Electricity: measuring and testing – Of geophysical surface or subsurface in situ – Using electrode arrays – circuits – structure – or supports
Reexamination Certificate
2000-05-30
2001-11-20
Snow, Walter (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
Using electrode arrays, circuits, structure, or supports
Reexamination Certificate
active
06320386
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods of geological exploration employing electromagnetic field transients and may be used to prospect for local geological formations, particularly, on the world ocean shelf.
SU Inventor's Certificate No.1062631 discloses a method of geological exploration comprising the steps of exciting electromagnetic field in the studied medium by axially symmetrical injection of electric current to the Earth, using a circular electric dipole formed of feeding electrodes, wherein one of the electrodes is grounded in the center of a circle formed by the other uniformly grounded feeding electrodes, measuring parameters of a field electric component transient along profiles radiating from the circle center, and assessing, based on the measurement results, the structure and properties of the studied medium.
However, it is difficult to recover local nonuniformities by the above method as a signal of a transient in the electromagnetic field produced by influence of the country medium influence can be completely compensated only at the ideal axial symmetry of the current injection to the Earth, i.e. when the number of feeding electrodes grounded in a circle is infinitely large.
Mogilatov V.S., Balashov B. P., Sounding by Vertical Currents.—Physics of the Earth, 1944, pp.73-79, teaches a method of geological exploration using a circular electric dipole, comprising the steps of exciting an electromagnetic field in the studied medium by series of sounding current pulses generated by injecting electric current to the Earth by feeding electrodes, one of the feeding electrodes being located in the centre of a circle formed by the other feeding electrodes. The parameters of a field electrical component transient are measured along profiles radiating from the circle centre and the measurement results are used for assessing a structure of the medium being studied. The parameters of a field magnetic component are measured and the results are used to decide on presence or absence of a local formation. However, the above prior art method does not allow a decision to be made on which particular geological formation (karsts, kimberlite bodies, etc.) caused the horizontal nonuniformity of the section, detected by the measurements. Using this method, a preliminary construction of a geoelectric section of a country medium is impossible as the method is directed at obtaining an abnormal signal and, hence, the investigations are not highly informative.
An apparatus for implementing the aforementioned method comprises a current generator having a first output coupled to a feeding electrode which is located in the centre of a circle formed by the other feeding electrodes which are coupled to a second output of the current generator by respective supply line radial lengths. Each of the supply line radial lengths includes a current controller. The starting time of magnetic and electric component meters is synchronized with the time of turning the current on, using quartz-crystal clocks in the current generator and in the meters.
A problem with the prior art apparatus above is that when a number of radial lengths or a current in the radial lengths is increased, a higher current pulse amplitude is required in the current generator as the current pulse amplitude of the latter is a sum of current amplitudes in the radial lengths and, accordingly, a generator having a higher output power is required, making operation of the apparatus less reliable. Second, the prior art apparatus does not provide regulation of the current pulse amplitude in the radial lengths, which results in violation of axial symmetry in the current injection to the Earth and, hence, an increased background of uncompensated normal magnetic field. Third, to prospect for geological formations laying at small depths, i.e. when a field magnetic component is measured at the early stage of a transient, a highly accurate quartz synchronization is required between the current generator and the measurements of electric and magnetic components, which is hardly feasible in field conditions.
SU patent No.1819354 discloses a method of marine electrical exploration which is applicable in search and exploration for mineral deposits on water areas and involves exciting alternating electric field by a horizontal electric dipole and selecting optimum sounding spacing, taking into account variations in the sea bottom structure and in water layer thickness when conducting electric exploration operations from a moving ship. However, the method is only suitable for studying a sedimentary mantle of a small thickness (up to 50 m) at small depths (up to 30 m), and, in principle, is not capable of detecting elongated local formations such as formations of a “deposit” type, at large depths, insofar as with increase in a sounding spacing, a response signal is more and more effected by longitudinal conductivity of seawater.
The aforementioned method of sounding by vertical currents (J. Physics of the Earth, 1994, No.6, pp. 73-79) can be employed in the marine geological exploration. However, the employment of this method in the marine geologic exploration suffers certain problems. First, the measurement results will be essentially influenced by a seawater layer above the shelf surface, and this layer can be up to 300 m. Second, salinity of the seawater complicates the axially symmetrical injection of current to the Earth.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of prospecting for geological formations, wherein a magnetic component transient signal of a field produced by the country medium, is sufficiently compensated, using a practicable number of feeding electrodes grounded in a circle.
It is another object of the invention to provide a method and apparatus for prospecting for geological formations, which ensure higher information content of investigations, reduced background of uncompensated normal magnetic field and extend the range of studied depths towards both small and high values.
Additionally, it is an object of the invention to provide a method and apparatus for prospecting for geological formations on the world ocean shelf, using a circular electrical dipole.
In accordance with the invention the above objects are attained by a method of prospecting for geological formations, comprising the steps of
using grounded feeding electrodes including a set of first feeding electrodes and at least one second feeding electrode to inject electric current to a studied medium and excite a signal of electromagnetic field transient in the studied medium;
locating said first feeding electrodes at equal distances in a circle at a site on the studied medium;
locating said at least one second feeding electrode in the centre of said circle;
more precisely defining a number of the first feeding electrodes such that a predetermined signal of a magnetic component transient of a field produced by the country medium does not exceed a predetermined level;
axially symmetrically injecting electric current to the studied medium by periodically coupling the electric current to said optimum number of the first feeding electrodes and to said second feeding electrode;
area measuring a signal of the electromagnetic field transient caused by the studied medium, upon each turning the electric current off at said first feeding electrodes and said second feeding electrode;
determining a time of maximum manifestation of a magnetic component transient signal of a field produced by the studied medium;
mapping, based on the results of area measuring the magnetic component transient signal, isolines corresponding to the time of maximum manifestation of the magnetic component transient, to decide on presence or absence of a geological formation on the studied medium.
The employment of grounded feeding electrodes arranged in a circle in accordance with the invention, wherein the number of the electrodes is defined so that a magnetic component signal of a normal field, i.e. the field produced by the country m
Balashov Boris Petrovich
Mogilatov Vladimir Sergeevich
Sachenko Georgy Vasilievich
Sekachev Mikhail Jurievich
Zakharkin Alexandr Kuzmich
Ostrolenk Faber Gerb & Soffen, LLP
Snow Walter
Tovarischesivo S Ogranichennoi Oivetsivennostiju Nauchotekhniche
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