Longline Gravity

For longline gravity surveys, Excel owns and operates Model-U LaCoste and Romberg remote-reading longline gravity meters 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. Longline gravity surveys are designed and used as a cost effective and time efficient method to evaluate vast regions including heavily treed areas, mountains, swamps and difficult ground-access areas as well as environmentally sensitive zones with virtually no environmental impact.

 

Excel has collected over 15,000 gravity stations using remote-reading gravity meters in many areas of British Columbia, the Yukon and the Northwest Territories. Longline gravity surveys are conducted by helicopter with the meter suspended on a 50 meter longline below the helicopter. The gravity meter is lowered to the ground and read remotely by an experienced operator in the helicopter, while the helicopter hovers in place. The helicopter is not required to land; therefore, a natural clearing of only a few meters in diameter is required for the gravity meter to be lowered to the ground. An onboard GPS system installed in the survey helicopter is used to accurately determine the aircraft position and height above ground. 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 land gravity station is always acquired at all longline field base locations to provide extra gravity data and to be used as a measure of quality control.

 

Longline gravity surveys have proven to be an excellent geophysical tool for identifying anomalies of interest for various industries. Longline gravity surveys are designed to cover vast areas in order to map and detect the depth and geographical extent of geological bodies such as faults, reservoirs, and rock types in the subsurface. Gravity station spacing is tailored according to the geological goals of any particular project, which generally should be no greater than half the expected width of the targets being explored for. From Excel's vast experience with the oil and gas exploration industry, a station spacing of about a fifth of a mile or less has become the standard spacing used for gravity exploration programs. Infill stations are often added to the planned survey to detail the anomalies associated with targets of interest to assist in creating complex geological models during interpretation. The resulting datasets are then modelled using proprietary 2D and 3D gravity modeling software and integrated with other existing geophysical, well and surface geology data. Variable density terrain correction techniques are also used for areas of complex geology.