Wells – Processes – With indicating – testing – measuring or locating
Reexamination Certificate
2002-12-30
2004-03-30
Schoeppel, Roger (Department: 3672)
Wells
Processes
With indicating, testing, measuring or locating
C166S254200, C702S008000, C324S346000, C073S152350, C342S153000
Reexamination Certificate
active
06712140
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention, according to a first aspect thereof, relates to a borehole radar tool for directionally sensitively locating transitions in the subsurface surrounding the borehole radar tool in use, comprising a generating assembly for generating the electromagnetic radiation having a frequency of between 10 MHz and 200 MHz, signal processing means for processing the received electromagnetic radiation, and a housing having a substantially cylindrical wall and a central axis which accommodates at least:
a transmitting antenna assembly for emitting the electromagnetic radiation generated by the generating assembly, comprising a transmitting antenna and an electroconductive reflector; and
a receiving antenna assembly for receiving the electromagnetic radiation reflected by the surrounding subsurface, comprising a receiving antenna and an electroconductive reflector,
wherein both the transmitting antenna and the receiving antenna extend parallel to the axis of the housing and both comprise a dipole antenna, each reflector comprising a reflective surface which extends abreast of and parallel to the respective dipole antenna at a distance therefrom, and the space between each reflective surface and that section of the housing which in each case is located opposite thereto being filled with a medium having a dielectric constant of at least 10.
2. Background Art
A borehole radar tool of this type is described in WO-A-89/03053. This publication describes a radar instrument for use in a borehole for locating fractures in a geological formation, comprising directionally sensitive transmitting and receiving antennae. Emitting means generate a pulsed radar signal with an output in the frequency range of between 30 and 300 MHz. This signal is radiated via a reflector, in particular a reflector consisting of two electroconductive plates arranged in the shape of a V. A receiving antenna, provided with a similar reflector, is used to intercept the radar signal reflected by the formation. The space through which the radiation propagates within the housing is filled with a barium titanate powder/air mixture, whose dielectric constant matches that of the surroundings and in particular has a value of about 80.
Using a borehole radar tool of this type, it is possible to achieve a certain directional sensitivity. With many applications there is a need for both the highest possible directional sensitivity and for as much radiation as possible penetrating into the subsurface. Given the constraints to which borehole radar tools are subject, including a preferred diameter of at most 20 cm and the electromagnetic characteristics of the subsurface to be surveyed, it has so far not proved possible to increase the directional sensitivity.
SUMMARY OF THE INVENTION
The object of the first aspect of the present invention is to provide a solution to the abovementioned problem, said aspect being characterized, to this end, in that the transmitting antenna and receiving antenna extend near the wall of the housing and in that at least that section of the reflector which is located diametrically opposite the transmitting antenna and receiving antenna, respectively, likewise extends near the wall of the housing.
Using a borehole radar tool of this type, it is possible to achieve a higher directional sensitivity in conjunction with high power of radiation penetrating into the subsurface. Two variables have to be considered in this context. The first variable is the ratio between the emitted power in the target direction and the emitted power in the other direction(s). This ratio should be as large as possible. The second variable is the total power emitted in the target direction. Obviously, this should be as large as possible. This is desirable to achieve the highest possible penetrative power. Thus a good image of the subsurface can be obtained with as small a number as possible of expensive boreholes.
A configuration according to the first aspect of the invention was found to function satisfactorily. In such an arrangement, the reflector should have a certain surface area. In practice, optimal dimensions exist for the reflector, which predominantly depend on the constrained dimensions of the housing.
The first aspect of the invention provides, in addition, for the distance from the transmitting and/or the receiving antenna to the reflective surface to be as large as possible. This is achieved by means of a borehole radar tool according to the first aspect of the invention. The term “distance” in this context refers to the mean distance between the reflective surface and the transmitting or receiving antenna.
The term “near the wall of the housing” in this context means that the relevant section of the reflector, or the centre of the transmitting and/or the receiving antenna, respectively, is located at a distance from the inner wall of the housing which is at most a quarter of the inside diameter of the housing. In particular, the distance between the inner wall of the housing and the relevant section of the reflector, or the transmitting and/or the receiving antenna, respectively, is at most 2 cm, advantageously less than 1 cm. Thus, as large a distance as possible between antenna and reflective surface can be achieved in a simple manner. In special conditions, for example at very high frequencies and when the apparatus is filled with a dielectric having a very high dielectric constant, the distance can alternatively be greater than 2 cm.
In a particular embodiment, at least one of the reflectors forms part of the wall of the housing. Thus it is possible for the dimensions of the housing, i.e. of the borehole, to be optimally utilized to the advantage of the reflector. For example, the housing is fashioned as a cylinder from a nonconductive material, for example plastic, part of the cylinder being formed by the reflector section which is fabricated from metal, for example.
In another embodiment, at least one of the reflectors comprises a thin plate. In this embodiment, a narrow space exists between the reflector and the housing, said space accommodating, for example, the cabling to the transmitting and/or receiving antenna. It is important for said cabling to be screened against electromagnetic radiation present in the space between the reflective surface and that section of the wall of the housing which is situated opposite thereto. The reflector can serve as such a screen.
The shape of the reflector, and in particular of the reflective surface, is not subject to any particular constraint. Suitable, for example, are two straight plates touching one another at one end, each end of both plates being situated near the wall of the housing. This affords a V-shaped reflector.
Advantageously, at least one reflective surface is a substantially smoothly curved surface which, as seen in the axial direction of the housing, at least substantially forms part of a conic. Giving the reflective surface such a shape makes it possible to ensure that the reflective surface at least largely follows the shape of the wall of the housing, thus making the mean distance from the reflector to the transmitting and/or receiving antenna as large as possible.
The term “substantially smoothly curved” in this context means that it is acceptable for the reflective surface to exhibit deviations in its shape whose dimensions are much smaller than the wavelength used, in particular at most {fraction (1/10)} of the wavelength of the electromagnetic radiation. Owing to the wave properties of the electromagnetic radiation, the reflective surface will then still appear to be smoothly curved. Examples of such deviations in shape are holes for wiring, or bending edges in a faceted reflector.
The conic is preferably chosen to be as advantageous as possible. Advantageously, the conic is a circle having a radius which is substantially equal to half the inside diameter of the housing. In this way, it is ensured that the mean distance between the reflective surface and the transmitting and/or rece
Budko Neil Vladimirovich
Fokkema Jacob Tjeerd
Remis Robert Frans
van den Berg Petrus Maria
van Dongen Koen Willem Anton
Brooks & Kushman P.C.
Schoeppel Roger
T & A Survey B.V.
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