Azimuthal Variation of Rg Energy From Quarry Blasts in Central Texas

We present the results of a study of short-period surface waves from chemical explosions at a limestone quarry 40 km west of Waco, Texas. Our purpose is threefold: (1) to obtain dispersion curves for the surface waves generated by the explosions, (2) to investigate the azimuthal variations of Rg wave (fundamental-mode, short-period Rayleigh wave) energy at azimuths around the quarry, and (3) to determine the quarry pit's influence on these energy variations. Observed dispersion curves were interpreted using theoretical dispersion curves that consider the estimated values of Poisson's ratio (0.35-0.4) for the upper crust of central Texas. The Rg dispersion essentially has three different sections: (1) an inversely dispersed branch complicated by the presence of higher modes (at periods less than 0.6 seconds), (2) a group velocity minimum (at periods between 0.6 and 0.7 seconds), and (3) normally dispersed Rg (at periods between 0.7 and 3.0 seconds, with an inflection point near 1.25 seconds). For periods between 0.7 and 1.5 seconds, multipathing complicates the interpretation of the dispersion curves by creating spectral holes in the results of the Multiple Filter Analysis (MFA), a single-station routine for calculating group-velocity dispersion. Spectral holes must be dealt with when interpreting the final dispersion curve, which is then used to extract Rg from the complex wave train by phase match filtering (PMF). Rg extracted from paths not traversing the open pit of the quarry have more energy than paths do that cross the open pit. Likewise, higher-mode energy exceeds fundamental-mode energy only when the propagation path includes the open pit. Implications for these observations are discussed.