Single-station attenuation measurements of high-frequency Rayleigh waves in Scotland

Abstract

Summary. Amplitude spectra derived from fundamental-mode Rayleigh waves generated by small underground and underwater explosions in Scotland are analysed using a technique for measuring attenuation for a single sourcestation path. The procedure estimates the average Qi' profile as a function of depth along an isoleted propagation path by correcting for known effects and mapping the function representing the remaining phenomena onto the corresponding data. Two methods of inversion are employed, both giving similar results. The technique is applied to explosion data recorded on the LISPB and LOWNET arrays in Scotland with epicentral distances of up to 85 km. This yields well-resolved single-station Qj' values of between 0.02 and 0.09 in the top 400 m of the crustal area traversed by the waves. The values at greater depth are weakly constrained but the Qj' estimates imply a decrease to <0.01 between 400 m and 800 m depth, followed by an increase to 0.04 below 800 m. The underground explosion spectra are modelled most accurately by a source-time action in the form of an exponential step, with an inverse timeconstant of 4s-' being optimum. The mean source strength value of 1012Nm agrees with a theoretical prediction based on the explosive yield. A model consisting of a shock wave and ensuing negative pressure of 0.8 s duration is optimal for the underwater explosion data. The mean source-strength value is 107Nm, in close agreement with the theoretical prediction using known pressure data. In both cases, attenuation coefficients calculated using the Qj' models estimated from this analysis are consistent with the coefficients previously obtained in Scotland, using surface waves, for pure geological provinces.