The Different Methods of Performing Water Bottom Surveys
Riley Kooh | August 12, 2022
What are Hydrographic Surveys?
Hydrographic surveys (aka seafloor surveys) refer to the gathering of data on ocean, lake, river, or estuary floors. Hydrographers use the data collected on depths and physical configurations in order to create nautical charts and precise models. This information is critical for planning maritime construction, identifying transportation routes, flood management or for insights into benthic ecosystems like cold water coral reefs and kelp forests.
Additionally, survey techniques are commonly used as an emergency response for military or search and recovery operations.
- Explosive Ordnance Disposal (EOD): EOD teams will utilize boats or unmanned vehicles like AUVs, USVs, and ROVs mounted with side-scan sonars to sweep bodies of water for mine detection. If the mooring becomes damaged or removed, mines can drift to unidentified locations, posing serious threats to passing cargo or civilian ships. Once identified, the EOD team can remove or intentionally detonate the dangerous explosives. Most recently; the Ukraine-Russia War has resulted in Soviet-era mines being cut loose in an effort to disrupt global trade. Until these mines are successfully removed or detonated in a controlled environment, maritime transport can not resume.
- Search and Recovery (SAR): Identifying drown victims or discarded evidence underwater can be extremely challenging. Drifting currents and turbid conditions can compile for near-zero visibility to search for an ever-moving object. By completing floor surveys using side-scan sonar, SAR teams can identify anomalies across large search areas to narrow down search targets. Interested in reading more about search and recovery? Check out our in-depth article here.
The Origin of Seafloor SurveysHydrographic surveying dates back hundreds of years via the use of sounding rods, however the earliest renditions of modern vessel techniques began in the early 1900s. The method known as ‘wire dragging’ required two vessels to pull a system of wires, weights, and buoys to collide with exposed rocks for depth calibrations. This symbolized a major milestone in bathymetry (underwater topography) as it streamlined surveys and set the stage to develop modern sonar and lidar bathymetry techniques.
Modern HydrographyModern bathymetry and seabed mapping is accomplished using echosounders, sonar, or lidar systems. Advancements in these imaging technologies allows for quicker, easier, and more effective surveys at greater depths. Additionally, hydrographers can opt for Acoustic Doppler Current Profilers (ADCP) to identify current patterns or sub-bottom profilers to analyze sediments and build a more comprehensive understanding of water bodies.
Multibeam Imaging SystemsMultibeam sonar, lidar, or echo sounder systems offer a wider survey area in comparison to their single beam counterparts for greater surface coverage. This lends itself as the prime technique for large-scale surveys of oceans or lakes. Multibeam systems work by sending multiple simultaneous sound or light pulses in a cone-pattern shape rather than a single direct pulse. Hydrographers will use the return time and intensity of these pulses to calculate water depth as well as the floor material to generate detailed maps. These systems would generally be mounted to, or dragged by a boat or other vehicle for panning floor surveys.
Single Beam Imaging SystemsShallow or small water channels can utilize single beam imaging systems. Utilizing just a single pulse, these systems are effective at measuring water depths directly below a boat. In smaller water environments, this can be beneficial for more targeted analysis, and as a lower cost methodology. For smaller bodies of water, these single beam systems can also be mounted to ROVs or AUVs for improved navigational ability.
Side Scan SonarSide scan sonars are effective tools which can be used for large hydrographic or bathymetric surveys. They utilize side mounted imaging sonars to paint pictures of the seafloor over a wide surface area. If being towed, they can provide greater detail than a boat mounted sonar since depth can be adjusted. The tradeoff of such a large footprint is fine detail, which can be lost at the ends of the side scan “fan”. In practice, side scan sonars are commonly used to identify large anomalies like shipwrecks, submerged structures/vehicles, or mines (source).
Acoustic Doppler Current Profiler (ADCP)An ADCP is a hydroacoustic current meter that oceanographers and hydrographers can use to measure different flow rates at given depths. It functions by sending high frequency ‘pings’ through the water and calculating the current and sediment levels based on the frequency of the return pings. Used in conjunction with echosounders or sonar, ADCPs are a useful tool for building a more comprehensive understanding of water bodies.
Seafloor SamplingRetrieving physical samples enables hydrographers and other marine scientists to study past, current, and future trends in aquatic environments. These can be used to predict flood patterns, monitor erosion, study marine life, inspect for construction projects, and more. With a vast array of different required samples, there are a variety of unique techniques, each with a different end goal in mind.
CoringIn its simplest terms, coring refers to the collection and compression of sediment from the seafloor into cylinders or “cores”. While there are multiple variations of coring methods including kasten, piston, multi, mega, and gravity coring, the same principles are applied in each. A sampling tube is lowered to the seafloor and the weight of the body allows it to dig into sediment for collection. Once gathered, the core can be brought to the surface for further analysis.
GrabbingGrabbing is one of the most common and simplest methods of collecting physical samples off the bottom of oceans, lakes, rivers, etc.. Using a device consisting of two “clamshells”, hydrographers can lower the clasps to floor level and collect topside sediment quickly and easily. Deep water extractions generally require a crane, pulley, or ROV to lower the clasps into the sediment, however in shallow waters, manual grabbers can be used.
DredgingDredging refers to the collection of rocks or other loose debris for removal from a water body. This is typically done as a countermeasure against the progressive accumulation of sediment in waterways to continue their usable life. Dredging often is focused on maintaining or increasing the depth of navigation channels, anchorages, or berthing areas to ensure the safe passage of boats and ships (source). With the vast majority of global goods being transported by large cargo ships, maintaining safe channel depths is a necessity for the world economy.
There are a number of methods to conduct dredging which vary depending on the size of the project and materials being collected. Dredging vessels generally use either a traditional clamshell mechanical device to scoop and remove sediment, or a combination of cutters/augers to loosen the sediment for removal by vacuum. Once lifted, this sediment and debris can be studied for environmental trends or repurposed for things like beach rehabilitation, construction materials, or topsoil.
TrawlingTrawling is a method of dragging a large open-faced net behind a ship and collecting all large marine life in its path. While trawling is primarily used as a commercial fishing practice, its yieldings can also be used to study ocean floors and overall marine health. A recent study evaluated the findings from 235 trawls off the coast of South Africa to study fish abundance as well as the concentration and distribution of pollution.
Uses of Photography and Video in HydrographyCapturing photos or videos during seafloor surveys is an effective method of studying marine species, physical features, or monitoring environmental changes over time. Removing physical samples can be difficult, or destructive to the environment. Photos can typically convey all the same information without requiring the removal of samples. Additionally, these photos and videos can easily be stored and shared around the world for enhanced knowledge pooling.
In the world of seafloor surveying, photography can add a visual medium for monitoring sediment buildup, degradation or improvement of natural habitats, animal behaviors, or as an inspection tool to evaluate the constituents of the seafloor. Underwater images can also be compiled and rendered into 3D photogrammetric models. These models can be extremely useful for topography, construction planning, or identifying waterways. However, the level of detail required to create these models makes it incredibly difficult to complete on a large scale.
Unmanned Systems for Seafloor Surveys
Unmanned Surface Vehicle (USVs)USVs are a remote operated topside ship capable of piloting with varying degrees of autonomy. Hydrographic surveys are the most common application of these vehicles, as they are well suited to navigate repetitive routes in lieu of an on-deck operator. Operating on top of the surface, USVs are more affected by wave conditions, and are best suited for shallow waters. Equipping these vehicles with imaging sonar technology allows for thorough surveys in turbid conditions, without manual staffing.
Autonomous Underwater Vehicle (AUVs)AUVs are untethered underwater vehicles capable of piloting underwater environments without any external controls. These vehicles can range dramatically in size, from just a few hundred pounds, to thousands. AUVs operate via preprogrammed mission controls, and are equipped with similar imaging technologies as a USV to store data internally as they travel along the mission path. With no need for tethered communications, AUVs can work at immense ranges. This is beneficial for surveying large lakes or oceans, however any breakdown in the system can result in a lost unit.
Remote Operated Vehicle (ROVs)Similarly to AUVs, ROVs operate beneath the surface to survey submerged assets. Where they differ is ROVs operate with a much smaller form factor, and rely on tether communications for manual precision piloting. Similarly to AUVs, ROVs range in size from work class units weighing thousands of pounds, down to observation or micro class vehicles that can be carried in one hand. Mounted with the same imaging technologies, ROVs are commonly used synchronously with autonomous systems for large surveys that also require precision navigation.
Benefits of ROVs for Seafloor SurveyingRemote Operated Vehicles (ROVs) are an incredibly safe, effective, and increasingly affordable method of capturing underwater photos, collecting samples, and gathering other data. They operate as tethered drones capable of diving to immense depths and housing hydrography specific addons to drastically simplify missions.
Easy to TransportObservation or micro class ROVs have made incredible leaps in the name of portability. These devices are small enough to fit inside just one or two Pelican cases and be deployed by a single operator. Battery powered units remove any need for topside power generators, reducing the footprint of the vehicle for quicker deployment and transportation. This makes visual surveys possible to be completed from both small or large vessels.
ManeuverabilityWhile piloting deep sea underwater drones can sound like a daunting task, intuitive controllers coupled with an increasing familiarity with video games and consumer electronics allow for a very accessible experience. Some advanced features may require additional practice or visual guides, however the core operations of driving, capturing images, and retrieving samples are simple and reliable. Units with multiple vectored and vertical thrusters offer an even smoother experience to instill confidence in nearly any pilot.
High Quality ImagingSome of the challenges of underwater photography is the cost associated with dive teams, adequate lighting, or complex or unsafe depths preventing diver access. Utilizing ROVs, surveyors can quickly and easily capture 1080p or 4K images/videos without any of these concerns. Additionally, maintaining a topside visual of what the ROV is seeing empowers the surveyors to ensure the images are precise, rather than relying on communication to a third party.
Add-onsThe ability to house project specific add-ons truly empowers an ROV to be an all-in-one surveying solution. On top of the integrated camera, ROVs can be equipped with grabbers and samplers for physical retrievals, sonar for bathymetry scanning through turbid conditions, as well as environmental sensors to gather precise measurements throughout different ocean zones.
How Deep Trekker ROVs are Beneficial to Sea Floor and Bottom SurveysDeep Trekker manufactures industry leading observation and micro class ROVs capable of a variety of ocean science tasks. With four different models including the DTG3, PIVOT, REVOLUTION, and DTPOD, there are available options to suit nearly every budget or environment. Capable of diving up to 1,000ft and comfortably operating in temperatures as low as -5 degrees celsius, Deep Trekker ROVs empower visual seafloor surveys to be conducted nearly anywhere.
With all units operating on battery power and functioning from a handheld BRIDGE controller, pilots can enjoy the portability of one or two Pelican cases depending on the model. Grabber arms can be equipped with sediment or water samplers, or simply be used for physical sample retrievals, and every unit is sonar integratable for navigation or scans through turbid water. With units still in service for over 10 years, Deep Trekker’s ROVs are a reliable investment for continuous visual surveys, rather than relying on repeated costs of contracted divers or rental equipment.
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