CONTINUOUS CALIBRATION OF MARINE SEISMIC SOURCES*

The success of signature deconvolution in optimizing both signal‐to‐noise ratio and time resolution in the seismic section depends critically upon obtaining an accurate estimate of the far‐field source signature. Various deterministic estimation schemes have been proposed in recent years, most of which involve direct monitoring of source output within the water layer.

As an alternative to elaborate and error‐prone source monitoring schemes during data acquisition, a simple modification to any source array permits subsequent estimation of far‐field signatures directly from reflected signal. The new method requires the inclusion within any chosen source array of a simple point source, the “reference” source. Initial experiments employed a water gun as the reference source, characterized by a concise implosive signature with peak‐to‐peak amplitude of approximately 2 bar·m within the seismic sprectrum.

In operation the reference source is fired shortly before the main array (typically 2 s during initial trials) and the usual record length is extended by a similar amount. Each recorded trace then comprises two results: the subsurface response to the reference source signal followed by the response of the same subsurface to the main array. The disparities in source amplitudes and NMO differentials ensure that interference effects are negligible in the main recording.

Time‐ or frequency‐domain methods can be employed to extract the main array signature from the dual dataset or to invert this to some preferred wavelet simultaneously. As an additional benefit the reference source yields excellent high‐resolution profiles of the shallow geology.