24. Experimental Study on Generation and Propagation of S-Waves : 6 S-Wave Prospecting by Means of Well Shooting
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概要
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The principle of the well shooting method, which is quite popular in reflection prospecting for oil, is used to obtain S-information. Seismic sources are generated by hitting a wooden plate horizontally. The main purpose of this study is to determine how closely the calculations of the interval velocity, within the shortest possible lateral distance from the borehole, can give an accurate interpretation of the underground structure. Travel times of waves travelling from a source point 5 meters away from the borehole, to a seismometer placed at a depth of 3 meters in the borehole, were recorded. The procedure was repeated, moving the borehole seismometer downwards every 3 meters, to a depth of 24 meters. These travel times were corrected to vertical travel times and the interval velocities were calculated. Using this velocity distribution with respect to depth, travel times from every surface source point to the seismometer placed at the different depths were calculated and were compared with the observations. Finally, with slight modification of the model in the places away from the borehole, we could determine the underground structure in situ. Therefore, S-prospecting can be carried out conveniently by means of the well shooting method using SH-source. This method is easier and faster than any other conventional method. Other merits are as follows: the first is that it can be applied in a limited area. The second is that it can be used for the detection of low velocity layers at depths, which are not easily avail able from the conventional method. From the vertical hitting, later phases appear on the seismograms. There is quite a similarity between these interval velocities and those of SH-waves. Further studies of such phases are necessary to see if we could obtain S-information from the well shooting method by hitting the ground vertically.|S波による小規模な地震探査が可能になり,軟弱地盤を構成する種々の地層内を伝わるS波速度に関する情報も次第に数をましてきた.いまゝでに開発されてきた探査法は,板叩き法(SH震源)と,ボーリング孔を利用する火薬震源操作法(SV震源)の2つに大別される.これ等の方法では,地盤深部におけるS波の情報を得たい場合,地表に長い側線をとって観測しなければならない.これは都会地などでこの種調査を実施するためには致命的な欠点である.そこでこれ等の方法にとってかわる,場所をとらない,かつ容易な探査法の開発がのぞまれる.これは,S波のdataをふやす意味でも大切なことである.今回上記両法の長所を生かした板叩き震源によるWell Shootingを実施する機会を得たので,これのみによって得られた地下構造が,他の地上展開による観測結果をどの程度までうまく説明出来るかをしらべることによりWell Shootingの有用性を調べた.この結果,板叩き法によるWell Shootingが充分に実用に供しうることがわかった.この方法の長所は,大変簡単であること,場所をとらないこと,低速度層の検出が容易であることなどである.
- 1968-07-25
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