Moving source dipole electromagnetic exploration device for...

Details Moving source dipole electromagnetic exploration device for... Moving source dipole electromagnetic exploration device for...

2002-03-26

2004-04-13

Decady, Albert (Department: 2133)

Electricity: measuring and testing

Of geophysical surface or subsurface in situ

With radiant energy or nonconductive-type transmitter

C702S007000

Reexamination Certificate

active

06720771

ABSTRACT:

FIELD OF INVENTION
The present invention relates to moving dipole source electromagnetic induction device for deeper and poorer electrically conducting subsurface bodies. The present invention also relates to a method for detecting deeper and poorer electrically conducting subsurface bodies.
BACKGROUND AND PRIOR ART REFERENCES
The constant search for new mineral deposits have become important to our civilization as the demand for metals, petroleum, and water increase with increasing industrialization. Geophysical methods, which take advantage of the various physical properties of the earth's material, have a well-established place in this under-ground exploration. Electrical conductivity, a very wide-ranging physical property of 10
−14
to 10
8
Sm
−1
, has been exploited in electromagnetic methods of exploration. Electromagnetic methods fall into two categories:
(i) natural field methods, and
(ii) controlled source methods
The first method relies on source fields generated by ionosphere and magnetotelluric current or spontaneous current associated with electrochemical activity of the earth's material. As the source distribution in the natural field methods is unknown, the number of variables increases. In the second method artificial field can be created by means of direct, alternating or pulsed current. The energizing source is under control of user and can be exploited to increase the resolution of data by using variable frequencies. Artificial fields can be applied conductivity (through electrodes) or inductivity (by means of coils or large wire loops). Finally, in both the methods different parameters of the resultant electric or electromagnetic field are measured. Electromagnetic induction methods are based on the well known principle of electrical induction i.e., a time varying primary field inducing electric currents in conductors. The secondary currents flow in such a way that the resulting electromagnetic field opposes the primary inducing field. The two fields will have the same frequency but will generally differ in direction, magnitude and phase with resultant field being elliptically polarized. A number of exploration systems have been conceived on the basis of geometry of plane of polarization. The secondary electromagnetic field is commonly termed in geophysical literature as the response or the anomaly due to the target. Its magnitude and variation in space and time also comprises the basis of some of electromagnetic systems. The anomaly characteristics are suitably interpreted in terms of geological and geometrical characteristics of the causative body.
In mineral prospecting, inductive electromagnetic methods have attained greater popularity than conductive electromagnetic methods as the former react to absolute conductivity, rather than conductivity contrast and can distinguish between highly and moderately conductive target. Also, in large regions of permafrost, deserts or arid tracts where conductive contact is not possible due to high resistivity layer on the surface inductive methods can be employed conveniently. An additional advantage of them over conductive methods is that they do not require electrical contact with the ground and thus can be moved rapidly over earth's surface and also adopted in airborne reconnaissance survey.
Usually inductive electromagnetic methods are further classified according to the type of energizing source, the receiving system employed and the quantity measured. The methods the distance between source and target is variable, giving rise to larger anomalies. Portability requirements dictate that in most instances the source should be three-dimensional (3D). On the other hand, in fixed source methods, the distance between source and receiver is variable and is fixed between source and target. The power of the source can be increased considerably which is not possible beyond a certain limit with the moving source methods.
Artificial moving source inductive Electromagnetic (EM) methods in frequency domain are widely employed as they can be used in field prospecting both on ground and airborne survey.
Most commonly used transmitter [T]—receiver [R] coil configurations are: (1) Horizontal—Horizontal [Horizontal Coplanar]; (2) Vertical—Vertical—[Vertical Coplanar]; (3) Vertical Co-axial and (4) Horizontal—Vertical—[cross coupled]. Horizontal Coplanar method has achieved steadily increasing popularity since its introduction in the late 1930s. This method, also known as Slingram, employs a variable frequency (100 Hz-4000 Hz) and separation (20 m-200 m). A systematic nomenclature for various moving transmitter—receiver configurations is given by Parasnis (1970). The quantities measured are inphase (IP) and quadrature (OP) (90° out of phase) components of the anomaly vector resolved in time phase with respect to the primary field.
Phase measuring techniques carry other advantages besides improved accuracy in locating anomalies. Firstly phase difference between the primary and resultant field is essentially a target conductivity phenomenon, which is not affected by geometric irregularities in the primary field. Secondly, the phase provides a clue to the conductivity of the target. Because of essential simplicity of instrumentation, operation and the progress in electronics in phase measuring techniques this method received further impetus both in direct application from air and in subsequent ground follow up. In a field survey the profile directions are laid perpendicular to the strike of the conductor. Two men crew is sufficient to conduct a survey. Survey can be done in INLINE configuration and BROADSIDE configuration.
The Transmitter [T] and Receiver [R] are moved in tandem along the profile direction in inline configuration. In this configuration T and R straddle the target one after other. In Broadside configuration the T and R are held parallel to the strike of the conductor and perpendicular to profile direction.
In frequency domain several EM mineral prospecting moving source-receiver systems have been fruitfully used for nearly six decades to detect shallow massive sulfide ore bodies. However, the ratio of the feeble response from the target to the strong primary field (which is the parameter measured with these systems) is very small in case of deeper and poorer conducting bodies.
Time domain EM methods were, therefore, developed wherein a repetitive pulsed primary field energizes the earth and the transient secondary field is measured while the primary field is zero. However, these methods require more intense primary field and the circuit design for transient signals is more difficult than for sinusoidal signals.
Frequency domain methods, however, are more suitable for faster reconnaissance and detail, are lightweight, portable, cheaper units and probably produce better anomaly resolution. They also have better developed data interpretation. The depth of exploration for all moving dipolar source methods is only 0.6-0.8 L (L=T−R SEPARATION).
OBJECTS OF THE INVENTION
The primary object of the present invention is to provide a moving dipole source electromagnetic induction device for deeper and poorer electrically conducting subsurface bodies.
Another object of the present invention is to provide a method for detecting deeper and poorer electrically conducting subsurface bodies.
Yet another object of the present invention is to provide a device for exploration of mineral, ground water, archeology and mapping of subsurface geology.
Yet another object of the present invention is to provide a device with transmitter coil placed strategically that is not affected by the primary field and measures only the secondary field.
SUMMARY OF THE INVENTION
The present invention relates to moving dipole source electromagnetic induction device for deeper and poorer electrically conducting subsurface bodies. The present invention also relates to a method for detecting deeper and poorer electrically conducting subsurface bodies.


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