Estimating the Firing Depth of Underground Explosions

Abstract

THE usual way of determining the depth of focus of an earthquake is to measure the difference, τ, between the arrival time of the direct P wave and the arrival time of the surface reflexions pP and sP. pP and sP are the signals that travel upward from the focus as P and S waves respectively are reflected at the free surface above the focus (with conversion of some S energy to P) and then travel steeply downwards (as P waves) passing close to the focus and following a path to the receiving station similar to that of P. Thus τ depends on the difference in path length between P and the surface reflexions (and hence on the depth of focus) and on the velocity structure in the crust and upper mantle above the focus. The depth of focus can be estimated because the velocity structure of the upper mantle is known. This method is difficult to use for determining the depth of firing of underground explosions using P signals with frequencies around 1 Hz: τ is small (often 1 s or less) and the effect of the recording instrument, and of absorption in the Earth's mantle, is to smear out P and pP (sP is not generated by a pure explosion source) so that the two arrivals usually overlap. Here we describe a method of applying corrections for absorption and recording instrument effects to the P signals recorded from explosions so that P and pP can be more easily identified as separate pulses and their difference in arrival time measured. Then for explosion signals recorded at long range where d is the depth of firing and v the velocity in the overburden.