Electricity: measuring and testing – Of geophysical surface or subsurface in situ
Patent
1996-10-18
1999-05-11
Snow, Walter E.
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
324344, 324347, 324334, 367 14, G01V11/00;3/12;3/30;1/00
Patent
active
059031530
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and equipment for detecting fluids underground and particularly relates to a method and equipment which can be used to detect liquids such as water or hydrocarbons in porous rocks.
The most commonly used method for detecting liquids underground is by means of seismic prospecting using acoustical waves from a seismic source at or near the surface. It is known that the seismic waves reflected by interfaces between rock layers and such reflections can be used to map subsurface structures.
Recently, in order to obtain better results, three dimensional seismic surveys have been used, particularly within the oil industry. However, these methods all suffer from drawbacks and can only perform poorly in detecting underground liquids.
In addition to hydrocarbons it is also important to detect other underground liquids such as water for civil engineering and other purposes.
Methods have also been proposed for the detection of underground liquids using existing equipment but relying on electrical phenomena. U.S. Pat. No. 2,054,067 describes a method in which the changes in the resistance of the earth between two electrodes resulting from an acoustical charge is measured and used to ascertain information about the underground structure. U.S. Pat. No. 2,354,659 describes a method which utilises the DC current which may be generated when fluids, displaced by a seismic shock, return to their original location. However these methods either generate signals which are too weak to detect or measure accurately or are only useful at very shallow depths.
Methods have also been proposed which measure the resistivity of the earth in various places and rely on the variation in electrical resistivity of the earth in the presence of conductive liquids to indicate the presence of such liquids; however this has proved to be of limited effectiveness.
U.S. Pat. No. 4,904,942 describes a method for detecting conductive fluids underground by generating a seismic impulse at or near the surface of the earth and using electronic or magnetic detectors to detect an electromagnetic signal generated by a dipolar movement in a region containing two immiscible fluids.
The preferred detectors disclosed are two rod-like electrodes spaced apart at a distance of about 15-2000 feet or 4.6-610 meters with the rods being driven down to the first water table beneath the surface with preferably an array of detectors being used. The electrodes are connected, optionally, in series with a battery to the primary coil of a transformer, the secondary coil of which is connected via a notch filter to a recorder which measures the electrical fluctuations generated.
The method disclosed in this invention measures the electrical potential between two electrodes placed in the earth and employs a conventional seismic array of receptors and uses a conventional arrangement, with the receptors being in the position for normal seismic work.
It has been found that this method does not give accurate results and, in particular, has a low signal to noise ratio which makes interpretation of the results difficult.
The present invention differs from the invention described in U.S. Pat. No. 4,904,942 in that the electrical potential between one location and a base point insulated from the earth is measured and compared with the electrical potential between another location and the base point to obtain signals. This suprisingly gives better and clearer results than in U.S. Pat. No. 4,904,942.
The invention provides a method for detecting an underground liquid which method comprises initiating a seismic impact which passes through the earth, measuring the electrical potential generated by the shock from the seismic impact interacting with sub-surface rock containing the liquid at least two different locations, and comparing the measured electrical potentials at two different locations so as to indicate the presence of the liquid by means of a comparison device which generates a signal in proportion to the difference between the electrical pot
REFERENCES:
patent: 2054067 (1936-09-01), Blau et al.
Clarke Richard Hedley
Millar John William Aidan
Court Services Limited
Sherer Ronald B.
Snow Walter E.
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