Underwater Gravity

For underwater gravity surveys, Excel owns and operates Model-U LaCoste and Romberg remote-reading longline gravity meters enclosed in a watertight submersible casing that work on the same principles and basic mechanics as the Model-G gravity meter used in land gravity surveys. Under field conditions, the meters have an absolute accuracy of ± 0.01 mGal and a repeatability ± 0.02 mGal. Underwater gravity surveys are designed and used to evaluate marine environments with depths up to 500 meters.

Excel has conducted underwater gravity surveys in a variety of marine environments worldwide. In the past two years, Excel has carried out underwater gravity surveys in parts of Eastern Canada and Western Africa. Underwater gravity surveys are conducted using a marine vessel equipped with a mechanical winch used to reel the gravity meter in and out of the water. The gravity meter is lowered to the sea-floor and read remotely by an experienced operator in the survey vessel, while the vessel is anchored in place. An onboard GPS system installed on the survey vessel is used to accurately determine the meters position. During each survey, a minimum of three GPS base stations are setup to precisely determine the location of the gravity meter during post processing. A boat depth sounder equipped with a GPS is also setup to continuously record depth data throughout the survey to provide high resolution bathymetry data for post processing. A land gravity station is always acquired at all underwater gravity field base locations to provide extra gravity data and to be used as a measure of quality control.

Underwater gravity surveys are carried out in a range of marine environments ranging from shallow lagoon to deep marine settings. Underwater gravity surveys have proven to be an excellent geophysical tool for identifying anomalies of interest for a variety of industries. For the oil and gas industry, Excel has performed multiple underwater gravity surveys to determine the location and geometry of salt bodies in the subsurface by measuring the change in density across the salt structure. Underwater gravity surveys have also been utilized in several sea floor mapping, mineral exploration and geotechnical projects. The resulting gravity datasets are modelled using proprietary 2D and 3D gravity modeling software and integrated with other existing geophysical, well and surface geology data.