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Such site surveys typically provide high vertical resolution, but lateral resolution may not be sufficient in the case of shallow gas trapped in narrow paleo scars and channels or sand pockets within debrites. Though the primary line spacing at the well location may be relatively small (50 m or 164 ft), it usually increases to 100 m (328 ft) or more away from the well. Traditional site surveys in shallow water typically are conducted using a grid of 2D high resolution seismic lines over a limited area, usually 1 sq km (0.39 sq mi).

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By eliminating the receiver and source ghost recordings in towed marine seismic, GeoStreamer technology delivers broadband seismic data free of ghost notches, which improves resolution of events and allows the reliable interpretation of shallow geohazards. This was eliminated by the advent of broadband marine seismic acquisition, introduced in recent years by PGS. The general rule of thumb is that the 3D seismic data should be recorded at 2 ms or smaller sample interval and the near offset acquired should be less than half the water depth.Īnother issue with utilizing 3D seismic data for shallow geohazards may be the position of the notch in the frequency spectrum, which is optimized for deeper targets. The 3D seismic acquisition geometry is optimized for much deeper targets and not as much of an issue there. In shallow water, multiples and refractions can be a problem. Essential criteria are the frequency bandwidth recorded and the near offsets relative to the water depth. Most modern exploration 3D seismic surveys are likely to be suitable as the basis for shallow geohazard analyses. The second reprocessing flow was focused on the near surface section, approximately 1.5 seconds TWT beneath the seafloor, and included generating data attributes such as amplitude, coherency, and angle-versus-offset (AVO) diagnostics to identify shallow gas.

The first was designed to accurately image the seabed, yielding high resolution data at a spatial sampling of 6.25 m x 12.5 m (20.6 ft x 41 ft). Two independent reprocessing sequences were applied. Short offset data from an existing conventional 3D seismic survey was reprocessed through customized processing flows to yield high spatial and vertical resolution data suitable for accurate seabed mapping and near surface geohazard studies. Such a rendering immediately flags steep slope areas that can be problematic for the installation of a seabed drilling templateĮni Muara Bakau contracted PGS to undertake a study to better understand the regional seabed morphology with respect to potential geohazards to field development. In this case yellow corresponds to a 7º slope. (bottom) An illuminated Digital Terrain Model with the steepness of the slope color-coded. (top) An illuminated Digital Terrain Model with seabed amplitudes draped over, together with a profile of the seafloor. In addition, shallow gas accumulations are present. Debris flows originating at the shelf edge also may pose a risk. Seafloor stability is an issue as evidenced by seabed slumping. The steep shelf edge leads into a generally eroded and canyonized slope. Water depths vary from 70 m (230 ft) on the shelf to more than 1,000 m (3,281 ft) at the east edge of the survey.

Production Sharing Contract (PSC) block is on the East Kalimantan shelf edge and the west slope of the Makassar straits, Indonesia.

Petroleum Geo-Services (PGS) highlights one case study for operator Eni in Indonesia. In deepwater, modern 3D seismic surveys intended primarily for exploration and development objectives are proving valuable for investigation of seabed morphology and analysis of shallow sub-seabed geohazards. Eni An example from the Makassar Straits, Indonesia