Measuring and testing – Gravitational determination
Patent
1998-10-26
2000-07-04
Chapman, John E.
Measuring and testing
Gravitational determination
G01V 714
Patent
active
060821949
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an apparatus for measuring gravity gradients and in particular to a gravity gradiometer suitable for use in prospecting.
There are generically only four types of measurements available for use in prospecting for subterranean orebodies. Magnetic field mapping has been extensively used over the past twenty years with great success for ferromagnetic orebodies. Aerial electromagnetic surveys have recently indicated very great potential for identification of conductive orebodies while still more recently the potential of airborne geochemical analysis of surface material by spectral reflectance has been demonstrated. The remaining parameter known to be of significance in surface observations, the gravitational field, has not yet been used with sufficient precision to provide a reliable prospecting method.
The presence of subterranean structures such as orebodies, gas and oil deposits gives rise to minute variations in gravitational field at the earths surface.
The magnitude of a typical anomaly relative to the unperturbed gravity field is proportional to the total mass excess (or deficit), and is inversely proportional to the square of the distance from its effective centre to the point of observation. Most traditional gravity measurements are performed on stationary platforms fixed to the earth surface, and precision is limited by vibration noise sources common in the earth.
The gravitational anomaly of an ore body of density contrast 300 kg m.sup.-3 and of dimension say 200 m buried below a depth of say 100 m of overburden is typically 2.times.10.sup.-6 ms.sup.-2, which is 0.00002% of the normal earth gravity field. This relatively small effect is normally measured in units of micro gals .mu.Gal, and would represent approximately 200 .mu.Gal.
To this time most resource significant measurements have been made using instruments of the LaCoste/Roberg type which are essentially ultrasensitive spring balances detecting the small difference in weight caused by the gravity anomaly. The measurements are subject to a wide variety of environmental influences, and measurements must be performed relative to a standard point which is used regularly during the survey as a fixed reference for the removal of drifts in the instrument. With great care, measurements over reasonable areas can be achieved to about 5 .mu.Gals, making this technology appropriate for mapping regions of known potential. The procedure is slow, and requires extensive information on local topography and geology by reason of the fact that the normal variation of gravity with height is approximately 300 .mu.Gal per meter.
On mobile platforms, it is not generally possible to distinguish the accelerations acting on a body due to variable gravitational effects from those due to kinematic effects associated with changes of the body's velocity. Aerial prospecting, which is the most economic method of covering large areas, cannot be achieved with adequate precision with instruments normally used for measuring the gravitational field from fixed platforms at the earth's surface.
This suspended mass type of relative gravity instrument has in fact been used with great difficulty from moving platforms and in particular from aircraft where altitude control using for example precision radar altimeters and pressure sensors can be used to achieve vertical position to fractions of a meter. This still imposes limitations of the order of a few hundred .mu.Gal on the gravity data. For this reason emphasis for large scale geophysical prospecting has moved towards gradiometry. In principle, measurement of the gradient of the gravity field over a known baseline allows one to cancel out the accelerations due to the motion of the platform itself. Gradient measurements also have some advantages in detection of boundaries of anomalies.
The vertical component of the gradient above the orebody discussed above and measured from an aircraft 100 m above the surface is approximately 6.times.10.sup.-9 ms.sup.-2 per meter, which is 6 Eotvos (the Eotvos is a unit of
REFERENCES:
patent: 2217123 (1940-10-01), Malmqvist
patent: 3727462 (1973-04-01), Stone et al.
patent: 4419891 (1983-12-01), Browning
Derwent Abstract Accession No. 84-305915/49, SU 1086397A (Ryazan Wireless Eng Inst), Apr. 15, 1984.
Derwent Abstract Accession No. C3057 D/11, SU 74416A (As UKR Low Temp Phy), Jun. 30, 1980.
Chapman John E.
The Commonwealth of Australia Commonwealth Scientific and Indust
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