An Electrostratigraphic Study of the Formations in the Coastal Area of Yunlin Hsien, Westcentral Taiwan

Abstract

A geoelectrical survey was conducted in the coastal area of Yunlin Hsien, westcentral Taiwan, for the study of electrostratigraphy as well as seawater intrusion. The direct current resistivity method was used, and forty-six vertical electrical soundings with Schlumberger array were carried out in the study area. The sounding data were interpreted using the 1-D inversion method. The results indicate that the shallow part of the study area can be divided into three electrostratigraphic units. They are designatd, from top to bottom, the A, B and C formations.

The A-formation is a combination of thin layers of medium resistivity and is correlated with the layers of soil and fine sand on the top.

The B-formation is characterized by a thick layer or layers of low resistivity and is correlated with the layers of clay, mud and fine sand with saline groundwater.

The C-formation is characterized by high resistivity and is correlated with the layers of pebble, sand and clay with fresh groundwater.

The parameters in Archie's equation are evaluated with the resistivity interpreted from the VES data, and the results are a=0.858 and m=l.367 . The critical resistivities of salt}' strata are also evaluated.

There is seawater intrusion in the study area, but it is locally distributed within a small area and is near to or on the surface of the ground. The intrusion was caused by the flooding of seawater during typhoons, storms and storm surges. The main saline groundwater body is the formation water in the B-formation, which is not caused by seawater intrusion but is the connate water in the stratum sedimented in marine environments.

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