Beachrock, a type of sedimentary bedrock composed of beach deposits, is expected to restrain coastal erosion by preventing the outflow of sand. To artificially create beachrocks, we need data on the chemical and physical properties of this rock. In this study, we conducted a Direct Current (DC) electrical survey and a seismic surface wave survey to investigate the underground structures of the beachrock in Yagaji Island, Okinawa, Japan. In order to estimate the effectiveness of the geophysical surveys, we conducted laboratory tests to measure the resistivity, the shear wave velocity, and the porosity of beachrock samples collected from the study site. On the basis of these results, we estimated the beachrock structure as follows: The beachrock is detected as a high resistivity and high velocity zone, which is 1–2m thick. A sand layer under the beachrock is represented as a low resistivity and low velocity zone.

1 Introduction

Sea-level rises, triggered by global heating, cause coastal erosion owing to the outflow of sand across the world. Concrete armor units have been used as a revetment for preventing coastal erosion, but this method has the drawback that it affects the environment and landscape adversely. In addition, the continuing decline in the expenditure on Japanese public works projects means that new methods of bank protection are required to minimize the maintenance and operation cost of such projects. For the above reasons, we studied the formation process of beachrocks to develop a new method of bank protection that has the ability to repair itself.

Beachrock is a type of sedimentary formation that generally occurs in tropical and subtropical beaches as result of intertidal lithification of loose beach sands and gravels by carbonate cementation (Ginsburg 1953) (Fig. 1). The cement materials of beachrocks are mainly high-magnesium calcite and aragonite. The most widely held theory of is that the deposition of CaCO3 between coral sand and gravel by evaporation of seawater leads to the formation of bedrocks (Ginsburg 1953). Bedrocks are formed over a long period of time, but it has been reported that the formation periods of some beachrocks were only a few decades (Danjo & Kawasaki 2012). Many studies, both in Japan and in other countries, have reported on beachrocks, but the majority of these studies focused on their geochemistry. In particular, the underground structures of beachrocks are not yet well understood. One of the few studies of this aspect focused on the application of electrical resistivity tomography to map the beachrocks in Thassos Island, Greece, by Psomiadis et al. (2009). In spite of the general claim that it is absolutely imperative to comprehend beachrocks' underground structures to elucidate its formation process, existing research has been restricted to areas around the surface of seashores out of consideration for avoiding damage to the landscape from boring exploration. In this study, we conducted a seismic surface wave survey and a DC electrical survey in Yagaji Island, Okinawa, Japan.

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