The authors applied time-term method in a seismic refraction survey to determine the variation of the<br>thickness of a gravel layer overlaying a weathered granite basement in a 100 x 400m area. The reason why we used<br>time-term method in this survey was that the ground surface at the site was covered with boulders as large as 1<br>meter in diameter, it was difficult to apply conventional refraction survey in which sources and receivers have to be<br>in a line. Before doing field measurements, it seemed that the velocity structure of the site satisfied all<br>requirements for the applicability of time-term method. However, when analyzing observed seismic refraction data,<br>we found that the structure was not satisfied one of the requirements that the basement had no lateral variation in<br>velocity. To determine the variation of basement velocities as well as the distribution of the thickness of gravel<br>layer, we incorporated a tomographic technique into a conventional method and successfully applied it to the survey<br>data.


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