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The effect of sea swell on seismic data

Marine seismic surveys are often run in rough sea conditions. While this issue is not necessarily regarded as significant for single surveys, it is more important to quantify these effects for use in time-lapse differencing for monitoring of hydrocarbon reservoirs. Sea swell causes the vertical displacement of both the seismic source and the receivers (mainly the receivers). There is also the consideration of the ghost signals that reflect off the water surface above the source and the receivers that can be displaced relative to the primary signal. These displacements can affect the seismic signals so that there is a significant mismatch in the data from different surveys. A synthetic survey was made on a physical modelling system over a horizontal acrylic block immersed in water in a tank. A shot record was collected using piezoelectric transducers as the source and receiver containing 36 channels. From this, 144 shot gathers were generated to form a 2-D line with two planar seismic horizons. Using this as the master dataset many sub samples were generated, each of which had varying degrees of swell statics introduced. Each sub set was differenced with the master 2-D line to generate difference 2-D sections showing the effects of swell. The statistics analysed for each sub set were the maximum positive, negative and root mean square (RMS) trace amplitude values. Results indicate that there is a significant effect on the data with a swell of over 2-3 metres. The maximum effects of sea swell appear toward the sea wave amplitude consistent with approximately half the seismic wavelength of 100 Hz showing evidence of "notching" at this frequency. Thus a moderate swell can have a significant effect on data to be differenced in time-lapse studies.