Inversion of explosive source land seismic data to determine source signature parameters

다국어 초록 (Multilingual Abstract)

In seismic land exploration, where terrain conditions limit the use of vibrator sources, explosive sources are the preferred choice, and in situations where low‐frequency energy is essential, explosive is the preferred source, because it contains energy down to DC (0 Hz). I present a new method to estimate the source time functions of explosion source seismic data, using a modified acquisition method, data processing, modelling and inversion. Two shots of different sizes are fired into the same geophone spread with source–receiver geometry arranged such that the Green's functions are essentially the same. The spectral ratio of corresponding seismic traces is then the spectral ratio of the source time functions. A corresponding synthetic ratio filter is calculated from Blake's explosion source model for the sources within each pair. Each modelled source is defined by five parameters: the minimum radius a of the elastic zone, the internal pressure p0 at a, the density of the rock ρ and two elastic constants for an isotropic rock, the P‐wave velocity c and Poisson's ratio σ. A grid search finds the source parameters that minimise the difference between the measured and synthetic filters for each source pair, thus yielding the two source time functions, but not their absolute amplitudes. Deconvolution of the shot records within each pair for the minimum‐phase source time functions recovers the estimated earth impulse response gathers, convolved only with the response of the recording system. The improved knowledge of the source signature is then helpful in subsequent processing, especially for reverse time migration, full waveform inversion and amplitude‐versus‐offset analysis. I consider shots at different depths in the same shot hole and shots at the same depth in different shot holes. The former configuration minimises differences in geophone coupling, and, in operations, quality control procedures must be met. There is considerable variation in recovered source parameters from place to place, caused by variations in charge size, soil or rock properties and source coupling.