airborne EM data. However, it hasn't been yet applied to the interpretation of controlled-source audio-frequency magnetotelluric (CSAMT) data. In this paper, we apply the LCI method for CSAMT data inversion by preconditioning the Jacobian matrix. We apply a weighting matrix to Jacobian to balance the sensitivity of model parameters, so that the resolution with respect to different model parameters becomes more uniform. Numerical experiments confirm that this can improve the convergence of the inversion. We first invert a synthetic dataset with and without noise to investigate the effect of LCI applications to CSAMT data, for the noise free data, the results show that the LCI method can recover the true model better compared to the traditional single-station inversion; and for the noisy data, the true model is recovered even with a noise level of 8%, indicating that LCI inversions are to some extent noise insensitive. Then, we re-invert two CSAMT datasets collected respectively in a watershed and a coal mine area in Northern China and compare our results with those from previous inversions. The comparison with the previous inversion in a coal mine shows that LCI method delivers smoother layer interfaces that well correlate to seismic data, while comparison with a global searching algorithm of simulated annealing (SA) in a watershed shows that though both methods deliver very similar good results, however, LCI algorithm presented in this paper runs much faster. The inversion results for the coal mine CSAMT survey show that a conductive water-bearing zone that was not revealed by the previous inversions has been identified by the LCI. This further demonstrates that the method presented in this paper works for CSAMT data inversion.

Frequency and time domain three-dimensional inversion of electromagnetic data for a grounded-wire source

We present frequency- and time-domain three-dimensional (3-D) inversion approaches that can be applied to transient electromagnetic (TEM) data from a grounded-wire source using a PC. In the direct time-domain approach, the forward solution and sensitivity were obtained in the frequency domain using a finite-difference technique, and the frequency response was then Fourier-transformed using a digital filter technique. In the frequency-domain approach, TEM data were Fourier-transformed using a smooth-spectrum inversion method, and the recovered frequency response was then inverted. The synthetic examples show that for the time derivative of magnetic field, frequency-domain inversion of TEM data performs almost as well as time-domain inversion, with a significant reduction in computational time. In our synthetic studies, we also compared the resolution capabilities of the ground and airborne TEM and controlled-source audio-frequency magnetotelluric (CSAMT) data resulting from a common grounded wire. An airborne TEM survey at 200-m elevation achieved a resolution for buried conductors almost comparable to that of the ground TEM method. It is also shown that the inversion of CSAMT data was able to detect a 3-D resistivity structure better than the TEM inversion, suggesting an advantage of electric-field measurements over magnetic-field-only measurements.

Cooperative constrained inversion of multiple electromagnetic data sets

We evaluated a method for cooperatively inverting multiple electromagnetic (EM) data sets with bound constraints to produce a consistent 3D resistivity model with improved resolution. Field data from the Antonio gold deposit in Peru and synthetic data were used to demonstrate this technique. We first separately inverted field airborne time-domain EM (AEM), controlled-source audio-frequency magnetotellurics (CSAMT), and direct current resistivity measurements. Each individual inversion recovered a resistor related to gold-hosted silica alteration within a relatively conductive background. The outline of the resistor in each inversion was in reasonable agreement with the mapped extent of known near-surface silica alteration. Variations between resistor recoveries in each 3D inversion model motivated a subsequent cooperative method, in which AEM data were inverted sequentially with a combined CSAMT and DC data set. This cooperative approach was first applied to a synthetic inversion over an Antonio-like simulated resistivity model, and the inversion result was both qualitatively and quantitatively closer to the true synthetic model compared to individual inversions. Using the same cooperative method, field data were inverted to produce a model that defined the target resistor while agreeing with all data sets. To test the benefit of borehole constraints, synthetic boreholes were added to the inversion as upper and lower bounds at locations of existing boreholes. The ensuing cooperative constrained synthetic inversion model had the closest match to the true simulated resistivity distribution. Bound constraints from field boreholes were then calculated by a regression relationship among the total sulfur content, alteration type, and resistivity measurements from rock samples and incorporated into the inversion. The resulting cooperative constrained field inversion model clearly imaged the resistive silica zone, extended the area of interpreted alteration, and also highlighted conductive zones within the resistive region potentially linked to sulfide and gold mineralization.

Regularized inversion of controlled source audio-frequency magnetotelluric data in horizontally layered transversely isotropic media

We present an algorithm for inverting controlled source audio-frequency magnetotelluric (CSAMT) data in horizontally layered transversely isotropic (TI) media. The popular inversion method parameterizes the media into a large number of layers which have fixed thickness and only reconstruct the conductivities (e.g. Occam's inversion), which does not enable the recovery of the sharp interfaces between layers. In this paper, we simultaneously reconstruct all the model parameters, including both the horizontal and vertical conductivities and layer depths. Applying the perturbation principle and the dyadic Green's function in TI media, we derive the analytic expression of Fr?chet derivatives of CSAMT responses with respect to all the model parameters in the form of Sommerfeld integrals. A regularized iterative inversion method is established to simultaneously reconstruct all the model parameters. Numerical results show that the inverse algorithm, including the depths of the layer interfaces, can significantly improve the inverse results. It can not only reconstruct the sharp interfaces between layers, but also can obtain conductivities close to the true value.

Mineral Exploration using CSAMT data: Application to Longmen region metallogenic belt, Guangdong Province, China

A controlled-source audio-frequency magnetotelluric (CSAMT) survey has been carried out to investigate potential iron (Fe) and polymetallic (Pb-Zn-Cu) deposits in Longmen region, which is one of the main metallogenic belts in southern China. Conducting geophysical surveys in this area is quite difficult due to mountainous terrain, dense forest, and thick vegetation cover. A total of 560 CSAMT soundings were recorded along twelve surveying lines. Two-dimensional Occam’s inversion scheme was used to interpret these CSAMT data. The resulting electric resistivity models showed that three large-scale highly conductive bodies exist within the surveying area. By integrated interpretation combined with available geologic, geophysical, and geochemical data in this area, three prospective mineral deposits were demarcated. Based on the CSAMT results, a borehole penetrating approximately 250-m depth was drilled at the location of 470 m to the northwest end of line 06, defined with a massive pyrite from the depth of 52–235 m with 7%–16% Fe content, as well as locally high-grade Pb-Zn- and Ag-Ti-bearing ores.

Geophysical Evidence through a CSAMT Survey of the Deep Geological Structure at a Potential Radioactive Waste Site at Beishan, Gansu, China

This paper describes a controlled-source audio-frequency magnetotellurics (CSAMT) survey conducted in August 2009 over a potential high-level radioactive waste (HLRW) disposal site in northwestern China. The site is primarily covered with well-developed metamorphic rocks. The purpose of the CSAMT survey was to map the outcropped faults to depth and identify any hidden faults or weakened zone in the subsurface. The site is located in the arid Beishan area, Gansu Province. Substantial challenges were encountered in acquiring quality electrical field data because of the highly resistive ground. Satisfactory electrode contact conditions were generally maintained by applying salt-saturated water to both the transmitting electrodes and the receiving electrodes. The excitation frequency ranges from 9,600 Hz to 1 Hz with a target depth of investigation of 1,000 m. The CSAMT data were processed in several steps. Low-pass filtering was applied to remove the static effect caused by the local electrical inhomogeneities near the ground surface. An optimum filter length was found through experiment to yield the maximum static effect reduction. The pre-processed data are inverted for geoelectrical cross sections using a 2-D inversion method. Inversion artifacts were suppressed by imposing a model smoothness constraint. The inversion reveals several important results. First, the inversion cross sections correctly recognized the fractures and deformation bands mapped at the surface. The cross sections also identified four new faults that were not observed in the geological survey. The inversion profiles suggested that the narrow factures and deformation bands observed along survey line 1 extend to a great depth. The profile helped identify a possible weak mineralization zone along survey line 2. Considering the regional tectonic stress direction, the lower resistivity zone suggests that the faults parallel to survey line 2 are subject to an extensional or transtensional force that produces a broad and broken alteration zone. Although subject to further drilling confirmation, these interpretation results greatly enhance the understanding of the deep geological hazards at the Beishan site.

Geophysical exploration for geothermal resources: An application of MT and CSAMT in Jiangxia, Wuhan, China

We present a case study of applying MT (magnetotellurics) and CSAMT (controlled source audio-frequency magnetotellurics) for geophysical exploration in Jiangxia (江夏), which is located in new industrial developing suburb, where artificial noises are severe. In order to know deep buried structure, fracture status, and characteristics of underground geothermal development about 2 km, we acquired MT and CSAMT data to image subsurface structure through inversion and joint interpretation. The electrical terms of the 2D MT inversion can be divided into three ranges of resistivity values: (1) a highly resistive (>350 Ω·m) layer mainly characteristic of limestone, dolomitic limestone, leuttrite, silicarenite, and packsand; (2) an intermediate resistivity (250–350 Ω·m) layer mainly constituted by siliceous shale, siltstone, battie, and ampelitic limestone; and (3) a low resistivity (20–250 Ω·m) layer, from surface to −100 m, which is related to lacustrine alluvium of Quaternary period; the deep low resistivity layer is interpreted to be representative of the geothermal field. The result of the 2D CSAMT inversion reveals two layers of different electrical resistivities: (1) the first resistive layer (20–250 Ω·m), which is related to lacustrine alluvium of Quaternary period and the heat source, and (2) the second resistive layer (250–3 000 Ω·m). The heat source appears to be bounded within the middle of exploration area and shows the N-S trend. Its depth ranges from more than 1.2 to less than 0.7 km, and its resistivity values range from 20 to 250 Ω·m in the northeast part of Jiangxia. Comparing the results of MT and CSAMT method, the positive anomalies are similar and can be assumed to be generated by the same source.

Simulated annealing for controlled-source audio-frequency magnetotelluric data inversion

Simulated annealing (SA) is used to invert 1D controlled source audio-frequency magnetotelluric (CSAMT) data. In the annealing process, the system energy is taken as the root-mean-square fitting error between model responses and real data. The model parameters are the natural logarithms of the resistivity and the thickness in each layer of the earth. The annealing temperature decreases exponentially, while the model is refreshed randomly according to the temperature and is accepted according to a Boltzmann probability. We first tested the SA on synthetic data and developed a cooling schedule of model updates specifically for CSAMT data inversion. The redesigned cooling schedule reduces the magnitude of the model updating, and makes the solution converge rapidly and stably. For a three-layer model whose resistivity increases with depth, SA has difficulty in obtaining the global solution for the middle layer. However, the solution for such a layer can be significantly improved by using the mean value of the estimates. The inversion of field data from a northern suburb of Beijing, China, demonstrates that starting from a 1D smooth inversion to determine the range of SA parameters permits the SA to obtain very good results from the CSAMT survey data.

Sensitivities of generalized RRI method for CSAMT survey

This paper presents sensitivity analysis for generalized rapid relaxation inversion (GRRI) algorithm, which was originally developed by modifying the RRI algorithm to recover a two-dimensional (2-D) conductivity structure from MT data, to interpret controlled-source audio-frequency magnetotelluric (CSAMT) data. The GRRI algorithm can be extended to invert CSAMT data if it is combined with 2.5-D forward modeling including a source effect. GRRI approximate sensitivities are validated by comparing with exact 1-D and 2.5-D sensitivities. The comparison shows that the GRRI sensitivity is a good approximation to the exact sensitivity and has about half magnitude of the RRI sensitivity. Although the magnitude of the GRRI sensitivity is still slightly larger than that of the 2.5-D sensitivity, both sensitivities are broadly similar in shape when source-receiver offsets are greater than one skin depth in the earth. This suggests that the GRRI approach can be successfully used to invert CSAMT data through some scalling of sensitivities.

CSAMT exploration at Sellafield: Characterization of a potential radioactive waste disposal site

The long term disposal of radioactive waste in an underground repository requires the detailed geological evaluation of a potential site. Owing to their inherent sensitivity to the presence of fluids in rocks, electromagnetic (EM) methods have an important role in this assessment. Controlled‐source EM techniques are especially useful in strong anthropogenic noise environments such as industrial locations. However the complexity of modeling and inversion can limit the quantitative interpretation of controlled‐source EM data. A potential radioactive waste disposal site at Sellafield in Great Britain has been investigated using a variety of EM exploration techniques. Controlled‐source audio‐frequency magnetotelluric (CSAMT) data have given the best subsurface information in an environment that has a high level of cultural noise. One‐dimensional inversions of the Sellafield CSAMT data were found to be inadequate; 2.5-D forward modeling and inversion were used to interpret the data. The resulting resistivity models show good agreement with well log data collected at the site. These resistivity models show the presence of a large zone of hypersaline groundwater extending 1 km inland towards the potential repository and indicate the effect of faults on the hydrogeology.

Inversion of controlled source audio‐frequency magnetotellurics data for a horizontally layered earth

We present a technique for inverting controlled source audio‐frequency magnetotelluric (CSAMT) data to recover a 1-D conductivity structure. The earth is modeled as a set of horizontal layers with constant conductivity, and the data are apparent resistivities and phases computed from orthogonal electric and magnetic fields due to a finite dipole source. The earth model has many layers compared to the number of data points, and therefore the solution is nonunique. Among the possible solutions, we seek a model with desired character by minimizing a particular model objective function. Traditionally, CSAMT data are inverted either by using the far‐field data where magnetotelluric (MT) equations are valid or by correcting the near‐field data to an equivalent plane‐wave approximation. Here, we invert both apparent resistivity and phase data from the near‐field transition zone and the far‐field regions in the full CSAMT inversion without any correction. Our inversion is compared with that obtained by inverting near‐field corrected data using an MT algorithm. Both synthetic and field data examples indicate that a full CSAMT inversion provides improved information about subsurface conductivity.

Rapid relaxation inversion of CSAMT data

SUMMARY In this paper an inversion algorithm for controlled-source audio frequency magnetotelluric data is presented. This algorithm combines 2.5-D finite element forward modelling with the concepts of rapid relaxation inversion of magnetotelluric data. The inversion uses the same technique to compute sensitivities as the rapid relaxation inversion, and these approximate sensitivities are validated by comparison with exact 2.5-D sensitivities. The comparison shows that the approximate sensitivities are similar in shape to the exact sensitivities when transmitter‐receiver oVsets are greater than one skin depth in the Earth. The magnitudes of the two sensitivities diVer but the variations with depth are similar. Tests of the algorithm on synthetic data and field data provide promising results.

Measurement of static shift in MT and CSAMT surveys

Controlled source audio-frequency magnetotelluric (CSAMT) surveys have found frequent application in mineral exploration, and there have also been a number of attempts to use magnetotelluric (MT) soundings in the same application. The advantages of CSAMT over conventional electromagnetic methods lies in its ability (through measurements of the electric field) to discriminate resistive as well as conductive zones, and in its depth-of-penetration capability.“Static shift” however can be a common problem in many MT and CSAMT soundings. It severely distorts any depth calculations and degrades the resolution of conductivity structure inferred from scalar CSAMT measurements. Static shift also significantly restricts the MT-method in its use for geological studies of the upper and lower crust; and its application to mineral, hydrocarbon, and geothermal resource exploration. Theoretical investigation using modelling has shown that measurements of an Inductive Source Telluric Field (ISTF) may closely predict and thus potentially remove the static shift at MT and CSAMT stations.Applications of the method in the field require the additional setup of a low-power transmitter near the survey line. The main survey electrodes, cables and receiver are then used to collect additional data for corrections. This process can be run in parallel with the MT or CSAMT data acquisition, and requires little additional survey time. Initial tests of the concept in field studies near Sydney and over a skarn prospect in the Parkes vicinity proved very encouraging. The ‘along-line variation’ indicative of static shift was reduced by the correction by a factor of about 5 in the Parkes case, and the ‘corrected’ CSAMT section appeared smooth and regular as opposed to having the vertical striping typical of static problems.

Controlled‐source audiofrequency magnetotelluric responses of three‐dimensional bodies

Modeling the controlled‐source audiofrequency magnetotelluric (CSAMT) responses of simple three‐dimensional (3-D) structures due to a grounded electric bipole confirms that the CSAMT technique accurately simulates plane‐wave results in the far‐field zone of the transmitter. However, at receiver sites located above large conductive or resistive bodies, the presence of the inhomogeneity extends or reduces, respectively, the frequency range of the far‐field zone. Measurements made on the surface beyond a large 3-D body display a small but spatially extensive effect due to decay of the artificial primary field. Situating a 3-D inhomogeneity beneath the source permits an evaluation of “source overprint” effects. When such a body is resistive, a slight shift in the near‐field response to higher frequencies occurs. When a body below the transmitter is conductive, it is possible to make far‐field measurements closer to the transmitter or lower in frequency. However, as the size of the conductor and its secondary‐field response increases, large transition‐zone responses distort the data. For both a plane‐wave source and a finite source, current channeling into a 3-D conductor from conductive overburden enhances the response of a target. The modeled response of a dike‐like conductor shows no better results for either the broadside or collinear configuration. The location and extent of such a body are better defined when measuring the electric field perpendicular to the strike of the prism, but resistivity estimates are better when using the electric field parallel to the strike of the prism, irrespective of transmitter orientation. Models designed from data collected at Marionoak, Tasmania, yield results which indicate that the thin, vertical graphitic unit intersected by drilling is detectable by the CSAMT method, but probably is not the sole source of the large anomaly seen in the CSAMT data.

Structure mapping at Trap Spring Oilfield, Nevada, using controlled-source magnetotellurics

The seismic reflection method has been a highly successful tool in oil and gas exploration for half a century, and it presently accounts for about 98% of all geophysical expenditures world-wide. However, the relatively high cost of seismic exploration and its limitations in certain geologic environments are continuing problems. Some help has been provided by the magnetotelluric (MT) sounding technique, but the cost of MT is also quite high due to the low natural signal strengths being measured. The controlled-source audio-frequency magnetotellurics (CSAMT) technique is a shallower-penetrating variation of MT which uses an artificial signal source. This permits faster and more economical data acquisition. CSAMT has a penetration of about 2 km in typical petroliferous environments. CSAMT does not replace seismic but functions in three specific roles: (1) as a reconnaissance tool to help focus seismic coverage, or to help avoid 'no-record' zones; (2) to assist in statics corrections and in interactive seismic interpretation; (3) as a primary tool in certain environments (volcanics, complex thrust areas) where seismie data acquisition is limited. An example of the application of CSAMT to structure mapping comes from data taken over Trap Spring Field, located in the frontier Great Basin of the western United States. The field produces oil from fractured volcanics at the edge of a major graben fault. The CSAMT data delineate the major subsurface faulting and stratigraphic relationships in the area. The resolution of the CSAMT survey is significantly better than previously obtained induced polarization (IP) data. Detailed comparisons with electric log, drillhole, and air-photo data show an excellent correlation between the CSAMT features and known geology. The work suggests that CSAMT could be used in this area for reconnaissance mapping to develop seismie prospects, at approximately one sixth the cost of seismic.