IV. Background

This post's intention is to provide a technical brief for background on how a search of this kind is possible and how it all works together.

The technical details of any ocean exploration project largely depends on the purpose of the survey. Common purposes include: exploration for science and general mapping, military applications (unexploded ordnance detection), marine construction such as for submarine cables, pipelines, offshore energy infrastructure, offshore wind-farms, offshore Liquefied Natural Gas (LNG) facilities, bridges, tunnels... etc), marine mining and drilling, searching for downed aircraft, lost shipwrecks, marine salvage, and in this case...missing persons.

Typical tools of the trade to image the water column, seafloor, and what's below the seafloor include (but are certainly not limited to) the following:

Side Scan Sonar - A sonar system used to efficiently image large areas of the sea floor in terms of bathymetric features and character, and detection and identification of underwater objects. A side scan sonar emits a fan-shaped wide angle pulse of sound towards the seafloor perpendicular to the path of the sensor through the water. The sound pulses reflect off of relief or objects that are laying on the seafloor. The strength and travel time of those reflected sound pulses are recorded and processed by a sonar computer to produce an image of the seafloor.

Magnetometer - An instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument. A magnetometer is used for detecting iron-bearing debris, cables, and pipelines. We are using a SeaSpy Overhauser marine magnetometer. An Overhauser magnetometer utilizes some interesting quantum physics effects (Nuclear Magnetic Resonance-NMR) that takes advantage of the physical phenomenon based upon the quantum mechanical magnetic properties of the hydrogen atom's nucleus, but probably beyond the scope of this writing.

Echo Sounder - (or fathometer) uses pulses of sound directed from the sea surface vertically down to the seafloor to measure the distance to the bottom. The water depth is measured by multiplying the half-travel time of the outgoing pulse to it's return by the speed of sound in water (about 1,500 meters per second). We commonly use a multi-beam echo sounder (MBES) to produce a high-resolution 3-dimensional map of the seabed.

Subbottom Profiler - An acoustic system used to create cross-sectional images below the seafloor. The relatively low frequency sonar (typically 3.5kHz) penetrates the upper 10s of meters of sediment below the seafloor (penetration dependent upon geology, power, and frequency) and reflects on density contrasts (acoustic impedance contrasts) within the sediment which is recorded and processed to produce a "subbottom profile".

Remotely Operated Vehicle (ROV) - Unoccupied tethered underwater robot. ROVs are typically highly maneuverable, operated by a pilot aboard a vessel to which the ROV is tethered, and are useful for video, sampling, salvage, or construction.

Autonomous Underwater Vehicle (AUV)- Unoccupied, untethered, underwater robot typically used for acquiring high-resolution sonar data (side scan, subbottom, and multi-beam), video, or sampling. AUVs are programmed to fly along a specific route or to perform a set of specific tasks during a dive. They navigate by acoustic beacons referenced to a support ship with GPS along with an internal inertial navigation system and Doppler velocity imaging system.

Human Occupied Vehicle (HOV) - Manned deep-ocean submersibles used for in situ exploration, sampling, and observation. These versatile submersibles are capable at bringing scientists, researchers, and workers to great depths. The submersible Alvin (DSV-2) is perhaps the world's most famous, known for surveying the wreck of the RMS Titanic in 1986. Alvin carries two scientists and a pilot as deep as 4,500 meters (about three miles).

This search will utilize a side scan sonar, echosounder, and a remotely operated vehicle (ROV). Once the R/V Persistence is on site in Aruba, we will perform a sound velocity profile (SVP) to measure the sound velocity of the local sea water. A bathymetric survey will be performed to basically map out the seabed. This will consist of gridding the search area with the echosounder so that we know what to expect during the side scan sonar survey. After the bathymetry throughout the survey is at least roughly known, the side scan sonar search will be performed. After the sonar data is processed and analysed for sonar targets, the ROV will perform a dive series on the sonar targets.

During the side scan survey, the side scan sonar will be towed behind the boat. The height of the towfish will be controlled to stay a fixed height off the bottom. Typically, a side scan sonar is towed 10% of its range per side off the sea floor. For example, if we set the side scan to cover 100m (50m to either side), then we will tow the fish 5m off the bottom. Maintaining this height off bottom can be challenging and stressful in unknown or quickly changing conditions. Flying too high will provide poor data and flying too low also provides poor data and worse... risks a catastrophic collision with the seabed. The towfish depth is only controlled by varying the length of cable out (layback) and varying tow speed with the vessel.

GPS positioning is recorded into the sonar data during acquisition. In shallow water (less than around 300ft) the position of the towfish will be calculated based on the GPS provided position of the boat and the amount of cable out. In deeper water, towfish position (and the ROV when in use) will be tracked acoustically by an ultra-short baseline (USBL) tracking system. A transducer / hydrophone mounted on the R/V Persistence sends a low frequency "ping" into the water column. A transponder beacon mounted on the towfish or ROV submersible hears the ping, then sends two reply pings on different frequencies. The first ping gives range and bearing information, and the second ping gives water depth. This occurs at a regular interval throughout the dive. The information from these acoustic signals are then converted into horizontal distance and bearing points indicating the position of the submersible in relation to the support ship.

The accurate positioning of the side scan sonar towfish is vital to properly locating any sonar targets it detects. At the same time, the ROV must be properly positioned and tracked so it can be guided to those targets. If for example the towfish was positioned 1/2 degree off, with about a mile of cable out (expected layback in the deeper portions of the search), we would mis- locate the sonar target by 50ft. This error would make it difficult and time consuming to find the target with the ROV (if it isn't missed completely).