| Abstract: | An experiment was undertaken at BP's Fulbeck Geophysical test site to compare the viability of various simultaneous vibroseis recording techniques, which are often recommended as a means of improving data acquisition production rates for 3D seismic surveys. Of particular interest were: (a) the ability to separate the signals from each source during processing, (b) the generation and suppression of harmonics and (c) the effects of any source interaction. Two vibrators were deployed with a baseplate separation of 10 m, about a borehole containing a vertical array of geophones. Our analysis concentrated on the groundforce signals measured at each vibrator and the far-field signatures measured using a vertical geo-phone at a depth of 204 m. By comparing single vibrator records with similar but separated records from a simultaneous recording sequence, signal separability, harmonic suppression and vibrator interaction could be fully studied. Separated far-field signatures from simultaneous vibroseis methods using combinations of up and downsweeps exhibited unsuppressed harmonics and substantial energy from the undesired source which leaked through the correlation process. The ‘up/down’ method was capable of separating the signal from each source by only 12.7 dB, and is therefore unsuitable as a field technique. The variphase simultaneous vibroseis methods studied afforded some harmonic suppression and gave signal separations of about 30.0 dB. Use of variphase simultaneous vibroseis methods will compromise the quality of the data recorded, when compared with single-source acquisition methods. None of the simultaneous vibroseis methods tested provided adequate signal separation and, therefore, cannot be recommended as data acquisition techniques. The ‘alternate sweeping’ method coupled with multispread recording will give the desired improvement in data acquisition rates, while preserving the necessary quality of our seismic data. |
