Geological structure of beachrock by geophysical exploration: application to Yagaji Island in Okinawa, Japan

Creating artificial rock with self-repairing ability is expected to become a new method of bank protection in coastal areas. Toward one such method, beachrock, a type of sedimentary bedrock composed of beach deposits, is studied from the viewpoint that the structural material is a natural one that can tolerate climatic conditions. To artificially create beachrock, data on the chemical and physical properties of this rock are necessary. In this study, a direct current (DC) electrical survey and a seismic surface-wave survey were conducted to investigate the underground structures of the beachrock on Yagaji Island, Okinawa, Japan. In addition, the resistivity sections were converted into porosity sections by Archie’s law. In order to estimate the effectiveness of the geophysical surveys, laboratory tests were conducted to measure the resistivity, shear-wave velocity, and porosity of the beachrock sample collected from the study site. As a result, the beachrock structure is estimated as follows: the beachrock is detected as a zone of high resistivity, high velocity and low porosity, which is 1 m thick, increasing toward the coastline: and the sand layer under the beachrock is represented as a zone of low resistivity and low velocity. This paper describes the features of beachrock in the study area and discusses the knowledge obtained from this study of geophysical prospecting for beachrock.