Eel

Key Takeaways

• Ghost notch problem and eel solution reflect the fundamental physical limitation of conventional surface-towed streamers in shallow water, where the sea surface acts as a perfect pressure reflector for upgoing seismic waves, creating a downgoing ghost wavefield that arrives at the hydrophone with opposite polarity and a time delay equal to twice the streamer tow depth divided by the water velocity, causing destructive interference at frequencies where the ghost delay equals half the signal period: for a streamer at 5-meter tow depth in 1,500 m/s water, the ghost notch falls at 150 Hz (1,500/2/5), severely limiting the high-frequency content of the recorded data and reducing vertical resolution; at 10-meter depth, the notch falls at 75 Hz; the eel suspended at 0.5 to 2 meters above the seabed in 20 meters of water places its hydrophones at approximately 18 to 19.5 meters depth, pushing the ghost notch to 750/(18 to 19.5) = 38 to 42 Hz, which is lower than the 75 to 150 Hz range needed for shallow high-resolution imaging; this counterintuitive result (the near-seabed eel has a lower ghost notch frequency than the near-surface streamer) means that the eel must be interpreted with ghost deconvolution processing rather than simply relying on ghost notch avoidance, but the higher signal-to-noise ratio near the seabed (less ambient sea noise and better coupling to the seabed-reflected upgoing wavefields) provides compensating benefits that make the eel system attractive for specific shallow high-resolution applications.
• Eel deployment mechanics require a mechanical system that holds the eel below the main streamer at a controlled height above the seafloor while the seismic vessel moves at survey speed (2 to 5 knots), with the eel's neutral buoyancy and drag properties balanced so that it maintains a stable position without excessive depth variation from vessel speed changes or seafloor topography: the eel is typically attached to the main streamer by a flexible tether at intervals of 12.5 to 25 meters, with the tether length controlling the height of the eel above the seafloor (the tether plus the main streamer depth above the seafloor determines the eel height); when the vessel passes over seafloor topography that brings the bottom closer to the streamer depth, the eel must rise with the topography to avoid seabed contact, requiring either a passive buoyancy design that allows the eel to rise on the tether as the seabed approaches or an active depth control system; in shallow water with flat seafloor, the eel height control is straightforward, but in areas with irregular seafloor (sand waves, buried channel edges, boulders) the eel may contact the seabed if the vessel does not reduce speed or the tether is not adjusted to provide more clearance; the recording bandwidth of the eel system is limited to the hydrophone and preamplifier specifications of the eel cable (typically optimized for higher frequencies than the main streamer), with the data merged and processed together with the main streamer data to provide a complementary shallow and deeper subsurface image in a single survey pass.
• Eel system data integration with the main streamer acquisition requires careful attention to the depth of each hydrophone element in both the eel and the main streamer, the timing synchronization between eel and streamer recording channels, and the different noise environments experienced by the two systems (the eel is subject to seabed coupling noise and current noise near the bottom, while the main streamer is subject to sea surface noise and flow noise from the turbulent boundary layer around the cable): the integrated eel-streamer dataset is processed with the eel data serving as the shallow-target receiver record and the main streamer data serving as the deeper-target record, with the two datasets combined in the processing workflow through amplitude scaling, dip filtering to separate upgoing and downgoing wavefields, and joint deconvolution to remove the different ghost functions of each receiver system; the integration also allows the shallow high-resolution data from the eel to be merged with the deeper penetration data from the main streamer, producing a single seismic image that characterizes both the shallow hazards (gas pockets, shallow water flows, mass-transport deposits) and the deeper geological structure of interest for development planning in one survey operation; the quality control for the integrated dataset includes comparing the amplitude and frequency content of the same reflectors in both the eel and main streamer data (they should be consistent within the expected ghost and noise corrections applied in processing).
• Archaeological and environmental applications of eel systems leverage the high-resolution shallow imaging capability to detect buried cultural features (ancient coastal settlements, shipwrecks, and artifacts) and environmental hazards (buried cables, pipelines, and contaminated sediment layers) that lie within 1 to 10 meters of the seafloor at depths too shallow for conventional seismic and too deep for direct visual or diver observation without extensive search: the vertical resolution of eel seismic data (typically 0.5 to 2 meters for dominant frequencies of 200 to 1,000 Hz achievable near the seabed) approaches the scale of individual archaeological features in coastal and near-shore environments, making it possible to detect and map buried stone wall structures, floor surfaces, organic deposit layers, and other archaeological indicators without excavation; in marine environmental assessment for offshore infrastructure installation (cables, pipelines, turbine foundations), eel surveys provide detailed characterization of the upper sediment column (including the presence of shallow gas, fluid migration pathways, peat layers, shell beds, and other features that affect foundation design and installation risk) that cannot be obtained from conventional higher-altitude streamer data; in UXO (unexploded ordnance) surveys for pre-installation clearance of former WWII combat zones, eel data combined with magnetometry provides the most comprehensive detection capability for both metallic and non-metallic buried ordnance.
• Eel versus ocean bottom cable (OBC) and ocean bottom node (OBN) comparison for near-seabed recording shows that eel systems provide higher operational efficiency (continuous acquisition during vessel transit) while OBC and OBN systems provide better coupling and repeatability (the receiver is in contact with the seabed rather than floating above it), with the choice between the two approaches depending on the area of coverage required, the seafloor type, the required resolution, and the project budget: OBC systems that lay a cable directly on the seabed provide better coupling to the seabed's elastic wavefield (including shear waves that are not transmitted through the water column and not recorded by hydrophone-only streamers or eels), allowing recording of converted PS waves that give information about rock and fluid properties not captured by PP reflection data alone; the eel's advantage over OBC is the absence of cable laying operations (which require a separate cable lay vessel and significantly more time than a streamer acquisition pass), making the eel system cost-effective for large-area reconnaissance surveys where the site-specific quality of OBC data is not required; in autonomous ocean bottom node surveys, the nodes are individually deployed and recovered by ROV, providing the deepest water and highest-repeatability 4D seismic capability but at the highest cost per square kilometer surveyed, while eel systems provide a cost-intermediate option between conventional streamers and node-based surveys for specific high-resolution applications.