Geoelectrical Surveys of Southwestern Taiwan

Abstract

Using the Transient Electromagnetic (TEM) method, the resistivity pattern of the southwestern Taiwan was mapped used as part of an integrated coastal environmental change study. A SIROTEM in loop configuration with square transmitting loops about 50 m on each side was used in this survey. More than 300 soundings, covering an area of about 100 km by 20 km, were collected between 1992 and 1995.

In addition to lD ridge regression inversions, the trend surface analysis technique was used to gain a quantitative interpretation. The results were organized into both depth slice and profile maps to study 3D environmental changes.

The regional resistivity trend is found to decrease southwestwards in the Coastal Plain. In the sediments, sand is similarly found to have gradually changed to clay along the SW direction. The two trends, in fact, seem to be well correlated. Moreover, the sediment transportation directions varied systematically from NS to WE during its deposition. On the other hand, the vertical variation patterns of the resistivities also disclosed paleosea level changes. Based on the resistivity profiles, the paleo-coastal lines progressed as far as 20 km land-ward from the present location once during 16 to 6 kyr. B. P. in the northern part of the survey area; during the same period, the southern region was still covered with seawater. All of these findings are supported by drillings as discussed in this paper.

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