Our Search Technology
In addition to diving, UHG Members use a variety of search technologies and methods for their shipwreck search and investigation projects. These include; metal detectors, magnetometers, Remote Operated Vehicles (ROV) and sonar systems.Magnetometers:
Understanding magnetometers. Loosely speaking the world is one big magnet, complete with a North Pole and South Pole. This is why a compass works. All a magnetometer does is measure the local magnetic field. However iron and steel objects have their own magnetic field outside of the earth’s natural field. You could try a small test by standing outside with a compass and finding magnetic north. Then introduce an iron or steel (but not stainless steel) object close to the compass and look at the compass needle. Note it moves or reacts violently. This is because the compass is confused as to which magnetic reading is the correct one that represents magnetic north. Once the magnetometer is pressed into action it measures, in this example, every 2 seconds the magnetic field, whist you conduct your survey. Readings remain relatively the same until you approach an object made from iron or steel. What's represented on the LCD display is dramatic with a sudden rise or drop in readings. GPS is linked to the magnetometer and with each reading, information in sent to the magnetometer's memory linking that position with a magnetic reading for post analysis. Shipwrecks, even of timber construction, contain many items made from either iron of steel such as; anchors, ballast blocks and cannon, hence it's a valuable tool in locating shipwrecks. The magnetometer our members use is an Aquascan AX200 marine type designed to be towed behind a boat. We have built an aluminium frame to mount the sensor head on; this allows the system to also be used as a land-based application for searching intertidal zones for buried targets.
UHG members use Minelab Excalibur and Aquascan AQ1B metal detectors to aid the search for targets of interest. These units are hand-held and can be used underwater by divers or for searching land or beach areas for metal objects. Normally the initial survey of a search area is carried out using either the magnetometer or a side scan sonar system: once a target has been located the metal detector is used to locate buried or small artefacts.
Remote Operated Vehicles (ROV)
ROV systems are used to investigate, survey and record targets in depths beyond safe dive limits or to record diver activities on a wreck site. The remotely operated vehicle sends real-time video images through a tether cable to the surface team. The ROV pilot, using telepresence as a guide, controls the vehicle by manipulating controls to operate thruster motors and other systems. Sonar is used to navigate the ROV and to register target details; video is recorded on digital format for later study. The UHG use a Benthos MiniRover MkII system, depth rated to 330 metres, and a smaller SeaBotix LBV system which is rated to 150 metres.
Side scan sonar and scanning sonar systems are acoustic imaging devices, both are used by the UHG for search and survey of the ocean. The side scan sonar involves towing a sonar transducer sensor behind a boat, which uses sound waves to transmit details of bottom features to an onboard processor unit. The signals are converted to images displayed on a monitor screen, resembling an aerial view of the bottom features and submerged targets. The sonar beam swath covers an area on both side of the sensor tow "fish" up to distances of 300m. The scanning sonar transducer is either ROV mounted or diver held. The rotating sonar head transmits an acoustic beam into the water column and detects targets at distances up to 100m; these are displayed on a monitor screen. The system is particularly of benefit in low visibility when searching for a target or wreck. Both systems can register displays in a variety of colours which assist in identifying the composition of the target. The UHG uses an Imagenex 586 330khz sonar system.
Searching the Intertidal Zone:
The remains of many shipwrecks around the NZ coasts are hidden beneath the intertidal zones presenting problems for investigation and survey. UHG members have developed equipment to aid them in their search for these lost shipwrecks. Using compressed air probes and water pressure pumps, they are able to investigate targets, discovered using magnetometers, buried deep beneath the sands.