Gravity Surveys

 

ClearView Geophysics Inc. carries out Gravity Surveys. We maintain Scintrex CG6 and Trimble R12i/R12 equipment and provide experienced field operators to provide high quality gravity data.

Gravity surveys are carried out to map subsurface density contrasts caused by geological structures.  A gravimeter is an instrument used for measuring the local gravitational field of the Earth. It is a type of accelerometer, specialized for measuring the constant downward acceleration of gravity, which varies by about 0.5% over the surface of the Earth. Gravimeters are designed to be highly sensitive in order to measure very tiny fractional changes within the Earth's gravity of 1 g, caused by nearby geologic structures or rock types. Gravimeters display their measurements in units of gals, milligals or microgals instead of ordinary units of acceleration.

There are two types of gravimeters: relative and absolute. Absolute gravimeters measure the local gravity in absolute units (gals). Relative gravimeters compare the value of gravity at one point with another. They must be calibrated at a location where the gravity is known accurately, and then transported to the location where the gravity is to be measured. They measure the ratio of the gravity at the two points.

RTK GPS is acquired at each gravity station using ClearView's Trimble R12i with extended pole and 35W radio-link repeater station for real-time sub-centimetre vertical positioning in rugged terrain.  The inertial feature from the R12i allows faster accurate readings without needing to level the GPS antenna.  A standard Trimble R12 receiver with radio link is used for the base station.

For diamond exploration, it is common to complement gravity data with magnetics (e.g., snowmobile mode magnetics), Electromagnetics (e.g., MaxMin to 56 kHz) and GPR (e.g., 25 MHz, 50 MHz, 100 MHz and 250 MHz).  For bedrock escarpment investigations, it is common to complement gravity data with seismic refraction data.  Gravity surveys carried out in urban settings are often done during the night to minimize noise caused by traffic vibration.

Our personnel have been carrying out gravity surveys since the inception of the first CG3 Gravimeter introduced by Scintrex in the late 1980's.  Mr. Joe Mihelcic carried out his first Scintrex CG3 survey around this time while Dr. Andrew Hugill (designed core components) was providing technical support and guidance at Scintrex.  In the early 90's, operators carried out gravity surveys while Mr. Mihelcic completed survey planning and post processing.  Now ClearView owns and operates its CG6 state-of-the-art Gravimeter.

Gravity data are post-processed to obtain 1D, 2D and 3D models of the regional and residual Bouguer and Terrain-corrected gravity data. These can be presented as 3D Voxels and contour maps using Geosoft and Voxler software licensed by ClearView.

Contact ClearView Geophysics Inc. to discuss whether this method is ideal for your site specific case.  In most cases we would suggest letting us complete a Forward Model to see what sort of anomaly to expect.  Please also see our gravity blog entry.

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