Efficient surface wave method for investigation of the seabed
Takahiro KUMAGAI, Tomokazu Yanagibashi, Ayato Tsutsumi, Chisato Konishi, Kazuhiko Ueno
Abstract
In the design and construction of structures, information on the SPT N-values and the undrained shear strength is very important for investigating problems related to stability, deformation, etc. Since the shear wave velocity (S-wave velocity) is correlated with these properties, geophysical exploration methods, such as surface wave methods, which can estimate the properties densely over a wide area, are effective for creating better geotechnical designs and building better structures. In terms of investigating the seabed using a surface wave method, the method of arranging hydrophones on the seabed and dropping a weight from a boat to generate surface waves was proposed in previous studies. The present study proposes a new method using a distinctive weight-dropping source, which causes the weight to fall vertically with little resistance from the fluid region and to generate a desirable surface wave efficiently at the impact of landing. Underwater dropping tests were carried out at two field sites to find the desirable shape for the weight-dropping source. In a fundamental field test at a tideland, the effectiveness and applicability of the proposed weight-dropping method were demonstrated through a comparison with reliable on-land survey methods using geophones. Moreover, the accuracy of the proposed method was demonstrated through a practical field test conducted at sea by comparing the SPT N-values converted from the S-wave velocity, obtained by the proposed method, with those measured by standard penetration tests. Recognizing that calm conditions are required for the waves and the currents in order to apply the proposed method, the method is expected to be an effective tool for geotechnical investigations of the seabed, and to be applicable for creating better designs and constructing better coastal and marine structures.