Deep Tow
Deep tow in marine geophysics is a seismic acquisition method in which the seismic source (typically an air gun or sparker array) and/or the receiver (a hydrophone streamer or ocean bottom cable) are towed by a surface vessel at a water depth of tens to hundreds of meters below the sea surface rather than at the conventional near-surface tow depth of 5 to 15 meters, placing the equipment significantly closer to the seabed and to the shallow subsurface features that are the target of the survey; the primary advantage of deep tow is the reduction of the travel path between the source-receiver system and the shallow target, which reduces the two-way travel time and therefore increases the temporal and spatial resolution of the seismic images of shallow features such as pockmarks (seabed depressions caused by gas venting), shallow gas accumulations (bright spots and flat spots in the first 200 to 500 meters below the seabed), hydrate-related features (bottom-simulating reflectors, BSR, and gas hydrate stability zone boundaries), slope instability and mass transport deposits (submarine landslide headwalls, debris flow deposits), and pipeline burial depth and integrity (in high-resolution pipeline inspection surveys where the deep tow equipment is operated close to the seafloor to image the immediate pipe environment); deep tow is also used for ultra-high-resolution seismic profiling of the water column above the seabed (water column imaging for gas bubble plumes from seabed venting, fisheries habitat mapping, and oceanographic studies) where the near-bottom placement of the transducer array eliminates the geometric spreading loss that reduces resolution when the same measurement is made from the surface vessel.
Key Takeaways
- The resolution advantage of deep tow over surface tow for shallow subsurface imaging arises from two geometric effects: first, the shorter slant range (the distance from the source-receiver system to the target) reduces the spherical divergence energy loss (which increases with the square of the range), allowing the system to use lower-power sources (sparkers, boomers, chirp transducers) that emit higher frequencies than the large air gun arrays required for surface-tow surveys in deep water; second, placing the source and receiver near the seabed reduces the water column travel time contribution to the two-way travel time, allowing a shorter record length and finer time sampling to resolve the shallow features without being swamped by the surface multiples (reflections that bounce multiple times between the sea surface and the seabed, which arrive at the receiver after the target reflections for surface-tow surveys in deep water but are reduced or eliminated in deep-tow surveys where the water column above the system is minimal); typical resolution of a deep-tow sparker or chirp survey is 0.1 to 1.0 meter vertically and 1 to 5 meters horizontally, compared to 5 to 20 meters vertical resolution for a surface-tow air gun survey in the same water depth.
- Autonomous underwater vehicle (AUV) seismic acquisition is the modern implementation of the deep-tow concept, using battery-powered self-navigating vehicles programmed to follow prescribed survey lines at a constant altitude above the seabed (typically 20 to 100 meters) while towing a short hydrophone streamer or operating an integrated multi-channel seismic system; AUVs eliminate the surface tow vessel constraint that historically required the deep-tow vehicle to be connected to the ship by a long umbilical (which generated hydrodynamic drag, limited the survey speed to 1 to 2 knots, and introduced motion noise from the ship's heave and pitch that degraded data quality); AUV-based seismic surveys achieve better horizontal positioning accuracy than cable-towed deep-tow systems (because the AUV navigation system uses Doppler velocity log, inertial navigation, and acoustic positioning to maintain a precise track, while a cable-towed vehicle meanders unpredictably in the current), better depth control (because the AUV actively maintains its altitude above the seabed), and quieter acoustic performance (because the battery-powered thrusters generate less noise than the umbilical cable vibrations and surface vessel machinery noise transmitted down a tow cable); commercially deployed AUV seismic systems include Kongsberg Maritime's Hugin, OceanServer's Iver, and SeaBed Geosolutions (now Shearwater Geoservices) AUV systems used for shallow hazard surveys, pipeline inspection, and near-seafloor imaging in deepwater oil and gas fields.
- Bottom-simulating reflector (BSR) imaging using deep-tow seismic is the primary application for investigating gas hydrate stability zones in continental margin sediments: the BSR is a seismic reflector at the base of the gas hydrate stability zone (GHSZ) that parallels the seabed topography (hence "bottom-simulating") and is caused by the acoustic impedance contrast between hydrate-bearing sediments above (high velocity, high impedance) and free gas-charged sediments below (low velocity, low impedance); because the BSR is typically at 100 to 600 meters below the seabed in water depths of 500 to 2,000 meters, surface-tow seismic with 30 to 60-meter vertical resolution cannot reliably identify the detailed stratigraphy at the GHSZ base; deep-tow surveys with 0.5 to 2.0-meter vertical resolution can image the BSR and the thin free-gas zone below it in detail, providing data for hydrate volume estimation and for geohazard assessment of deepwater drilling sites where drilling through a BSR can release free gas from below the GHSZ, creating a potential blowout risk; the Integrated Ocean Drilling Program (IODP) and its predecessor ODP routinely used deep-tow seismic data to select hydrate drilling sites and to plan the drilling strategy for approaching the BSR from above to minimize geohazard risks.
- Seabed geohazard surveys for deepwater oil and gas infrastructure use deep-tow acoustic systems (sub-bottom profilers, side-scan sonar, and multibeam echosounders operated by AUV or towed at 20 to 100 meters above the seabed) to characterize the shallow geomechanical hazards that can affect seabed installations (manifolds, wellheads, templates, umbilical trenches, pipeline routes): shallow gas pockets (identified by acoustic turbidity and enhanced reflections in the first 50 to 200 meters below the seabed) can blow out during drill bit penetration or pipe driving operations; gas hydrates (identified by the BSR or by increased acoustic velocity in the hydrate zone) can decompose during pipeline operation if the line is warm enough to elevate sediment temperature above the hydrate stability boundary; slope instability (identified by headwall scarps, debris lobes, and chaotic seismic facies) can cause submarine landslides that damage seafloor infrastructure; the high resolution of deep-tow geohazard surveys (1 to 5 meters horizontal, 0.1 to 0.5 meters vertical) is essential for engineering foundation design and pipeline route selection because these hazards are typically too small and too shallow to be imaged adequately by the regional 3D seismic volumes acquired for subsurface exploration purposes.
- Ocean bottom node (OBN) acquisition and deep-tow source surveys represent a hybrid approach in which the receiver is placed on the seabed (providing full vector wavefield recording with both hydrophone and three-component geophone data) while the source (typically a deep-tow air gun array or a vibroseis source towed at 50 to 100 meters depth) provides high-frequency, low-noise energy with a reduced direct-wave-to-ghost notch frequency compared to surface-tow sources; the direct wave ghost (the reflection of the downgoing source wave from the sea surface, which arrives a fraction of a second after the direct wave and creates a destructive interference notch in the source spectrum at frequencies of c / (2 * source depth), where c is the sound speed) has a notch frequency of 37 Hz at 20 meters source depth and 750 Hz at 1 meter source depth; by towing the source deeper (50 to 100 meters), the ghost notch moves to 7 to 15 Hz (below the seismic bandwidth of interest) and the signal bandwidth is improved at frequencies of 20 to 150 Hz, recovering frequency content that is missing from conventional near-surface tow sources; this deep-tow source concept is used in combination with OBN receivers in the "mirror imaging" technique where the notch-free downgoing wavefield is separated from the ghost using dual-sensor data and used to image the subsurface with a flat spectrum across the full recording bandwidth.
Fast Facts
The deep-tow seismic technique was developed in the late 1960s and 1970s by oceanographic institutions including Scripps Institution of Oceanography (the DEEP TOW system, developed by Fred Spiess and colleagues in the mid-1960s) and Woods Hole Oceanographic Institution, primarily for scientific investigations of the deep ocean floor including mid-ocean ridge morphology, hydrothermal vent fields, and turbidite systems that were too far below the surface vessel to image adequately with surface-tow acoustic systems. The commercial adaptation of deep-tow seismic for petroleum industry applications (particularly shallow hazard surveys for deepwater drilling and pipeline route clearance) followed the rapid expansion of deepwater drilling in the Gulf of Mexico, West Africa, and Brazil in the 1980s and 1990s, when operators recognized that the regional exploration 3D seismic data was inadequate for resolving the shallow geohazards that controlled drilling safety and seafloor infrastructure design. The development of AUV-towed seismic systems in the 2000s and 2010s reduced the cost and improved the positional accuracy of deep-tow surveys, making them the standard acquisition method for pre-drill seabed surveys in deepwater field development.
What Is Deep Tow?
Deep tow is a marine geophysical acquisition method in which seismic sources and/or receivers are towed at water depths of tens to hundreds of meters below the sea surface, placing the equipment closer to shallow seabed targets and improving imaging resolution compared to conventional surface-tow acquisition. Deep-tow systems reduce travel path length, suppress surface multiples, and allow higher-frequency sources (sparkers, chirp transducers) that achieve centimeter-to-meter vertical resolution for imaging shallow gas, hydrates, BSRs, slope instability, and pipeline burial. AUVs carrying integrated seismic systems represent the modern implementation, providing precise navigation and seabed altitude control independent of the surface vessel.
Synonyms and Related Terminology
Deep tow is also called deep-tow seismic, sub-bottom profiling (for single-channel acoustic profilers), or AUV seismic (for autonomous underwater vehicle-based systems). Related terms include bottom-simulating reflector (BSR, a seismic reflection at the base of the gas hydrate stability zone in continental margin sediments that parallels the seabed topography and is caused by the impedance contrast between hydrate-bearing sediments above and free-gas-bearing sediments below; the BSR is the primary target of deep-tow high-resolution seismic surveys for gas hydrate characterization and deepwater drilling geohazard assessment), sub-bottom profiler (a shallow seismic system (sparker, boomer, chirp) that emits high-frequency acoustic energy (500 Hz to 50 kHz) to image the stratigraphy in the first 1 to 50 meters below the seabed at centimeter-to-decimeter vertical resolution; sub-bottom profilers are commonly mounted on deep-tow vehicles or AUVs to achieve the near-seabed positioning that maximizes their penetration and resolution in deepwater environments), autonomous underwater vehicle (AUV, a battery-powered, self-propelled underwater robot that follows a pre-programmed survey mission without a tether, used for seabed geophysical surveys, pipeline inspection, and oceanographic data collection; AUVs carry multibeam sonar, sub-bottom profilers, and in some configurations short-offset seismic streamers, providing deep-tow equivalent positioning and data quality with improved efficiency and positioning accuracy compared to cable-towed deep-tow systems), geohazard (a geological feature or process that creates a safety or engineering risk for drilling, pipeline, or seafloor infrastructure operations; shallow geohazards relevant to deepwater operations include shallow gas pockets, gas hydrate instability zones, fault scarps, submarine landslide features, and active venting; deep-tow high-resolution seismic surveys characterize these hazards at sufficient resolution for engineering-grade risk assessment), and pockmark (a crater-like depression in the seabed surface caused by the expulsion of gas, fluids, or gas-charged sediments from the shallow subsurface; pockmarks range in diameter from meters to hundreds of meters and indicate active or paleoventing of gas from the seabed, representing a geohazard for seafloor infrastructure and a potential indicator of hydrocarbon migration from the deep subsurface; deep-tow acoustic systems and AUV-mounted sonar provide the resolution needed to map pockmark morphology and distribution for geohazard assessment).