PROCEEDINGS, Fortieth Workshop on Geothermal Reservoir Engineering

Stanford University, Stanford, California, January 26-28, 2015

SGP-TR-204

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Application of Geophysical Methods to the Geothermal Investigation

WANG Xuben 1, HE Lanfang 2, ZHAO Xiaoming 2 and YU Nian 3

( 1 Chengdu University of Technology, Chengdu 610059, China, 2 BGP, China National Petroleum Corporation, Zhuozhou 072750,

China, 3 Institute of Geophysics, China University of Geosciences, Wuhan 430074 China)

wxb@cdut.edu.cn

Keywords: Geophysical prospecting, geothermal field, gravity imaging, broadband frequency EM, CSAMT

ABSTRACT

Two case histories have been briefly reviewed to present the application of comprehensive geophysical to geothermal field investigation.

One example from the delineation of the geothermal field in a basalt zone by the AMT method with the one-meter hole temperature

measurement; the other is about the exploration of the geothermal water in a sediment strata area using the CSAMT method with the

interpretation of seismic data. All examples show that the effective usage of geophysical methods or combination is the key to success

in geothermal field exploration. In addition, we introduce three methods in the test of geothermal exploration: gravity imaging, high

power CSMT and broadband MT. It proves difficult to geothermal geologists and engineers for few direct ground information could be

used to concept the geothermal model. Research has indicated that the geothermal reservoir characterizes as relative low density, higher

linear gravity gradient, low resistivity and seismic anomaly. These provide possibility to use indirect information acquired by non-

invasion methods. Comprehensive geophysics is such a way to illustrate the pattern of geothermal reservoir by measuring and analyzing

the geophysics data which carried the information from media subsurface.

1. INTRODUCTION

With the rising of global warming and public health risksthat result from overusing fossil energy, using of the clean and renewable

energy is attracting the attention all through the world. Geothermal is considered as one of the best choice which agrees the bad needs of

the energy, for some renewable energy such as wind and solar works heavily relied on the weather condition and face the challenges of

energy storage. Geothermal energy has been utilized in more than 100 countries or regions, and it increases around 12 percent each year

(Bertani,2010). With the increasing of the utilization of geothermal energy, deep buried geothermal comes to be a major prospecting

target for geothermal geology. These challenge the geothermal geology which was traditionally relied upon the availability of point

measurements: hot spot, water sample analysis and results from boreholes (Binley et al, 2010). The critical problem to the geothermal

geology is how to concept the geothermal model which needs information about the cover layer, the reservoir and heat source of

geothermal system. Poorly known in terms of geometry, geology and hydraulic properties of the geothermal system often forces the

conceptual models to be black boxes (Binley et al, 2010). In order to concept a reasonable model, indirect information of the subsurface

of the geothermal system acquired on the ground is always exploited. Geophysical measurements account for the major means of

providing such information.

In general, geophysical methods are used to provide data for the following purposes: (1) structural characterization, (2) lithological

boundary or reservoir delineation for geothermal exploration and (3) geothermal reservoirs monitoring under exploitation. Gravity

surveys (Faulds et al., 2010) are widely used for geothermal structural characterization. Seismic reflection and imaging(Riedel et al.,

2014) are powerful tools for charactering the structure but features as higher cost. Induced seismic (Fehler et al., 2001) or micro-seismic

(Xu, 2012) are also considered as techniques for characterizing reservoir structure. Geo-electrical methods such MT or AMT

(Gasperikova et al, 2011), DC methods, TEM-soundings is used for both and structural characterization and reservoir delineation, for

resistivity is sensitive temperature, porosity and brine saturation of the reservoir (Wilt and Alumbaughe, 1998). Self-potential (SP)

method, induced polarization (IP) method, spectral induced polarization (SIP) or complex resistivity method are once considered as new

approaches for geothermal investigation but they generally has limit exploration depth, so the utilization is limited. Magnetics is used as

tool for identification of Curie point temperatures and depth magnetization as well as definition of field boundaries in geothermal

prospecting (www.gns.cri.nz). Data from time-lapse MT, AMT or CSAMT, gravity are now utilized for monitoring the geothermal

development such as hydro-fracturing, water injection or subsurface geothermal migration. In most cases, it is often quite difficult to

accurately characterize are fervor's quality with just one measurement. It is necessary to correlate different geophysical techniques

collecting different signals in order to accurately identify the useful anomalies from the geothermal system (Brunoet al, 2000;

Castañedaand Brunkal, 2009).

2. CASE HISTORIES

2.1 Geothermal prospecting in Emei Mountain

2.1.1 Backgroud and geothermal geology

The working lies in Emei Mountain, southwestern Sichuan province of China. This area has all the strata of each geologic age except for

the lack of the Silurian, Devonian and Carboniferous. It mainly features as for the shallow sea face sediment from Silurian to Middle

Triassic, and the Upper Triassic transits from sea to land, the Jura–Cretaceous is fluvio lacustrine faces sedimentary Shiziping area is

located in the middle lower segment of Qing Yinge streams and its adjacent areas in Emei mountain. It is located in the Guihuachang

synclinal axis and north–east wing flank province in the tectonic position (Figure 1). The basement rocks of the study area is the granite

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of Jinnin period in Presinian. The exposed strata is mainly basalt of Mount Emei, sandshale of Shawan Formation, shale of Feixianguan

Formation and the limestone and marl of Jialingjiang Formation.

According to the fracture structure, lithology, strata structure, groundwater cycling conditions, surface geothermal spot and many other

data in the study area, it indicate that the region has three main heat storage layer that has exploitation value, that is the limestone of the

Leikoupo–Jialingjiang Formation in the Middle and Lower Triassic, limestone of Maokou–Qixia Formation of the lower Permian,

limestone and dolomite of Hongchunping Formation in the Upper Aurora.

Figure 1: The geological map of Shiziping geothermal field

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2D Inversion Risistivity Map of Line 6

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Muyushan Figure 3: 2D inversion interpretation of

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2.2 Geophysical prospecting and result

The favorable geothermal exploration target is delineated by surface geology and water chemistry survey. EM sounding was used to

confirm the structure and fracture for the development of aquifer. There have six electromagnetic profiles in the inference favorable

geothermal area. The pseudo-section of EM sounding revealed the underground low resistivity zone, high resistance abrupt change fault

zone, as well as cap-rock distribution, and verified the geological infer the existence of a new fault. Comprehensive with geological,

geochemical and geophysical analysis, we fixed the final target position on Shiziping area which located in the triangle zone surrounded

by Duiheba–Lianghekou–Muyushan. Electromagnetic sounding data show that the fracture structure formation of the favorable

conditions includes: the existence of multilayer low–resistivity zones and good sealing conditions, which indicates that the upper and

lower multilayer water development, and the deep faults developed is conducive to deep hot spring circulation. The interpretation of the

results are shown as (Figure 2 and Figure 3):

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1) There are seven electrical layers in the surveyed area. In addition to the development multi–layer shallow water, the deeper

inversion result reflected the existence of two major aquifer systems. Due to the fluctuation is not fully connected, so there

may be upper and lower aquifer system.

2) There is a second low resistivity zone below 1200 meters in this area, which inference for fracture development or water-

bearing structure zone.

3) The second of layer of river systems located in the two cross–sectional surveyed area is related that constitute the favorable

situation of deep underground water system in this area.

Drilling result in section showed of Figure 3 saw aquifers in the depth of 174 m, 450 ~ 510 m and 900 ~ 1120 m revealed the target heat

reservoir, which agreed to the geophysical result.

2.2 Geothermal exploration of Chengdu Plain

2.2.1 Backgroud and geothermal geology

The working area is covered by quaternary. According to the regional geological data, the strata consisted of Quaternary, tertiary,

Cretaceous and Jurassic from up to downward. The Quaternary is gravel pebble based, multiple sedimentary cycles with sand, gravel

and clay layer. The Tertiary Miocene Dayi Formation is pebbly sand–based. The underlying bedrock is Cretaceousstratum,

mainly brown mudstone, argillaceous sandstone, lithic feldspathic sandstone (Figure 4).

The study area is located in Wenjiang dome fold belt in Chengdu depression between western Sichuan depression Longmen Shan thrust

belt and Longquan Mountain thrust belt. According to previous research, think Wenjiang is located in the NNE to Tang Chang–Dayi in

deep zone, Yongquan–Sanjiang trap and its holding Pixian–Chongzhou raised band. According to seismic data interpretation of

Wenjiang area, there is Wenjiang anticline at the top of the Penglaizhen, and it is divided by NEE saddle structure, appering a north

peak and a south peak, the north peak is located in the Wenjiang District, the south peak is located in the tourist area of Jinma. The

anticline structure affects the Triassic to Cretaceous.

Figure 4: The geology map of the working area

1,The Middle Pleistocene and Epipleistocene of Quaternary; 2, The Holocene of Quaternary; 3, The Tertiary; 4, The Guankou

Formation of Cretaceous; 5, The middle and lower of Cretaceous; 6, The Tianmashan Formation of Cretaceous; 7, The Penglaizhen

Formation of Jurassic; 8, The middle and lower of Jurassic; 9, The Triassic; 10, Fault; 11, Type–boundary section; 12, Geothermal

prospecting target area.

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Figure 5: The inversion interpretation map of AMT profile

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Figure 6: The resistivity imaging and electrical interface map of CSAMT profile

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2.2.2 Geophysical operation and result

We carried out AMT and CSAMT in the exploration area, found intense electrical changes of rock, and determined the related fault

tectonics. The results are showed in Figure 5-7:

Result from AMT of the survey area shows that there is low resistivity zone in the north east and north west, the intersection is near the

Mengqiao (Figure 5). Low frequency data can reflect lithology changes of strata which below 1000m, low resistivity zone seems narrow

strip, it extends to north east, and its arrangement seems echelon piecewise. Due to the attitude of deep stratum is nearly horizontal, so it

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is possible that these low–resistivity zone is water bearing fault zone. CSAMT was used in the preliminary selected target zone, which is

better than AMT method in detection of deep strata, we got the electrical characteristics data of the deep strata.

Based on the two-dimensional inversion results of CSAMT section, the resistivity imaging and electrical interface imaging (Figure 6) of

CSAMT section two–dimensional inversion shows that there is a steep fault in the cretaceous strata in the east of the survey line, the

fault cut the cretaceous and ends at the top of the Jurassic, its depth is about 1200~1900 m. The change characteristics of 1000 m deep

resistivity plane which is obtained by CSAMT data inversion (Figure 7) shows that there have obvious changes of electrical zone in the

detailed survey area, the survey area is divided into two portion, the distribution of resistivity is reverse in 1500 m deep, this indicated

that the fault strike is close north to south direction. Geological structure model can be established as follows: It can be divided into two

levels from the development features of the structure, there is fault development trending SN about below 1000 m depth in the survey

area. And the fault zone trending NE is well developed above the 1000 m depth, But there is fault development trending NW in the

shallow place and intersects with these faults which trending NE, it belongs to the shear fracture and is on the hanging side of the fault

which trends SN. The existence of this fault zone provides favorable conditions for the formation of underground hot water system, and

also provides bases for geothermal exploration wells.

According to the surface anomaly investigation and the method of AMT to confirm the target area, as well as the detection of deep

structure characteristics with CSAMT method in the target area, the study area has the conditions of the formation of underground hot

water progressively, we narrowed the scope of exploration and set a drilling in 1500 m depth in the CSAMT section finally, it was

proved by the drill after one year.

3. GEOPHYSICAL METHODS TEST IN GEOTHERMAL EXPLORATION

Changes in technology saw the deployment of modern geophysical techniques which made it possible for shallow and deep conductors

to be accurately imaged and thus more accurate geothermal models developed (Mariita, 2009). These renovate the geophysical

methods in two aspects: one is the improvement on data acquisition, high power transmitter system, broadband frequency EM, high

density EM has been used in field operation. The other is innovation in data processing including 3D inversion, gravity imaging, and

time-lapse data inversion. In this paper, we introduce three methods in these innovations: gravity imaging, high power CSMT and

broadband MT.

3.1 Gravity Imaging

Gravity Imaging Technology is a new branch of imaging technology, which was developed from gravity data processing technology and

image processing technology. There are two kinds of Gravity Imaging Technologies, one is planar imaging, the other is called pseudo-

depth imaging by us.

In order to emphasize the anomaly and extract more information for interpretation, the planar imaging method converted the gravity

anomaly with a suited grid density to a value of the grayscale or brightness, so that the anomaly could be displayed by image. Methods

specially used for image processing, such as pseudo-color processing, spatial shade processing, and convolution filtering, were used for

enhancing the imaging effect. The pseudo-color processing mainly assigned a kind of color to each greyscale, the information hidden in

the anomaly will turn to be more clear for recognizing, because our eyes are more sensitive to color than to gray. Spatial shade

processing firstly used in enhancing of the topography and physiognomy map, here we consider the grayscale which converted from the

gravity anomaly as the topography, if it was irradiated by the beam of light, there will have shade and bright as the real hill, and the

anomaly will be recognized more clearly (Figure 8).

The pseudo-depth gravity imaging is mainly based on wavelet analysis or other mathematical methods, such as analytic continuation.

Only the wavelet-based method is considered in this paper. Based on the wavelet analysis theory, a real serial could be converted to

wavelet coefficients of different scale. Each scale is related to a certain frequency. The gravity profile data could be considered as a real

serial, so we can convert it to wavelet coefficients. Different scale of coefficient has different frequency. The data of different frequency

is related to different depth information of the earth because of the ground filtering effect. Low frequency data contains deep

information and high frequency often contains shallow information of the earth. So we can get the pseudo-depth gravity imaging by

contouring its wavelet coefficients of different scale (Figure 8b). Of course, the planar imaging method could be used to pseudo-depth

gravity imaging result for further useful information.

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Figure 8: Comparison of the conventional result and imaging processing result of the gravity data. (Left) is the conventional

result of the residual anomaly and (Middle) is the planar imaging processing result, (a) is the conventional profile of the

gravity anomaly and (b) pseudo-depth gravity imaging processing result. (c) is the seismic result in the same section.

3.2 High Power CSMT

In order to improve the signal-noise ratio in the electromagnetic noisy area such as the urban or industrial area, a new approach of deep

groundwater prospecting, high power Control Source Magnetotelluric (CSMT), was developed to aim to the prospecting of the deep

geothermal reservoir. The power supply we use is T-200 current source which was developed by Phoenix Geophysics from Canada, it

has a power consumption up to 160 kW and works as a high-powered controlled source for geophysical applications. Its output

frequency ranged from 256Hz to 1024s, which is same to the conventional magnetotelluric sounding (MTS). This is why we call it

CSMT instead of CSAMT. We have studied the feasibility of the method, the response of different magnetic sensors (low frequency or

high frequency coil) and the response of the high-powered source. The low frequency result is acceptable by comparing to the MT result

at the same site. The result of the comparison of the result of CSMT and the MTS from the same station indicated that the resistivity

curve of the two method features as similar shape in frequency band ranges from 0.1 to 256 Hz. The resistivity of CSMT is higher than

that of MTS in frequency band lower than 0.1 Hz. Figure 9 shows the result of CSMT for geothermal favorite delineation. The survey

line located in the urban of Beijing, some 16 km northwest from Tian An Men Square. The transmit current we used in field operation is

up to 120A, and separation between the current dipole and receive station is 8.3 km. The inversion section showed in figure 9 provided a

reasonable resistivity profile, from surface to a depth of 3.5 km, there are a conduct layer, and then a relative resistant layer, a second

conduct layer, a resistant layer, and relative conduct layer. A conduct area was uncovered from offset 0.2 to 0.4 km, it is considered as a

fault, the deeper portion of the fault is interpreted as the favorite geothermal area, a borehole was drilled based on the result of CSMT

and the favorite geothermal area was verified. The borehole is still works as geothermal well.

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Figure 9: CSMT inversion section for geothermal prospecting in the urban of Peking.

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Figure 10: Resistivity curve of broadband MT

3.3 Broadband MT

MTS including audio magtotelluric (AMT) data are sensitive to the subsurface resistivity according to the fundamental Tikhonov-

Cagniard (T-C) model. The fundamental MT assumption is that the electromagnetic (EM) field due external sources at Earth’s surface

behaves as a plane wave. The two horizontal components of the electric field are linearly related to the two horizontal magnetic field

components. The proportionality between the E and H fields on the surface depends on the distribution of the electrical resistivity in the

subsurface (Eisel and Egbert, 2001 ). Generally, MTS has deep exploration depth because it acquires data with lower frequencies ranges

from 320-0.001 Hz. AMT, however, acquires data with lower frequencies ranges from 10000-10 Hz, as a result, it has relative high

resolution while shallow exploration depth. In some cases, we need the information from shallow to larger depth. This call for a new

MT operation method called broadband MT. In the field operation, both the AMT and MT data was acquired from the same station, and

we get a curve with frequencies band ranges from 0.001 to 10000 Hz (Figure 10). The advantage of broadband MT lies that it has

relative high resolution in the shallow portion and deep exploration depth which help outlines the deep geothermal reservoir and the heat

source which has a buried depth range from several kilometers to more than 20 kilometers. Besides, result of AMT could be used for

the shift correction in MT data processing. Result from Iceland and Hungary indicate that broadband MT act as a very important role to

help explore and characterize a geothermal reservoir (Yu et al, 2008, Tulinius,2009 ).

4. CONCLUSIONS

Geophysical measurements account for the major means of providing information of structural characterization, lithological boundary or

reservoir delineation for geothermal exploration and geothermal reservoirs monitoring under exploitation for several decades. Case

history from Emei and Chengdu plain indicated that geo-electrical methods could work well in geothermal prospecting from both

Sedimentary basin and igneous rock area. The innovations such as gravity imaging, high power CSMT and broadband MT sparks the

geothermal geophysical prospecting with better visualization, signal-noise ratio and resolution. These provide possibility to use indirect

information acquired by non-invasion methods. Comprehensive geophysics including gravity imaging, high power CSMT and

broadband MT is such a way to illustrate the pattern of geothermal reservoir by measuring and analyzing the information from media

subsurface. All examples show that the effective usage of geophysical methods or combination is the key to success in geothermal field

exploration.

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