The survey was performed in two stages: Stage A, before the commencement of infrastructure work along the seabed, was a visual underwater inspection along the route slated for the pipeline; Stage B, after the digging of the trench for setting the pipeline, was an underwater survey along the trench and an inspection of the site where the sand from the trench was temporarily deposited. After the gas pipeline was laid in the trench, the sand from the temporary deposition site was returned to cover the pipe.
The navigation utilized a GIS system, set on board boat Qadmonit, which was fed with vector layers of the gas pipeline (provided by the Ministry of Infrastructures), a bathymetric layer (depth map) and an aerial photograph. The system was connected to a Trimble type DGPS, at a level of accuracy less than 1 m. Marker buoys were placed at set distances (20 m) along the course of the pipeline and the team of divers navigated between them, using a compass and a guiding cable. Each discovered underwater artifact was documented and marked with a small marker buoy. After the dive, the position of the located buoys was recorded by the DGPS.
An underwater metal detector was utilized during the survey. For efficiency’s sake, namely increasing duration of diving, shortening of time intervals between dives and increasing safety, an oxygen-rich (Nitrox) mixture was used by appropriately trained divers.
Four geomorphological units typify the survey region:
Unit 1 (Holot Ashqelon). A low, narrow strip of sandy beach at whose rear is a low kurkar cliff covered with sand dunes. This narrow strip is completely inundated up to the coastal cliff during winter storms.
Unit 2 (distance of c. 150–200 m from shoreline and depth of 2–3 m below sea level). Seabed is characterized by a noncontiguous covering of kurkar rocks, which are the remains of the destroyed western kurkar ridge edge and slanting platforms of coastal rock.
Unit 3 (distance of 0.2–3.0 km from shoreline and depth of 3–32 m below sea level). Sandy seabed that gently slopes to the northwest; protruding bedrock blocks appear occasionally through the sand. The seabed covering becomes muddy and clayey the deeper one goes to the northwest.
Unit 4 (distance of c. 3 km from shoreline and depth of 30–32 m below sea level). Seabed is muddy and clayey; bedrock blocks that remained from a sunken kurkar ridge protrude from seabed. This ridge has a more impressive form c. 1 km southeast of the survey region, at the same depth and parallel to the coastline of Ashqelon.