The High Resolution Measurement of P and S Velocity
Abstract
Seismic explorations, which give seismic velocity, such as seismic refraction method and the down hole - PS logging, are generally applied to the large - scale area. Typically, at these seismic explorations, the receivers spacing ranges from 1.0m to 20.0m and resolution which means a minimal area required to determine seismic velocity is 10 to 50m depending of the receivers spacing. On the other hand, recently, seismic exploration to the smaller area has been applied with increasing frequency. For the large-scale constructions which require severe safety, such as the power station, dam, tunnels, bridges, the rock physical properties in wide area of several hundred meter square, are necessary in order to assess the safety when those are built and an earthquake comes. However, field tests which give the physical properties are almost applied to the area of around 1 m square. In this case, the issue exists whether or not the small field test area is representative of the whole rock property in the site. For this issue, seismic explorations to the small area are adopted for the purpose of the comparison between seismic velocity in the field test area and in the whole site area. It is generally recognized that the accuracy of seismic velocity decrease with decreasing seismic measurement length and number of receivers. To achieve high accuracy with the seismic exploration to the smaller area, we should adjust the spacing closer between the receivers compared to the spacing used by the existing method, and increase the number of receivers. And also, by doing this, we can increase the resolution of velocity results. At first, before the investigation, we calculated the errors of velocity caused by picking error of the arrival time from slope of a straight line using the linear least squares method, based on the Theory of Errors. This method shows that we should use the high frequency seismic wave in order to achieve the increasing the accuracy with the short seismic velocity measurement method. Because utilizing of high frequency wave provides the decrease of the arrival time- picking error. In order to use high frequency wave and measure those data, we selected the suitable equipment, such as small 3-axis accelerometers, the high time-resolution data logger, and the high frequency seismic source. We named this measurement and analysis system the short length seismic velocity measurement method. And then, we calculated the error of velocity results using new measuring system at an actual field. These velocities are by in error by less than 5%. This fact proves that we could measure seismic velocity with high accuracy, although the measurement length was short. The short length seismic velocity measurement method is very useful to estimate the detailed physical properties of rocks in construction fields. It is necessary for this method to estimate error of the calculated velocity. However, in the past, the estimation of the error has not been conducted sufficiently. In this paper, we demonstrated from the main three points of view for accuracy in the short length seismic velocity measurements: (1) estimating the errors before an investigation, (2) selecting the optimum measurement system, and (3) assessing the errors of the velocity results. And we show the case study of the short length seismic velocity measurement method.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMNS31A1654K
- Keywords:
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- 0935 EXPLORATION GEOPHYSICS Seismic methods;
- 5100 PHYSICAL PROPERTIES OF ROCKS