Fault location by channel wave seismology in United Kingdom coal seams

Abstract

About 80 percent of British deep mined coal is obtained from advance longwall faces. This mining method is highly capital intensive. Therefore, it is important to know the geologic structure prior to mining. Small faults can have a large effect on production rates. Channel wave seismology offers a possible method for locating these faults. A coal seam resembles a dispersive acoustic duct, and energy from a seismic source within the seam can be coupled into channel wave modes. Scattered waves from defects within the seam can be recorded and the data analyzed to produce a map of the seam. It has been shown that dispersion does not present a fundamental limitation to the resolution that can be obtained. In addition, broadband numerical holographic mapping procedures in the form of lag sums can be implemented. These techniques have been described previously by Mason et al. (1980a). This paper is divided into two main parts. First, we show how the basic lag‐sum techniques have been extended to produce an adaptive‐lag‐sum (ALS) mapping procedure. Second, we describe a variant to standard common‐depth‐point (CDP) stacking. This variation, called dynamic trace gathering (DTG), stacks only those traces with similar reflection points. Both ALS and DTG are extremely flexible. Targets at large angles to the geophone line may be imaged. Both processes can be used to analyze transmission and reflection seismograms, with or without mode conversion at any target. Dispersive propagation may also be included. The techniques are illustrated using data from underground field trials at three collieries.