Spatial filtering of marine seismic data

Extended source and receiver arrays have proved to be an effective tool for improving the data quality in marine seismic exploration. The extended arrays may be implemented in the field, or in a computer by the summation of traces with a common receiver coordinate or a common source coordinate, respectively. A tilted source or receiver array may be used to enhance reflectors with a specific dip. A tilted source array can be implemented in the field by delaying the pulses at the source subarrays, or in the computer by time‐shifting the traces before implementing the long source array. A tilted receiver array can approximately be implemented in the computer by time‐shifting the traces before implementing the long receiver array. In areas with complex geologic structure, the data can be corrected for normal moveout prior to implementing the extended arrays. The theoretical response of reflected waves from dipping reflectors for different extended array filters is given. Vertical and horizontal stacking constitute a spatial filter which is similar to an extended array filter. Vertical stacking with linear time shifts between the traces can be used to enhance reflectors with a specific dip. The theoretical response of reflected waves from dipping reflectors for different vertical and horizontal stacking filter is given. In order to discriminate against coherent noise travelling in the cross‐line direction, areal arrays must be used. The theoretical responses of three‐dimensional spatial filters are derived in the appendices. These responses are based on quadratic traveltime approximations for reflections in inhomogeneous layered media. A data example is presented which demonstrates the practical use of extended array filters, both implemented in the field and in the computer. From this example and others have come the following conclusions. In areas with strong coherent noise, a field‐implemented extended source array gives a signal‐to‐coherent noise improvement which cannot be obtained in data processing. In other areas, computer implementation of the extended arrays gives signal‐to‐coherent noise improvement as effectively as a field‐implemented extended source array. In such cases, the extended array filters should be implemented in the computer due to greater flexibility in testing on data and to the possibility of producing different stacked sections. Noise reduction is done more effectively by extended array filtering than by vertical and horizontal stacking prior to CDP stacking (weighted or unweighted full‐fold horizontal stacking).