Geophysical exploration in the Tatun volcanic region, Taiwan

Abstract

This is an introduction to geophysical exploration in the Tatun volcanic region in Taiwan. Two prospecting methods of geophysical investigation, magnetic and electrical resistivity survey, were applied at both the Tahuangtsui and Matsao geothermal areas from March 1967 to August 1968 The magnetic survey was used as a reconnaissance investigation to delineate the outline of the hydrothermally altered zone on the surface, while the electrical resistivity survey was used to find the thermal fluid bearing low-resistivity zone, and to understand the subsurface structure if the VES (vertical electric sounding) data could be interpreted rather quantitatively. Both magnetic and resistivity surveys shows interesting results. High magnetic anomalies are often associated with andesites and pyroclastic rocks, and low magnetic anomalies appear in the hydrothermally altered zone and the area of Tertiary sediments as expected. Also the magnetic contours are somewhat consistent with the topographic relief which is generally formed by the steep-sided andesite lava flows and volcanic domes in the surveyed area. The electrical resistivity survey at the Tahuangtsui area was carried out with two methods, horizontal exploration and vertical sounding. The VES data were interpreted semiquantitatively and the subsurface structure of the surveyed area was then inferred accordingly. In the Matsao area, the subsurface geological conditions were little known before the resistivity measurement began; therefore the resistivity survey in this area was used to get information about the subsurface geology and structure; hence the vertical sounding was applied primarily. The VES curves obtained show very complicated figures indicating the high complexity of subsurface geology and structural features. However, a qualitative interpretation was made. Evaluated from the VES data and the experimental result, the resistivities of various rocks in the surveyed area are as follows: 1. Andesite 1200–3000 ohm-meters 2. Pyroclastics (including tuff, tuff breccia, agglomerate etc.) 50–1000 ohm-meters 3. Miocene sediments 20–100 ohm-meters 4. Hydrothermally altered rocks and thermal fluid bearing formation < 20 ohm-meters