INTERPRETATION OF 3-D MAGNETOTELLURIC DATA · PDF file140 mudstone. Lineaments from ... effect causes the split in apparent resistivities at low frequencies between xy- and yx- modes

Proceedings of the 6th Asian Geothermal Symposium, Oct. 26-29, 2004 Mutual Challenges in High- and Low- Temperature Geothermal Resource Fields, 139-143

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INTERPRETATION OF 3-D MAGNETOTELLURIC DATA INCLUDING SEA EFFECT FOR GEOTHERMAL EXPLORATION IN POHANG, KOREA

Tae Jong LEE1, Yoonho SONG1, Toshihiro UCHIDA2, Yuji MITSUHATA2, and Seokhoon OH3

1 Korea Institute of Geoscience and Mineral Resources (KIGAM), Korea, 2 Institute for Geo-resources and Environment (GREEN), AIST, Japan,

3 Korea Institute of Water and Environment (KIWE), Korea e-mail: [email protected]

ABSTRACT

A three dimensional (3-D) magnetotelluric (MT) survey has been made to delineate the subsurface structures and possible fractures for exploration of low temperature geothermal resources in Pohang, Korea. The site is located in southeastern part of Korea and is very close to the east-ward sea shore. Survey area lies in 3 ~ 8 km apart from the sea shore lined to the north/south direction. To infer the sea effect, 3-D MT modeling has been performed for a layered model including the sea water. Layering of the model and conductivities of each layer are roughly estimated by well logs and other geological information. Results showed that the conductive sea water affects the low frequency MT data, especially below 0.2 ~ 1 Hz depending on the distance from the sea shore. Most severe sea effect was observed from the southeast parts of the survey area due to Youngil bay. Two kinds of 3-D inversion have been performed with the MT data observed from the site; including the sea as a constraint in the inversion or not. The two inverted models showed very similar characteristics in shallow parts, roughly down to 2 km. At deeper parts, however, they showed very different characteristics as can be expected from the modeling study. Strong effect of the nearby sea seems to form a fictitious conductive structure in southeast part of the survey region in ordinary 3-D inversion. A conductive structure, however, was shown at about 3 km depth from both of the inversions. More geological and geophysical study will be needed to identify the structure. Keywords: 3-D MT survey, sea effect, geothermal exploration, Pohang geothermal field 1. INTRODUCTION The boundaries between the ocean and inland induce severe distortion of electrical fields due to its extremely high conductivity contrast. There have been several efforts to identify and to correct the ocean and coast effects in MT and geomagnetic depth sounding (GDS) data (Monteiro Santos et al., 1999; Bapat et al., 1993; Oh et al., 2003). In the year 2002 and 2003, a dense 3-D MT survey has been carried out for geothermal exploration in Pohang, Korea. Pohang is located in southeast margin of Korean peninsular and the survey site lies very close to the sea shore (Figure 1). Among the surrounding seas of Korean peninsular, East Sea is the deepest and shows most severe effects to the low-frequency EM measurements in eastern side of Korean peninsular including Pohang area (Oh et al., 2003). Most of induction arrows point to the east in Pohang area at low frequencies. Ordinary 2-D and 3-D inversion showed the layered structure beneath the site and some possible fractures run through the region (Uchida et al., 2004). At deeper parts, however, the inversion results can, at least partly, be affected by the sea effect. In this study, we first perform a 3-D modeling for the sea model to infer the sea effect. The 3-D model is set up including the rough coastal line nearby, layering of subsurface structure, average conductivity of each layer down to 1.3 km, average depth to the sea bottom, and average conductivity of the sea water, which are based on test drilling and various geophysical/geological results. We then perform a 3-D inversion with such geological settings as a constraint and compare the results with the ordinary 3-D inversion results. 2. GEOLOGY AND MT SURVEY IN POHANG MT survey area is located in the north of Pohang City, southeastern Korea. The sea shore lies in 3 ~ 8 km apart from the site and is lined to the north/south direction (Figure 1). The area belongs to Tertiary Pohang Basin overlying Cretaceous sedimentary rocks with biotite-granite, rhyolite, and trachybasalt intrusions (Song et al., 2003). Topmost layer consists of thick Tertiary sediment formation, which is characterized by semi-consolidated and highly conductive


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mudstone. Lineaments from Landsat image are mainly in NNE-SSW in the region and two major lineaments in NNE-SSW and WNW-ESE directions intersect within the MT survey region. Total 70 measurements were made using Phoenix MTU-5 and MTU-5A system. The measurement at each site was done for two days, 15 hours per day from 5 pm to 8 am the next day, at which it shows relatively low cultural noises. Korea suffers extremely strong EM noises due to the old formations that show high resistivity and power lines covering almost entire country. For the reason, a remote reference is installed in Kyushu, Japan (approximately 482 km apart from the survey area), where it shows relatively quiet in EM noise sense. Remote processing dramatically improved the quality of the data. 3-D interpretation is made for the data from 44 sites surrounded by a rectangle in Figure 1. Two test boreholes are drilled in the year 2003-2004 and one of them has reached to the depth of 1.3 km. Various well logging techniques including caliper, gamma ray, resistivity logs have been performed, so that the layered structures, resistivity of each layer, and fractures intersecting the boreholes are informed. The location of the test boreholes is also shown in Figure 1. 3. MT 3-D MODELING FOR SEA EFFECT To infer the sea effect, a model is set up including simplified coast lines and a layered structure as shown in Figure 2. The resistivity of each layer down to 1.5km is based on the drilling and logging results. Average values for the depth to the sea floor and the resistivity of the sea water are used. Three-dimensional modeling for this model is performed using the staggered-grid finite difference code by Mackie et al. (1993). Figure 3 shows the induction arrows calculated for frequencies 0.001, 0.005, 0.02, and 1 Hz, respectively. On the three figures of which frequencies are lower than 0.05 Hz, all of the induction arrows within the MT survey area point to the east direction to the sea. At 1 Hz on Figure 3d, however, it shows very small amplitude and the sea does not affect the response any more. Note that some arrows on the southeast margin of the survey region point to Youngil bay, the nearest sea at that point, which means that the sea affects the MT responses even up to 1 Hz at the southeast margin of the survey region. Apparent resistivity and phase curves for two sites, A and B in Figure 2, are compared in Figure 4. The two sites correspond to the west and east most measuring point in MT survey, respectively. Because the sea shore boundary is the only 2-dimensional structure in the model, the differences in apparent resistivities between xy- and yx-mode can be considered as the see effect. The split of the two modes in apparent resistivity appears at frequencies roughly below 0.2 Hz on site A and below 1 Hz on site B. Thus the nearby sea can affect the MT responses below the frequencies 0.2 Hz ~ 1 Hz depending on the measurement points. 4. MT 3-D INVERSION INCLUDING SEA FLOOR Among the 70 MT measurements, 44 measurements within the rectangle in Figure 1 are used to performed 3-D MT inversion. A linearized least-squares inversion with optimum regularization and static shift parameterization (Sasaki,

Figure 1. Location map of the MT survey stations. The remote reference is installed in Kyushu, Japan. Note that the survey site is very close to the sea shore.

Figure 2. A 3-D model to investigate the sea effect in MT responses. The box indicates the MT site in Pohang. Note the arbitrary vertical scale.


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2004) is used for the inversion. Some modifications for sea-water constraint are added to the code. Forward routine for the inversion uses the finite difference method. A 3% noise floor was assumed. The number of frequencies used was 13 ranging from 0.0159 Hz to 66 Hz. Cell size for forward modeling is 150 m in horizontal and 100 m in vertical directions on the surface of the target area, and gets bigger outward and downward. Inversion blocks are divided into 2100 (10 15 14× × blocks in x, y, and z direction respectively). Coordinate is set to the north-west most measurement point as origin, x to the north, y to the east, and z downward.

Figure 3. In-phase induction vectors around Pohang geothermal sites by 3-D MT modeling with parameters shown in figure 2. The box indicates the site under study and length of a bold arrow depicted on top-left figure indicates unit amplitude.

Figure 4. Apparent resistivity and phase curves from the 3-D MT modeling at site A and B shown in Figure 2. Note that the sea effect causes the split in apparent resistivities at low frequencies between xy- and yx- modes.


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(a) (b)

Figure 5. Resistivity models from 3-D inversions without any consideration of nearby sea. 13 frequencies ranging from 0.0159 Hz to 66 Hz are used.

(a) (b)

Figure 6. Resistivity models from 3-D inversions considering near by sea as a constraint. Sea shore boundary is located at y = 9 km and extended to north/south (x) direction. Depth to the sea floor is assumed to be 1.5 km. Conductivity of the sea water and the formation below is fixed to 0.3 ohm-m and 1000 ohm-m, respectively, during the inversion process. Figure 5 shows the inversion results without any consideration of the sea effect, while Figure 6 is applied a constraint of sea shore boundary at y = 9 km and extended as a straight line to x direction. Input data and all the other parameters for the inversion are the exactly same for both of the inversions except that we added some blocks to the eastern side of the model with fixing its resistivity values during the inversion for Figure 6. Conductivities of the sea water and the formation below are fixed to 0.3 ohm-m and 1000 ohm-m, respectively, during the inversion process. The root mean square (RMS) errors at final iteration were 2.78 for Figure 5 and 2.19 for Figure 6. Comparing the two results, it can be easily seen that the shallow structure in both inversion shows very similar features. The semi-consolidated sediment layer having resistivity less than 10 ohm-m is extended to 300 ~600 m deep, which is already verified by the test drilling. The layer appears thicker in southern part and gets thinner to the north direction as can be seen in Figure 5b and 6b. Beneath the sediments layer, it follows a layer of 400 ~ 600 m thick and of resistivity from several tens to a few hundreds ohm-m. And below the layer, it follows a layer with resistivity more than 300 ohm-m, which will possibly be granite formation. Depth to this layer is varying point by point from several hundreds meters to 1~2 km. The similarity between the two results comes from the fact that the sea affects only for low frequencies below 1 Hz as discussed in sea-effect modeling section. If we assume the average resistivity of the formation to be about 10 ohm-m,


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the skin depth for 1 Hz is approximately 1.6 km. The two results, thus in theory, will show similar structure down that depth. At deeper parts, however, the two results show quite different characteristics. Note in Figure 5 that a conductive (10 ~ 20 ohm-m) layer appears from 3 km depth and is extended to upward and southeast direction. We can also find the conductive layer from the same depth in Figure 6. But this time north-west direction is most conductive and east part shows resistive characteristics. Modeling results including the sea shore lines showed that MT data from southeastern part of the survey area would be strongly affected by the Youngil bay. When we do not consider the sea effect in the inversion, the sea effect in observed data will be projected to the deeper part of the image. In that sense, the low resistivity anomaly in 3 km depth in Figure 5, at least southeast part of it, seems to come from the sea effect. In either case, however, there seems to be a conductive structure in about 3 km depth. Further geological and geophysical study should follow to identify the structure. 5. CONCLUSION 3-D modeling and inversion are performed for sea effect consideration in MT measurements near the sea shore. The nearby sea in Pohang area affects the MT data for the frequencies below 1 Hz ~ 0.2 Hz depending on the distance from the sea shore. Most severe sea effect was observed from the southeast parts of the survey area due to Youngil bay. Such sea effects seem to affect the inversion results to the depth over roughly 2 km. 3-D inversion with and without the sea water constraint showed very similar results in shallow part, roughly down to 2 km. In deeper part, however, strong sea effect seems to form a fictitious conductive structure especially in southeast part of the survey region in ordinary 3-D inversion. Our study uses very simplified model for coastal lines and the depth to the sea floor. Further studies should follow including detailed constraints for the sea shore and see floor as well as the resistivity of each layer beneath the sea water. ACKNOWLEDGEMENTS This work was supported by Korea Research Council of Public Science & Technology (KORP) funding to Korea Institute of Geoscience and Mineral Resources (KIGAM). The reference data in Japan in 2002 were obtained under a project of Tottori Prefectural Government for earthquake studies. REFERENCES Bapat, V. J., Segawa, J., Honkura, Y. and Tarits, P. (1993) Numerical estimations of the sea effect on the distribution of

induction arrows in the Japanese island arc. Phys. Earth Planet. Inter., 81, 215-229. Mackie, R. L., Madden, T. R. and Wannamaker, P. E. (1993) Three-dimensional magnetotelluric modeling using

difference equations - Theory and comparisons to integral equation solutions. Geophysics, 58, 215-226. Monteiro Santos, F. A., Nolasco, M., Almeida, E., Pous, J., Marcuello, A. and Queralt, P. (1999) Correction of the

ocean and coast effects on the magnetotelluric impedance tensor. The 2nd Int. Sympo. on 3-D EM (3DEM-2), 309-312.

Oh, S., Yang, J., Lee, D. K., Kim, S.-K., Mogi, T., Nakada, M. and Song Y. (2003), Study on deep structure of the Korean peninsula by GDS and teleseismic data. Proceedings, 2003 Korea-Japan Joint Seminar on Geophysical Techniques for Geothermal Exploration and Subsurface Imaging, 68-76.

Sasaki, Y. (2004) Three-dimensional inversion of static-shifted magnetotelluric data. Earth Planet and Space, 56, 239-248.

Song, Y. (2003) Overview on geothermal development in Pohang. Proceedings, 2003 Korea-Japan Joint Seminar on Geophysical Techniques for Geothermal Exploration and Subsurface Imaging, 17-22.

Uchida, T., Song, Y., Lee, T. J., Mitsuhata, Y., Lee, S. K. and Lim, S-K. (2004) Noise removal and 3D interpretation of magnetotelluric data in the Pohang low-enthalpy geothermal area, Korea. Proceeding for the 6th Asian geothermal symposium (this volume).

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INTERPRETATION OF 3-D MAGNETOTELLURIC DATA · PDF file140 mudstone. Lineaments from ... effect causes the split in apparent resistivities at low frequencies between xy- and yx- modes