Field testing of multiple simultaneous vibrator sources controlled by filtered m-sequences
Joe Wong, David Langton
Theoretical analyses and numerical simulations predict that shifted m-sequences are suitable quasi-orthogonal pilot signals for driving multiple land vibrators in high-efficiency simultaneous source acquisition.
We conducted field tests that provided experimental confirmation for the theoretical predictions. Shifted m-sequences modified by a realizable time-domain filter were used successfully to control hydraulically-powered land vibrators. In one filed test, blended raw field data recorded with two vibrators driven simultaneously by two quasi-orthogonal filtered m-sequences were easily separated by crosscorrelation into individual common source gathers with little crosstalk.
However, in a second test involving four simultaneous vibrators driven by four quasiorthogonal filtered m-sequences, the separation of blended raw data by crosscorrelation into ordinary common source gathers resulted in a high level of crosstalk. The crosstalk comes from strong surface-wave arrivals generated by adjacent and nearby vibrators.
Numerical simulations indicate that the performance of filtered m-sequence pilots used in simultaneous multi-sourcing is improved if the filtering process does not alter very much the spectra of the original pure m-sequences in the frequency range of 0 to 250Hz. For such cases, the ability to isolate very weak signals from very strong signals without crosstalk interference is retained.
A useful figure of merit for a quasiorthogonal set is the ratio (expressed in decibels) of the maximum absolute value of the crosscorrelations to the autocorrelation peaks. Any set of filtered m-sequence pilots with a figure of merit less than -65dB would be suitable for simultaneous multi-sourcing.