Detecting Underground Cavities Using Seismic Ambient Noise: The Finite-interval Spectral Power Method

Abstract:

Natural and man-made cavities pose a serious geotechnical hazard if they are undetected. It is therefore important for increasing human safety to develop methods for detecting and locating underground cavities mainly in urban development and civil construction.

The problem of detection and location of cavities has special importance in relation of underground nuclear explosions. Detection and location of a cavity generated by an underground nuclear explosion is an important proof in case of suspicion of violating the Comprehensive Nuclear-Test-Ban Treaty (CTBT), an international treaty banning nuclear weapon test explosion or any other nuclear explosion which is yet to come into force.

We present a new method for detecting and locating a horizontal position of cavity which uses the Finite-interval Spectral Power (FISP) of seismic ambient noise. The method makes it possible to use single-station measurements at a set of potentially irregularly distributed points in the area on the Earth’s free surface over a suspected cavity. Because the method gives better results for undistorted segments of noise records, we also present a method of automatic identification of such segments. We tested our method using records of noise from a site near the Felsőpetény, Hungary, which were collected for the CTBT Organisation during a field test in the framework of developing on-site inspection (OSI) capabilities. The method is ready for further tests in different cavity conditions and applications.