||In most previous studies relating to the estimation of underground structure, the underground structure is assumed to be horizontally layered, which confines the estimation accuracy. Hence, we propose a method combining the merits of gravity anomaly and microtremor survey, which are usually used to identify the ground structure. In this method, at first we assume the gravity anomaly for 2-layer model to get the approximate thickness of the depth of the entire ground structure. Using the estimated depth, the results of microtremor survey is applied to specify the ratio of thickness of each layer. Finally, gravity anomaly is used again to obtain more realistic density and thickness of the site. The validity of the proposed method is examined using numerical examples. The conclusions drawn from this study are: (1) The accuracy of estimation for shallow region is higher than that for the deep region. (2) For the case of irregular bedrock, the error in the estimation increases with increasing depth and decreasing angle of bedrock. (3) It is difficult for the proposed method to provide enough accuracy for the region near abrupt change in the shape of layer interface. (4) The capability for identifying the shape of bedrock is three times of the distance between two observation sites. It means this method cannot identify the shape of bedrock less than this range. |
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