METHODOLOGY:

ADA(UK)has the facilities to conduct all aspects of underwater, littoral and foreshore surveys for any archaeological or scientific project. Using the collaborative data of many survey methods allows for the safe insertion of a diver into the water for videography and photographic survey for data interpretation. Below is a brief outline of some of the survey methods we use.

Side-Scan Sonar Imaging:
Side-scan sonar is a category of sonar system that is used to create an image of large areas of the sea floor. This tool is used for mapping the seabed for a wide variety of purposes, including creation of nautical charts and detection and identification of underwater objects and bathymetric features. It may be used to conduct surveys for maritime archaeology; in conjunction with sea floor samples it is able to provide an understanding of the differences in material and texture type of the seabed. Side-scan sonar imagery is also a commonly used tool to detect debris items and other obstructions on the sea floor that may be hazardous to shipping or to sea floor installations by the oil and gas industry. In addition, the status of pipelines and cables on the sea floor can be investigated using side-scan sonar. Side-scan data are frequently acquired along with bathymetric soundings and sub-bottom profiler data, thus providing a glimpse of the shallow structure of the seabed.

Side-scan uses a sonar device that emits fan-shaped pulses down toward the sea floor across a wide angle perpendicular to the path of the sensor through the water, which may be towed from a surface vessel or submarine, or mounted on the ship's hull. The intensity of the acoustic reflections from the sea floor of this fan-shaped beam is recorded in a series of cross-track slices. When stitched together along the direction of motion, these slices form an image of the sea bottom within the swath (coverage width) of the beam. The sound frequencies used in side-scan sonar usually range from 100 to 500 kHz; higher frequencies yield better resolution but less range.

Up until the mid-1980s, commercial side-scan images were produced on paper records. The early paper records were produced with a sweeping plotter that burned the image onto a scrolling paper record. Later plotters allowed for the simultaneous plotting of position and ship motion information onto the paper record. During the late-1980s, commercial systems using the newer, cheaper computer systems developed digital scan converters that could mimic more cheaply the analog scan converters used by the military systems to produce TV and computer displayed images of the scan, and store them on video tape. Now data is stored on computer hard drives.

Multi-Beam Sonar Imaging:
Multi-beam sonar systems provide fan-shaped coverage of the sea floor similar to side-scan sonars, but the output data is in the form of depths rather than images. Instead of continuously recording the strength of the return echo, the multi-beam system measures and records the time for the acoustic signal to travel from the transmitter (transducer) to the sea floor (or object) and back to the receiver.

Multi-beam sonars are generally attached to a vessel, rather than being towed like a side-scan. Therefore, the coverage area on the sea floor is dependent on the depth of the water; typically two to four times the water depth.

Sub-Bottom Profiling:
SONAR is a technique that uses sound propagation (usually underwater) to navigate, communicate or to detect other vessels. There are two kinds of sonar—active and passive. Sonar may be used as a means of acoustic location. The term "sonar" is also used for the equipment used to generate and receive the sound. The frequencies used in sonar systems vary from infrasonic to ultrasonic. The study of underwater sound is known as underwater acoustics or hydroacoustics.

Powerful low frequency echo-sounders have been developed for providing profiles of the upper layers of the ocean bottom.

Magnetometry:
Magnetometers are used in geophysical surveys to find deposits of iron because they can measure the magnetic field variations caused by the deposits. Magnetometers are also used to detect archaeological sites, shipwrecks and other buried or submerged objects.

A magnetometer can also be used by satellites like GOES to measure both the magnitude and direction of the earth's magnetic field.