Controlled Source Audio-frequency Magnetotelluric Method Research Articles

Application of the CSAMT method to groundwater exploration in a metropolitan environment

Urban and suburban areas of large cities impose great challenges to geophysical surveys because of high-level ambient noise. This is particularly true for controlled source audio-frequency magnetotellurics (CSAMT) because the frequency range of the method overlaps those of many civil and industrial noises. Among the different types of noise sources, perhaps the most noticeable one is the 50-Hz power grid. We have successfully applied the CSAMT method for groundwater exploration in a suburban Beijing area where the ambient noise level is about three times as high as would normally be encountered. Several steps were taken in assuring quality use of the method, including careful survey design, adequate frequency range selection, and large transmitter dipole length and transmitter-receiver spacing. The next step was static effect removal through low-pass filtering and topography correction to remove any nonhorizontal component in the electric field measurement. Finally, a 1D inversion method was applied to construct an (apparent) resistivity cross section. The survey revealed a low-resistivity, water-bearing layer sandwiched between a surface cover layer and the bedrock. The available well data suggested that the depth of the bedrock was accurately mapped, to within about 2 m, at a depth level of 150–165 m.

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.

Geophysical exploration for a long deep tunnel to divert water from the Yangtze to the Yellow River, China

To identify problematic geological structures which would be encountered when driving tunnels in the high mountainous area of South West China, a joint geophysical and engineering geological study was made. The tunnels will form part of a project to divert water from the Yangtze to the Yellow Rivers. A controlled source audio-frequency magnetotelluric method (CSAMT) was carried out in 2004 between the Ma-ke and Jia-qu rivers in high, steep terrain. The paper discusses the method of data collection, processing, and analysis for the West route. The faults/fractures identified will need to be taken into account in the design and construction of the tunnel.

Application of the CSAMT Method for Exploring Deep Coal Mines in Fujian Province, Southeastern China

Coal reserve volume in Fujian Province, China, ranks third among the nine provinces in southern China. Over the past decades, most coal mines in uncovered and semi-covered coal strata have been explored and developed. Present exploration mainly focuses on deep coal strata, which are generally covered by older strata and volcanic strata. Because of the terrain and deep targets, seismic prospecting is difficult. The major coal-containing strata that can be excavated in Fujian are within the Tungtzeyan Formation of the Lower Permian Series. The thickness of this formation is greater than [Formula: see text]. The CSAMT (controlled source audio frequency magnetotelluric) method isused to delineate four deep coal strata units. The aim of this prospecting is to delineate the upper and lower boundaries of the stratum containing the coal units. The prospecting target depth is approximately [Formula: see text], which required a frequency range between [Formula: see text] and [Formula: see text] to provide sufficient data coverage. The distance from the transmitter to the survey line was [Formula: see text], with a transmitting current of [Formula: see text] and voltage of [Formula: see text]. The time synchronization between the transmitter and receiver was controlled by a global positioning system. Major static problems were encountered during the CSAMT survey. Both terrain and static corrections were applied during data processing, which smoothed the inverted results for better interpretation. Comparison of the CSAMT interpreted results with available engineering geology information shows a clear difference in electrical properties between coal stratum, upper old stratum, red-beds and volcanic strata. The CSAMT method is useful in identifying the coal strata under these geological conditions.

Imaging 2D structures by the CSAMT method: application to the Pantano di S. Gregorio Magno faulted basin (Southern Italy)

A controlled source audiofrequency magnetotelluric (CSAMT) survey has been undertaken in the Pantano di San Gregorio Magno faulted basin, an earthquake prone area of Southern Apennines in Italy. A dataset from 11 soundings, distributed along a nearly N-S 780 m long profile, was acquired in the basin's easternmost area, where the fewest data are available as to the faulting shallow features. A preliminary skew analysis allowed a prevailing 2D nature of the dataset to be ascertained. Then, using a single-site multi-frequency approach, Dantzig's simplex algorithm was introduced for the first time to estimate the CSAMT decomposition parameters. The simplex algorithm, freely available online, proved to be fast and efficient. By this approach, the TM and TE mode field diagrams were obtained and a N35°W ± 10° 2D strike mean direction was estimated along the profile, in substantial agreement with the fault traces within the basin. A 2D inversion of the apparent resistivity and phase curves at seven almost noise-free sites distributed along the central portion of the profile was finally elaborated, reinforced by a sensitivity analysis, which allowed the best resolved portion of the model to be imaged from the first few meters of depth down to a mean depth of 300 m b.g.l. From the inverted section, the following features have been outlined: (i) a cover layer with resistivity in the range 3–30 Ω m ascribed to the Quaternary lacustrine clayey deposits filling the basin, down to an average depth of about 35 m b.g.l., underlain by a structure with resistivity over 50 Ω m up to about 600 Ω m, ascribed to the Mesozoic carbonate bedrock; (ii) a system of two normal faults within the carbonate basement, extending down to the maximum best resolved depth of the order of 300 m b.g.l.; (iii) two wedge-shaped domains separating the opposite blocks of the faults with resistivity ranging between 30 Ω m and 50 Ω m and horizontal extent of the order of some tens of metres, likely filled with lacustrine sediments and embedded fine gravels.

Underground Structure of Mud Volcanoes in Tokamachi City, Niigata Prefecture Determined by Electromagnetic Exploration, and Geographical and Geological Surveys

In Tokamachi City, Niigata Prefecture, where Tertiary sedimentary rocks are distributed, excavation of the Mt. Nabetachiyama tunnel took a long time to complete due to the presence of a swelling mudstone zone. This swelling zone is distributed at a depth of 180m under the mud volcanoes. Geological and geographical surveys as well as electromagnetic surveys applying the Controlled Source Audio-frequency Magneto-Telluric Method (CSAMT), were carried out to clarify the geological structure under the mud volcanoes and the relationship between geographical features and the swelling zone. The laser scanner survey revealed that the geological structure around the topographic depression is intensely disturbed and the geological survey showed that the mud volcanoes and eruption points of groundwater and natural gas are located mainly along the outside edge of the geologically disturbed zone. This suggests that the area was active in the past due to the uplift and eruption of groundwater. The extremely low resistivity zone (ELR) at a depth of 400m with a diameter of 500m was detected below the geologically disturbed zone by the CSAMT survey. The ELR is estimated to correspond to a mud chamber filled with saline groundwater and mud, based on laboratory tests measuring the electrical properties of several rock samples obtained from the survey area. The low resistivity zone (LR) was also detected; it continues from the ELR to the mud volcano at the ground surface, indicating that saline groundwater, mud, and natural gas may be ascending via a concentric path to the mud volcanoes. On the basis of all the results obtained, it is interpreted that the swelling zone in the tunnel corresponds to the path of mobilized mud and saline groundwater in the fractured mudstone with abnormal pore water pressure.

An analytical method to estimate shadow and source overprint effects in CSAMT sounding

The controlled-source audio frequency magnetotelluric (CSAMT) method is well established in geophysical prospecting. By using a transmitter (usually a grounded electric dipole) to generate the source fields, the CSAMT method overcomes problems associated with weak natural EM fields used in conventional audio MT surveys. But the CSAMT method creates a new problem associated with active EM surveys: when anomalies are located between the transmitter and receiver, the measured data may be distorted so that the geological structure will be inferred incorrectly. Kuznetzov (1982) and Zonge and Hughes (1988) observe this phenomenon in 2D numerical model studies and term such behavior the shadow effect. This phenomenon has been simulated by numerical modeling and discussed in many papers (Newman et al., 1986; Zonge et al., 1986; Mitsuhata, 2000). Boschetto and Hohmann (1991) use numerical methods to study shadow and source overprint effects in CSAMT using a horizontal electric dipole source. For this problem, we separate the ground-wave and surface-wave terms from the closed-form expressions of the electric and magnetic fields in a uniform earth and use the ground-wave/surface-wave ratio to estimate the potential magnitude for producing shadow and source overprint effects. This is a new and effective analytical method.

Controlled Source Audiofrequency Magnetotelluric method applied in urban area of Central Osaka, Japan

A detailed study was carried out to find out the most suitable time for electromagnetic sounding in order to apply the CSAMT method in the urban area of Central Osaka. The preliminary data collected during the daytime were highly noise­ contaminated, which couldn't be attenuated even on increasing the sampling time. Because of this reason, all the soundings were obtained during the nighttime when the electromagnetic noises were drastically reduced. The changes in layer resistivities with depth, as obtained from the 1-D inversion of the observed data, correspond to the different reflectors as seen in the seismic section. It is found that the vicinity of Uemachi Fault, in the western part of the study area, is characterised by high conductivity.

Resistivity imaging of controlled‐source audiofrequency magnetotelluric data

The controlled‐source audiofrequency magnetotelluric (CSAMT) method is an electromagnetic (EM) method where a transmitter (a grounded electric bipole or a current loop) is placed far away from the receiver sites. If the transmitter is located at distances greater than 3–5 skin depths, the plane-wave approximation is valid and the techniques used for (natural source) MT interpretation can be applied (Goldstein and Strangway, 1975; Sandberg and Hohmann, 1982). The CSAMT method can be employed in a detailed survey by closely spacing a number of receiver sites along a traverse. The borehole CSAMT technique is proposed in West and Ward (1988) to enhance the ability of the surface CSAMT method to detect subsurface inhomogeneities. In these cases, two‐dimensional (2-D) smooth‐model inversion (Rodi et al., 1984; Sasaki, 1989; deGroot‐Hedlin and Constable, 1990) would be particularly useful for deriving a resistivity model from the far‐field data and for presenting the survey results in the form of an image.

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.