Electricity: measuring and testing – Of geophysical surface or subsurface in situ – By aerial survey
Patent
1993-10-29
1997-03-11
O'Shea, Sandra L.
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
By aerial survey
324334, G01V 316, G01V 317
Patent
active
056105231
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to an apparatus and method of interrogating target areas below the ground for exploration purposes such as, for example, geological surveying. The invention also relates to the application of TEM (Transient Electromagnetic Method), MMR (Magnetometric Resistivity) and MIP (Magnetic Induced Polarisation) analysis to a survey of this type.
BACKGROUND OF THE INVENTION
It is conventional, with TEM schemes, to use an airborne system in which a transmitter loop is located with or within an aircraft, and the associated receiver sensor is also carried by the same aircraft.
Such technique is very efficient in so far that a very large area can be covered in a relatively short time but because the size of the transmitter loop is inherently limited there is a limitation as to the effective depth that can be interrogated with such an arrangement.
On the other hand, by using a ground based transmission antenna, the size of the antenna can be very large indeed but it will inherently take significant time to move throughout the area to take appropriate readings.
Currently MMR and MIP surveys are conducted on the ground using a ground based transmitter and ground based receiver. As mentioned above, the main limitation of this approach is that it is slow and requires ground access to all portions of the survey area. This makes these survey methods relatively expensive.
One form of this invention can comprise a method of interrogating volume of material beneath the ground which comprises locating a ground based loop or grounded conductor across any area to be surveyed, effecting transient electromagnetic pulses into the ground based loop or grounded conductor, and detecting the results of such transient electromagnetic pulses from an airborne vehicle.
Such an arrangement has been found to provide very significant advantages over that which has gone on hitherto.
Nobody seems to have hitherto realized that by separating the transmitter loop from the airborne vehicle the loop can be made very large indeed can be located conveniently and extensively in a close position relative to the ground and significant time can be saved. Having a receiver carried by an airborne vehicle, such as either a helicopter or a fixed wing aeroplane, the gathering of information can be quickly achieved. The receiver itself can be highly sensitive and the detector can be carried either in an aeroplane or other airborne vehicle or preferably, in a bird towed behind the airborne vehicle. The receiving sensor can feed the signals into a memory means within the vehicle. All of these features in combination provide very significant advantages.
The advantages provided by this arrangement include an increased signal to noise ratio and, furthermore, the effects of the electromagnetic transient pulse can be measured out to longer time intervals which have the advantage of enabling a deeper depth to be interrogated.
One of the difficulties in using a ground based transmitter and an airborne sensor is that the signal power must be very high in order to give a sufficient signal over a loop that necessarily becomes very large. There has also been a major difficulty in being able to effect a sufficiently large power supply which, at the same time, can provide a signal of sufficient quality for such purposes.
A second problem has been that in order to effect useful readings, the sensor or the receiver must be able to be extremely and accurately synchronized or at least have access to synchronizing signals from the ground loop.
It is now possible to achieve a sufficiently accurate synchronization calibration which will hold at least over a few hours. The calibration is achieved by having very highly accurate crystal control oscillators which are first brought into an identical frequency oscillation and, then, prior to being separated, are calibrated to a sufficient extent that the receiver, which thereafter will be physically separated from the ground loop or the transmitter of the ground loop, will be able to predict
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O'Shea Sandra L.
Patidar Jay M.
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