Regional 3-D imaging of resistivity structure using GEMIT inversion

Mv. Space Rev, Vol. 13, No. 11, pp. (1 1)51—(11)54, 1993 Printed in Great Britain. All rights reserved.

0273—1177193 $6.00 + 0.00 Copyright © 1993 COSPAR

REGIONAL 3-D IMAGING OF RESISTIVITY STRUCTURE USING GEMIT INVERSION Sachio Nabetani Department ofEarth Sciences, Hirosakj University, Hirosaki, Aomori 036, Japan

AB ST RACT The analysis of LANDSAT imageries has been studied in association of geodetic, gravitational, geomagnetic and electromagnetic structures. Those physical parameters however differ from each other in observational range and resolution. GEMIT (Geo-electromagnetic Induction Tomography) is one of the methods to visualize a conductivity structure by inversion of electromagnetic induced signals to a prefered conductor as an imagery pattern under the ground. Several features of volcanoes, geological faults, or sei smic zones detected by satellite imagery analysis were examined from geophysi cal prospecting data. This report concerns to a practical case analyzed in the northern part of Japan, where the above mentioned topographic features were seen as extended to the conductivity structures at the deeper levels in the crust. INTRODUCTION The geophysical st ruct ure of the nort beast em region of Japanese Island Arc is one of the most active orogenet ic zone of the continent al margin around the Northern Pacific Ocean generated from a dynamic interaction between the Eurasian and the Pacific Plates. The area analyzed in this paper is featured as arrangement of several volcanic systems and active earthquakes foci in the geological periods after Paleozoic through Tertiary or Quaternary. Underground resources as like as iron, copper, hydrocarbon, geothermal fluids, etc. are extensively distributed in association with the above mentioned orogenetic systems. Geological features were created by Incessant movement of the continental margin with stresses from the Pacific Plate, when it submerged under the crust, and accomplished an arrangement of geological and geophysical structure from north to south in this region. Landsat imageries were analyzed only for extracting physical lineaments which gave a good indication to the above purpose of exploration. Besides, to identify a lithoface or a depositional face from the imageries was rather difficult because of homogeneous weathering and vegitation on the groundsurface except for a higher altitude of mountains. AREAL VIEW AND PROSPECTING Area shown in Figure 1 is covered with a large (11)51

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Fig. 1. Synthetic imagery map /left/ from KS—hO /1/, and lineament map /right/ from Landsat analysis. R indicates a ring structure, CF conjugate fault, HK Mt. Hakkoda, 1W Mt. Iwaki, and OS Mt. Osore. over the details of geological structures. A topographic anomaly however preserves the history of the crustal movement or evolutionary processes. Topographic data of KS-lb /GSI was converted to a visual imagery map as shown in Figure 1 by simulating the sun height and longitude /1/, and very high compatibility was obtained. Lineamental structures were thus taken out from both imagery maps with reference to the geological structures. The geological structure presents a high correspondence to the satellite imagery as shown in TM image or MSS cross section of Figure 2. A large topographic anomaly appeared on the longer wave images is good indication of the crustal activities, and then extended to the interior of the ground. The conductivity structure is determined by MT/GEMIT method /2/ as Fig. 3, in which 3—D inversion of the resistivity was projected on the section of 40°40’E at the central zone of this area. The distribution of resistivity on a horizon of several depths or a vertical section must be a visualized image of the physical structure of the earth’s crust. VOLCANO AND RING STRUCTURE Ring structures marked as R in Figure 1 are mostly created from volcanic activities in the periods of Tertiary and Quaternary. Among the volcanic areas, Mts. Hakkoda in the central part is the most active through the Quaternary Period, and accompanies earlier or secondary volcanoes. The rings join fa,ults at the tangential localities without extending beyond the structure. Generation and development of a volcanic structure is both controled by the structural belts originated from subduction of the plate. Viewing from the underground, the above mentioned structures are most connected to discontinuous zones of conductors /Figure 3/. The geothermal fluids as underground resources are frequently reserved within the fractured zones associated with the above mentioned structures. Those reservoirs develop around the volcanic magma in saturation of hot water of 200-300°C supplied from magma or heated in circulation of meteoric water. The reservoir may be therefore found in a conductive formation out of a resistive hot rock. To know the conductivity distribution and beat-

Imaging ofResistivity Structure

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ion of structural pattern is a necessary tool for geothermal exploration. Magmatic pocket near the surface is originated from the deeper source at boundary between the crust and the mantle or from an anomalous portion in the crust. The distribution of conductors as seen in the crustal cross section of Figure 3 shows a connection or a separation of conductors from the subcrust to the groundsurface, where volcanic mass and structure develop certainly. Figure 3 may presents a new idea of crust-mantle interaction in the same time because of an inhomogeous distribution of structures. Conjugate pair of faults, CF in Figure 1 is also located in the vicinity of volcanic structure, and joints frequently a ring structure. Geophysical parameters, gravity /conductivity /deformation are symmetric at origin of quadrants /3/ as resulted from zonab compression and migration of the volcanotectonic front towards west or north.

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KM Fig. 3. Horizontal distribution of resistivity at depth of 10-12.5 km /upper/, and Its depth section /lower/ of EW component projected on a traverse of 40°40’N. Resistivity in unit of dB scale to 1 rn/S is high at H, and low at L. REFERENCES 1. Y. Fukuda, S. Nabetani, H. Watanabe and M. Asada, Application of National Numerical Information (KS-lbO) to Landsat data processing, J.Geod Soc. Japan 27, 204—213 (1981). 2. S. Nabetani, Conductivity structure of crust and mantle in the northeastern Japan prospected by MT and GEMIT method, 1. Sci. Rep. Hir. Univ. 39, 37—46 (1992). 3. S. Nabetani and Y. Fukuda, Preliminary report on the Landsat analysis of geotectonic characteristics in the northeastern region of Japan, Sci. Rep. Hir. Univ. 28, 54—69 (1981).