On the ground surface, microtremors generated by various causes are existing, and those microtremors are affected by the structure and the characteristic of the ground. The microtremor survey method is the technique to grasp subsurface S-wave velocity structure by microtremors of the ground surface. The observation work of this exploration is only limited to observing the natural microtremor of ground surface by a few number of seismometers. No large scale instrument is necessary. These facts make the exploration method simple, easy and non-destructive. This exploration method can be applied for the depths of several meters to several thousand meters underground which is a very wide range. This exploration method has been used in deep part structure survey for earthquake disaster prevention as well as shallow ground survey. This lecture introduces some implementation case examples about applying microtremor survey method at alluvial ground, deep part structure survey and fault survey.
Geophysical exploration method uses the physical properties of ground in order to estimate the geological structure of ground. In the method of using elastic wave, refraction seismic survey or reflection seismic survey can be done. The kind of elastic wave to apply is body wave (Swave or P-wave) generated by an artificial vibration source and using many seismometers. Especially in the deep part seismic survey, in order to improve the S/N ratio, the hardware and software will be required at large scale and survey cost will become at large amount. In the microtremor of the ground surface, the surface wave is predominant compared to the body wave, which this surface wave is used in the microtremer survey method. The surface wave has dispersion characteristics (propagation velocity changes with frequency), which at low frequencies reflects the situation of the deep part of ground. As for the surface wave, seismometers are installed on the ground surface and microtremor will be observed. Then the dispersion characteristics of the surface wave will be extracted. The S-wave velocity structure can be obtained by inverse analysis of dispersion characteristics of surface wave. The advantages of microtremor survey method are as the followings
The observation instrument is small (there is no need to vibration source, there are a few simultaneous observation points)
Noise resistant (practicable in rural and urban areas)
Time period required for operation and analysis is short
In addition, from what can be obtained from the S-wave velocity structure, the estimation of ground's engineering elevation or dynamic characteristics is possible which indicates the extreme efficiency of the geophysical exploration method. Here, examples about seismic basement survey as well as active fault survey related to the estimation of engineering basement for alluvial ground, ground of Shinkansen line at the 2004 Niigata Chuetsu earthquake and the 2007 Niigata Chuetsu-oki earthquake are given.