Abstract
In addition to de-spiking, gun delay correction, correction for spherical spreading and earth’s absorption, gun and cable correction, zero-phasing, noise attenuation and correction for time shifts between sail lines due to changes in velocity of the water column, multiple elimination is the other important true relative amplitude processing routine desired to produce seismic gathers consistent with DHI and AVO analysis to de-risk the presence of hydrocarbon interpreted from seismic data. Multiple problems associated with seismic data acquired in the Niger Delta offshore are those due mainly to the free air-water interface and the seabed, and their presence constitute noise in the seismic dataset. In this study, we employed an approach in which we convolved all possible source and receiver peglegs for every multiple event that strikes the free surface irrespective of its path in the subsurface to model the multiple wave field in a 3D sense, in a partially processed pre-migration seismic data acquired in the Niger Delta deep offshore. The method of adaptive subtraction was then used to eliminate the modeled multiples from the dataset. Un-like other demultiple techniques such as tau-p deconvolution and radon, our multiple modeling and subtraction techniques do not require prior knowledge of the subsurface geology in terms of the velocity and reflectivity of the multiple wave field. The aim of the study was to improve the overall quality of the seismic data and signal-to-noise ratio, in addition to the DHI and AVO compliant gathers output from the process. The approach was successful and effective in removal of the free, air-water surface multiples from the dataset.
Highlights
Multiple reflections are considered noise in seismic data [1]; they reduce signal-to-noise ratio, interfere identification of primary reflections and cause difficulty in velocity analysis, resulting in spurious seismic images, amplitudes and stratigraphic interpretation uncertainties [2]
Multiples can be classified as surface related multiples and internal multiples based on where the downward reflection of the raypath occurs [3,4]
Multiples which have all of their downward reflections below the free airwater surface are classified as internal multiples
Summary
Multiple reflections are considered noise in seismic data [1]; they reduce signal-to-noise ratio, interfere identification of primary reflections and cause difficulty in velocity analysis, resulting in spurious seismic images, amplitudes and stratigraphic interpretation uncertainties [2]. Sometimes referred to as free surface or surface multiples, are multiples that have at least one downward reflection at the free air-water surface in marine and offshore operations These multiples are due to the large negative reflection coefficient at the free air-water interface, arising from the large acoustic impedance contrast across the interface. They include reflections generated from the water-bottom and reverberations which impact marine and offshore seismic data. Multiples which have all of their downward reflections below the free airwater surface are classified as internal multiples These multiples strongly affect land data, and only a few techniques exist to eliminate them from the seismic record [5]. They get more attention in marine and offshore operations when imaging subsalt targets
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