SEISMIC MAPPING OF UNDERGROUND CAVITIES USING REFLECTION AMPLITUDES

Abstract

Underground cavities (and similar marked lateral changes in seismic impedance) having dimensions of tens to hundreds of meters should markedly reflect artificial seismic waves and locally obstruct reflections from deeper horizons. This should be especially true for liquid‐filled cavities if shear waves are employed. Attempts have been made to apply these principles in delineating solution‐mined cavities in bedded salt. In one seismically reverberant area, results from a brine‐filled cavity 300 m below the surface were unclear. Strong seismic “shadows” were found, however, in a seismic reflection survey of solution cavities 500 m below the surface, in a seismically favorable area. At present, the method requires good reflecting horizons both above and below the stratum to be explored. The amplitude ratio of the two reflections is used to represent the seismic “opacity” of the stratum between, normalized for the effect of varying seismic efficiencies at the shots and detectors. Among other applications, the method should be useful in mapping natural caverns and nuclear‐blast cavities.