Refraction Interferometry and Free-surface Multiples for Large-offset OBS Data


We present successful results for applying refraction interferometry to ocean bottom seismic (OBS) data. The OBS data were recorded by six inline OBS stations with a 15 km station spacing and a source ship shooting every 150 meters; the maximum source offset from an OBS station is 181 km. Each virtual trace was created by correlating and stacking up to 150 trace pairs to create a super-virtual refraction arrival having a theoretical signal-to-noise ratio (SNR) enhancement of up to 12:1. Free-surface refraction multiples were also utilized to enhance the SNR of primary refractions and virtually double the number of OBS sites from six to twelve stations. Results show the successful reconstruction of far-offset traces out to a source-receiver offset of 120 km. The super-virtual traces increase the number of pickable first arrivals from approximately 1,600 to more than 3,100 for a subset of the OBS data where the source is only on one side of the recording stations. In addition, the head waves associated with the first-order free-surface multiples allow for the creation of six new common receiver gathers (CRGs) recorded at virtual OBS stations, located about 5 km from any of the actual OBS stations. These new traces double the number of OBS stations compared to the original survey and increases the number of pickable traces from 1600 to 6200. In summary, our results with this OBS data set demonstrate that refraction interferometry can more than triple the number of pickable first arrivals in long-offset traces. This suggests the possibility that the number of independent OBS stations can be increased by N-fold if the free-surface refraction multiples up to the Nth-order are of high SNR and if the recording time is appropriately lengthened.

AGU Fall Meeting Abstracts