“Optimum offset” seismic reflection mapping of shallow aquifers near Bangkok, Thailand

Abstract

The “optimum offset” shallow seismic reflection technique was developed for geotechnical and groundwater applications in the early 1980s. The method was based on equipment that was “state of the art” at the time: a 10-bit engineering seismograph, a simple inhole shotgun source, and Apple II personal computers for data processing. In 1985 and 1986, this technique was successfully used in Thailand to assist in understanding the land subsidence problem around Bangkok, which is directly attributable to excessive groundwater withdrawal. The seismic sections acquired in this study were used to map subsurface structure to a depth of about 180 m, which includes several major aquifers beneath the Bangkok Clay. Much of Bangkok’s domestic water is pumped from the so‐called “second aquifer,” 80–130 m below surface, which proved to be an excellent seismic marker horizon showing lateral continuity over distances on the order of kilometers. This observation suggests that there may be good hydraulic connection within this aquifer over large areas. While the technique and equipment used in this study are dated now, the results provide a valuable demonstration of the application of inexpensive shallow reflection methods to a major engineering and groundwater problem that is of concern in many urban centers around the world. It is presented here for its societal relevance in this regard and for its historical aspect as perhaps the first application of shallow seismic reflection profiling in Southeast Asia.