Where in the earth are the low frequencies?: comparison of sources at Hussar
David C. Henley
Recent increased interest in full-waveform inversion of seismic reflection data has motivated attempts to broaden the spectrum of reflections recorded at the earth’s surface. Of particular importance is the low frequency portion of the spectrum, which contributes significantly to the ‘character’ of reflections and to the ability to tie seismic data to well logs. An obvious way to boost the low-frequency spectrum is to increase the amount of low frequency source energy penetrating the earth during seismic acquisition, and to employ sensors optimized for detecting low frequencies. The objective of the Hussar experiment, performed by CREWES and some of its sponsor companies in 2011, was to test acquisition configurations designed to enhance low-frequency energy content of seismic surveys. Early results for these data compared the low-frequency spectra of the various sources on unprocessed trace gathers or on fully processed stack images. Here, in a different approach, we compare two low-frequency-optimized surface sources (Vibroseis) with buried dynamite on single source gathers from two different arrays of sensors. By separating the coherent surface-wave noise from the source gathers, we show that a larger proportion of the low-frequency source energy for the two surface sources appears as surface-wave energy, compared to dynamite. Single-fold dynamite data demonstrate both higher S/N and a broader reflection spectrum than corresponding Vibroseis data. Subsequent processing, including high-fold stacking and migration may diminish these differences, but it appears that dynamite is more effective in propagating low frequency energy into useful reflections than the surface sources tested, much of whose low-frequency output seems to preferentially excite surface waves.