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.
High-temperature geothermal deposits are concentrated in areas of modern volcanic activities and fault zones. Existence of sources of thermal fluid is connected with the presence of the high-temperature zone at depth and tectonic faults that allow circulation of thermal fluid. Several geophysical techniques can be used to image the subsurface geothermal fluid and determine its electrical resistivity, for example, direct current, MT, audio-frequency magnetotellurics, CSAMT, and transient electromagnetics (EM). Such information is critical in characterizing the geological structure that controls geothermal fluid flow.
One of the main impediments to using EM surveys is the complexity of the data. Unlike gravity or magnetic surveys, where the data can sometimes be used to infer geology or possibly to identify drill targets, EM data images are complex and are not connected with geology in a simple way. In order to know deep buried structure, fracture status, and characteristics of underground geothermal development in new industrial developing suburb, it may be necessary to integrate multiple techniques to better interpret reservoir parameters from geophysical data. In this case, we applied the MT and CSAMT methods to explore geothermal field.
The CSAMT method utilizes artificial and natural EM fields to measure resistivity variations in the ground, which overcomes the weak signals motivation with the conventional MT method, although its depth of exploration is lower than the MT. The original motivation to develop CSAMT was to improve the signal strength in MT method (Goldstein and Strangway, 1975), particularly frequency band, wherein the naturally occurring EM field is too weak to use. Sandberg and Hohmannt (1982) firstly used CSAMT to explore hydrothermal system. Then, CSAMT has emerged as a powerful exploration tool and has found its application in geothermal investigation (Sinharay et al., 2010; Spichak and Manzella, 2009; Park et al., 2006; Savin et al., 2001; Wannamaker, 1997a, b; Bromley, 1993; Bartel and Jacobson, 1987).
The main purpose of this article is to get realistic geophysical data in new industrial developing suburb by combining the advantages of these two methods. Firstly, using the MT method prospected the deep buried structure and fracture above 2 000 m and ascertained the heat source; then, ongoing exploration effort has relied on the use of MT. Utilizing the CSAMT method validates the result of MT method and provided useful information for sitting geothermal wells.
GEOLOGICAL SETTING
Jiangxia is located in Wuhan, Hubei Province in central China (Fig. 1). In the exploration area, hills are E-W zonal extension, hillsides are gradual, and valleys are wide. The elevation of residual hill is from 54.2 to 97.4 m a.s.l. and the elevation of valley is from 30.4 to 42.3 m a.s.l.. Generally speaking, the geomorphology of residual hills is composed of strong weathering silicolite stratum and the valley is composed of weak weathering stratum. The terrain is relatively flat, wide, and undulating, and vegetation cover is better. Overall, the sounding topography is relatively simple and geomorphology is complex.
Figure
1.
Geological map of Jiangxia area showing the major geological and tectonic features, including distribution of the measurement lines of MT and CSAMT. Lines 1 and 2 are MT measurement lines, and Line 3 is the measurement line of CSAMT. In the exploration area, the geological strata are composed of Quaternary, Triassic, Permian, and Devonian. In Quaternary, it is mainly lacustrine alluvium composed of gray yellow clay, silty clay, sand and silt clay. At the top T1-2j, the lithology is purple-gray thick massive dolomitic limestone and brecciated limestone; in the middle of T1-2j, it is dolomitic limestone and dolomite; and at the bottom of T1-2j, it is massive crystalline limestone. From top to bottom, the lithology of T1d is respectively thin-limestone, marlstone, yellow-green sandy shale, yellow-green argillaceous limestone, and shale. P1g is mainly made up of black thin-silicalite and claystone interbedding, thin silicalite with siliceous shale and claystone, and gray white thick silicalite. The lithology of P1q is principally divided into two parts: the first part is the top P1q, which is consisted of black massive carbonaceous limestone, and the second is the middle and bottom of P1q, which is composed of cherty limestone, carbonaceous limestone, and gray brown sheet claystone. The lithology of D3w is mainly gray quartz sandstone with claystone; at the bottom of D3w is thick quartz conglomerate.
Tectonic movement occurred many times, cut by surrounding faults, and formed raised block since Paleozoic Era at this region. EM exploration and natural radioactive survey had been taken at Jiangxia in 2006; they deduced that there are four faults (F1, F2, F3, and F4) crossing the exploration area. The geological and tectonic features of F1 and F2 have an NWW orientation, and F1 and F2 are pressure-shear fractures. The trend of F3 and F4 is NNE; these faults were caused by NS extrusion pressure and they cut Shirenshan anticline, F1 and F2 as shown in Fig. 1. The terrain of the exploration area is higher than the surrounding, and there is a perennial water system, rivers are underdeveloped, and there are sporadic ponds and small reservoirs. Subsurface geological survey shows that geological stratum are consisted of Devonian, Permian, Triassic, and Quaternary periods; the proper order of strata from bottom to top consists of shale, quart-sandstone, sand-shale alternating layer, shalelimestone alternating layer, and so on. Such layers are mainly folded in E-W orientation.
APPLICATION OF MT
Existence of sources of thermal water is connected with presence of the high-temperature deep center and tectonic faults wherein circulation of thermal water exists. Because there is no any volcanic activity at the exploration region, it is difficult to find a hot spring of high temperature at recent stage. Thus, we must prospect the deep buried structure and fracture to find a deep geothermal field. MT method has great exploration depth and is not shielded by the formation of high resistivity. It is an effective method to explore deep geoelectric structure.
MT Data Acquisition and Analysis
Geothermal resources are genetically related by water and heat. Since the electrical resistivity is affected by changes of temperature and permeability, the EM and electrical methods can provide a model for the subsurface resistivity variation. When geoelectric structure is in certain circumstance, the receiving frequencies determine the depth of exploration. So we can ascertain the different depth distributions of the electric property from the amplitude and phase of impedance at different frequencies by a single-site measurement. In this case, V5-2000 of Phoenix Geophysics Ltd. was used in the survey. It measured two orthogonal components of electric field (Ex, Ey) and two components of magnetic field (Hx, Hy) as shown in Fig. 2. The main purpose is to understand the depth buried structure and fracture above 2 000 m of the study area.
Figure
2.
Diagram of the MT station as deployment in Jiangxia.
Thirty-eight MT data sites acquired are indicated by the red crosses (lines 1 and 2 in Fig. 1). This case belongs to area of exploration, so the arrangements of measuring lines parallel to or perpendicular to the main structure trend are feasible. All profiles length is 3.6 km, there are 19 measuring points on each line, the distance between the two lines is 500 m, and the distance between adjacent sites is 200 m. The two components of the electric and magnetic fields were measured in N-S and E-W orientation in a frequency band spanning 320–0.000 55 Hz. The vertical magnetic field component was also recorded at every site. The measurements were carried out for 11 h at each site in the frequency bands.
All the data were processed using SSMT-2000 (Phoenix, Canada) software package. This processing facilitates robust single-site estimates of EM transfer functions. Time series with obvious disturbances were discarded prior to frequency domain conversion. Recently, several researchers published some papers about TE and TM models (Bologna et al., 2011; Key and Weiss, 2006). Zhu et al. (2009) compared the results of TE and TM model, which indicated that TM model enlarged the electrical difference about electrical body on lateral; especially, TM model is sensitive to reflection of fault about water prospecting in mountain; at the same time, in order to compare with the result of TM model CSAMT, TM model inversion was chosen in this article. Some MT apparent resistivity yx and impedance phase Zyx for the reference sites in exploration area are shown in Fig. 3. In Fig. 3, when the frequency is lower than 1 Hz, the apparent resistivity appears to jump. It means that data are not very reliable; however, in this case, the main purpose is to detect the geological structure about 2 000 m. Actually, the data with frequency lower than 1 Hz is not useful in real inversion.
Figure
3.
Apparent resistivity and impedance phase curves at TM (Ey, Hx) mode. These sites are near geothermal field, which are on Line 2.
Our penultimate aim was to construct a 2D resistivity model of exploration area and use it to understand its hydrothermal system by correlating it with independent geological and other geophysical information. Three major 2D inversion methods for MT interpretation have been developed recently. These include Occam, non-linear conjugate gradient, and rapid relaxation inversion method (RRI). They all belong to the class of minimum structure inversions. In this case study, we used the 2D RRI code written by Smith and Booker (1991). The RRI approach determines the resistivity model with least structure consistent with the data. Figure 4 shows the 2D section for the geothermal field along the two lines. The geological and tectonic features are predominantly having a W-E orientation in the survey region (Fig. 1). In view of these observations, the corrected TM apparent resistivity and impedance phase data were inverted using the 2D RRI for each profile line.
Figure
4.
Geoelectric profile of the 2D inversion of magnetotelluric data.
In Fig. 4, the 2D inversion was performed on the W-E profiles of lines 1 and 2 selected from the geological setting map shown in Fig. 1. The two pictures reveal three layers of different electrical resistivities: the first resistive layer (20–250 Ω·m) is good conductive fracture zone or lacustrine alluvium and conductive fracture zone that parallels anticline trend; the second resistive layer (250–350 Ω·m), the lithology of this layer includes sandstone, limestone, siliceous shale, siltstone, battie, and coaly limestone, it is relalithology of this layer includes sandstone, limestone, siliceous shale, siltstone, battie, and coaly limestone, it is relatively moderate resistivity response; and the third layer (> 350 Ω·m) consisted of low conductive bodies, the lithology includes gray quartz sandstone with claystone and thick quartz conglomerate. But in the exploration area, it is mainly high resistance reflection. From the surface to about -100 m, the low resistivity layer is related to lacustrine alluvium of Quaternary shown in Fig. 4. In Fig. 4a, the resistivity layer is discontinuous between 6th and 11th sites, which is the reflection of anticline fault F3; the west resistivity is lower than east, indicating that F3 cut Devonian, Permian, and Triassic, its lithology is mainly limestone, karst is very developed, and ground water is very active. In Fig. 4b, the resistivity layer is discontinuous between 6th and 11th sites, abnormal range is upright shape, and it is narrow in the deep of the abnormal range and wide in the shallow, and the west resistivity is lower than the east, and its property is similar to Fig. 4a. The anisotropy is typically accompanied with a fault, so speculating that the region is a water-filled fracture zone (anticline fault F3) in deep.
Weak anisotropy and orthogonal anisotropy of resistivity distribution in the horizontal plane at a measurement sites can be regarded as 1D and 2D structures, respectively. Comparing resistivity sections of the real data with the theoretical result, they are very close as shown in Fig. 5; error is very little, that is to say, the result of inversion is reliable and the method of inversion is true.
Figure
5.
Results of fitting curves of 2D MT of lines 1 and 2. (a) Diagram of real apparent resistivity; (b) diagram section of theoretical apparent resistivity.
Shown in the geoelectric profile of the 2D inversion MT data, low resistivity is visible between 7th and 9th sites in Fig. 4a and 6th and 11th sites in Fig. 4b. It means that faults traverse them. Because the measurement region is in suburb, railway and high-tension line is near to the area, the interferences are serious and the accuracy of MT is easy to be effected. Therefore, using CSAMT method validates the result of MT method. The survey lines were arranged around the abnormal region.
CSAMT Data Acquisition and Analysis
CSAMT method is a frequency-domain sounding technique that utilizes artificial and natural EM fields to measure the variation of resistivity underground. Typical CSAMT equipment consists of a transmitting station, the source of which is a dual horizontal electrical dipole, a receiving station that measures the orthogonal horizontal components of the electrical (E) and magnetic (H) fields. The ways of wild CSAMT measurements contain three methods: scalar, vector, and tensor measurements. In this study, we applied the first method to reconnaissance survey based on the economy shown in Fig. 6 using eight ways of electric field and a magnetic field observed, and the weight electrodes were distributed equidistantly along the survey lines. The magnet of the magnetic field was at the center of the eight electrodes and plumbed the extension direction of the electric field electrodes horizontally. The scalar CSAMT includes two measurements: TM (transverse magnetic) and TE (transverse electric). TM measurement was used in this case because the source position is out of the geological setting map; it is not shown in Fig. 1.
All the positioning of measuring and transmitting sites were completed by GPS, the accuracy positioning is sufficient for exploration, and the distance between adjacent stations is 40 m. In terms of the MT results, arranged one measuring line (Line 3 shown in Fig. 1). The 35 measurement sites are shown in Fig. 1. Rx distance is 8.1 km and Tx dipole is 1.2 km of Line 3. The range of frequencies is from 8 192 to 0.125 Hz (8 192.0, 4 096.0, 2 048.0, 1 024.0, 512, 256.0, 128.0, 64.0, 32.0, 16.0, 8.0, 4.0, 2.0, 1.0, 0.5, 0.25, and 0.125 Hz).
The effect of the non-plane-wave depends on several factors: the frequency of the signal, transmitter-receiver separation, length of the transmitter, and orientation of the Tx-Rx dipoles—all these factors alter the skin depth. Because the conductivity structure is unknown, the relationship of these factors to the non-plane-wave effect is difficult to determine. The region around the transmitter is generally divided into three zones: near-field region (r (the source-receiver offset) << δ (the skin depth)), transition zone (r≈δ), and far-field region (r >> δ). In theory,
where ρ is the grounding resistivity and f is frequency of exploration, if Tx-Rx distance is greater than 3δ, which means that region of survey is in far-field. In this case, the surface is good conductive bodies, the grounding resistivity is very low, which keeps that the skin depth is about 2 km, and the Tx-Rx distance is 8.1 km, which is greater than three times skin depth. In other words, in the exploration area, it belongs to far-field region, and there is non-near-field effect. Examples of CSAMT TM sounding curves along Line 3 are given in Fig. 7. We observed that near-field effect was not visible, and adjacent sites show similarity curves.
Figure
7.
Examples of TM model apparent resistivity and impedance phase curves of CSAMT soundings for sites are from 17 to 22 on Line 3. Note the non-abrupt change at any frequency, which indicates a far-field region or non-near-field effect.
The targets of geothermal exploration are to prospect the deep faults and fractures that control the movement of fluids. Alteration products observed along these faults and fractures are associated with chemical reactions caused by the hydrothermal system. Due to the alteration minerals and the brine, the faults zones respond as low-resistivity anomalies. In this article, SCS2D inversion software of USA Zonge Company was chosen. The software strives to make the difference between observed and calculated data and background and inverse modeling, and the roughness of inversion-simulation are minimized. The interpretation of 2D CSAMT results (Fig. 8) is similar to any other geophysical methods. The explanation mainly based on the following resistivity profile and geology setting, which determines the electrical scope of normal formation, can further define the abnormal electrical structure.
Figure
8.
2D inversion result of Line 3. The X-coordinate is the measuring sites, the Y-coordinate describes altitudes, and contours show the parameters of geo-resistivity.
Figure 8 shows the inversion result of Line 3. The measuring line is in W-E orientation, and the maximum inversion depth of exploration is -1 600 m. The electric profile is divided into two resistive layers: the resistivity of the first layer is 20–250 Ω·m, which are good conductive bodies, related to lacustrine alluvium of Quaternary period or the conductive fracture zone. The second resistive layer is 250–3 000 Ω·m, the lithology mainly contains siliceous shale, cherty limestone, packsand, battie, and silicate and its resistivity is moderate-high resistance response. From the surface to about -100 m, the low resistivity layer is related to lacustrine alluvium of Quaternary; in the middle of the profile, they are low conductive bodies, and the resistivity layer is obviously discontinuous between 18 and 28 sites, which manifested that there is a conductive fault; in terms of the geological setting map, the fault is F3. The fault in the depth forms fracture zone, its scale is large and extending is deeper, its obliquity is near to vertical, analyzed from the electrical profile, and the fault cuts each stratum of Devonian, Permian, and Triassic.
Comparing the real data with the theoretical result, they are very close as shown in Fig. 9, that is to say, the result of inversion is reliable and the method of inversion is valid. From the profile of CSAMT, we can get the following preliminary conclusions: there is a large-scale fracture. Integrating geological and geophysical data, considering the physical properties of underground water features, and combining with the information available fracture explained that the arrangement of a suitable measuring point wells was between 21 and 27 sites on Line 3.
Figure
9.
Results of fitting curves of 2D CSAMT. (a) Diagram of real apparent resistivity; (b) diagram section of theoretical apparent resistivity.
The exploration area is located in new industrial developing suburb, where artificial noises are severe, so the MT and CSAMT were applied at Jiangxia, enabling us to determine the structure of the first kilometers of the summit area. 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, which is 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 that from surface to -100 m is related to lacustrine alluvium of Quaternary period, and the deep low resistivity layer is interpreted to be representative of the hydrothermal system. 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 or the heat source, and (2) the second resistive layer (250–3 000 Ω·m), the lithology mainly contains siliceous shale, cherty limestone, packsand, battie, and silicate.
Comparing MT and CSAMT results (Figs. 4 and 8), a good correlation exists between MT positive anomaly and low-resistivity zone, which is located between 6th and 11th sites on Line 2 where a fault traverses them. The main anomaly was observed in the middle of exploration area as shown in Fig. 1 as indicative of the presence of heat sources. The application of CSAMT method provides information about the depth and position (between 21st and 27th sites on Line 3 in Fig. 1) location of this hydrothermal system and enables us to determine the MT anomaly. We can observe that the MT anomaly is clearly well correlated with low-resistivity zone inferred from CSAMT model.
CONCLUSIONS
The major purpose of MT and CSAMT survey is to define the location of the geothermal field of the Jiangxia: it appears to be bounded within the fault F3 and possesses the same N-S elongation; its depth ranges from less than 0.7 to more than 1.2 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 anomalies are similar and can be assumed to be generated by the same source. At the same time, it shows that the MT method is a useful tool for geothermal exploration in new industrial developing suburb.
ACKNOWLEDGMENTS
Thanks to Prof. Yaoguo Li and Drs. Oliver Ritter and Martyn Unsworth for constructive criticism and help in improving the manuscript. This work was supported by the National Natural Science Foundation of China (No. 40974040), the Deep Exploration in China (No. SinoProbe-01-03-02), and the Ministry of Land and Resources of China.
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Figure 1. Geological map of Jiangxia area showing the major geological and tectonic features, including distribution of the measurement lines of MT and CSAMT. Lines 1 and 2 are MT measurement lines, and Line 3 is the measurement line of CSAMT. In the exploration area, the geological strata are composed of Quaternary, Triassic, Permian, and Devonian. In Quaternary, it is mainly lacustrine alluvium composed of gray yellow clay, silty clay, sand and silt clay. At the top T1-2j, the lithology is purple-gray thick massive dolomitic limestone and brecciated limestone; in the middle of T1-2j, it is dolomitic limestone and dolomite; and at the bottom of T1-2j, it is massive crystalline limestone. From top to bottom, the lithology of T1d is respectively thin-limestone, marlstone, yellow-green sandy shale, yellow-green argillaceous limestone, and shale. P1g is mainly made up of black thin-silicalite and claystone interbedding, thin silicalite with siliceous shale and claystone, and gray white thick silicalite. The lithology of P1q is principally divided into two parts: the first part is the top P1q, which is consisted of black massive carbonaceous limestone, and the second is the middle and bottom of P1q, which is composed of cherty limestone, carbonaceous limestone, and gray brown sheet claystone. The lithology of D3w is mainly gray quartz sandstone with claystone; at the bottom of D3w is thick quartz conglomerate.
Figure 2. Diagram of the MT station as deployment in Jiangxia.
Figure 3. Apparent resistivity and impedance phase curves at TM (Ey, Hx) mode. These sites are near geothermal field, which are on Line 2.
Figure 4. Geoelectric profile of the 2D inversion of magnetotelluric data.
Figure 5. Results of fitting curves of 2D MT of lines 1 and 2. (a) Diagram of real apparent resistivity; (b) diagram section of theoretical apparent resistivity.
Figure 6. Layout of CSAMT method.
Figure 7. Examples of TM model apparent resistivity and impedance phase curves of CSAMT soundings for sites are from 17 to 22 on Line 3. Note the non-abrupt change at any frequency, which indicates a far-field region or non-near-field effect.
Figure 8. 2D inversion result of Line 3. The X-coordinate is the measuring sites, the Y-coordinate describes altitudes, and contours show the parameters of geo-resistivity.
Figure 9. Results of fitting curves of 2D CSAMT. (a) Diagram of real apparent resistivity; (b) diagram section of theoretical apparent resistivity.
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