2014 Volume 55 Issue 2 Pages 342-350
This paper describes the knowledge obtained from a study of geophysical prospecting for beachrock. Previous studies on beachrock worldwide have been focused on the geochemistry. However, more knowledge of the chemical and physical properties is needed to elucidate the formation mechanism. In the present study, a direct current (DC) electrical survey and a surface seismic survey were conducted to detect the underground structure of the beachrock on Yagaji Island, Okinawa, Japan. This was a first attempt at conducting multiple geophysical surveys to investigate beachrock. In each survey, one survey line was set perpendicular to the seashore and two survey lines were set roughly parallel to the seashore. The results of each survey were observed in section of resistivity and seismic wave velocity. Furthermore, in order to estimate the effectiveness of the surveys, laboratory tests were conducted on the beachrock samples collected from the study site to measure the porosity, the resistivity, and the velocities of primary- (P-) waves and secondary- (S-) waves. There was a superior correlation between the sections and with the data on the study site. Hence, the features of the beachrock at the site are as follows: the resistivity is about 4–16 Ωm, the S-wave velocity is about 325 m/s, the thickness is about 1 m, and the thickness has a tendency to become greater toward the sea. One beachrock formation mechanism obtained by this study is close a currently accepted mechanism.