Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Offshore prospecting
Patent
1987-10-02
1989-06-27
Tarcza, Thomas H.
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Offshore prospecting
367 87, 342 22, G01V 138
Patent
active
048435970
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Known methods for marinre seismic exploration are based on the emission of acoustic waves in the water from acoustic sources in the form of explosive charges, air guns and the like, whereby the corresponding acoustic signals from the depth are received by means of hydrophones, usually arranged ina seismic streamer which is towed behind a vessel.
This invention provides a completely new concept for such seismic explorations based on the merging of several more or less well-established fields of technology and science today, which gives a high degree of flexibility and makes it possible to design an optimum system for seismic explorations. Some main aspects of this new system are as follows:
New principles in radar technology involving matched radiation (illumination) make it possible to take advantage of apriori information regarding the object, i.e. the structures or subsurface layers under investigation. Based on such information the frequency bandwidth of the system can be narrowed and the sensitivity correspondingly increased. The aim of such a "self-learning" system is to have the appropriate seismic signatures stored in the transmitter unit processor and to structure this in such a way that only two bits of information is required: Is the particular structure present or is it not.
A specific aspect of the invention involves the fact that optimum illumination is achieved by coupling a set of sinusoidal acoustic waves to the wave propagation medium (i.e. water). By using matching networks in the form of resonant cavities, like what is known from microwave circuitry, there can be achieved a very high degree of matching over a narrow bandwidth.
Interaction mechanisms between acoustic and electromagnetic waves are well known in solid state physics and are utilized within electronics as a means of achieving high speed signal processing methods. As examples can be mentioned surface acoustic wave devices for implementation of filters and convolvers. Further can be mentioned opto-acoustic spectrum analysers based on surface acoustic waves in solids.
Thus, the present system is primarily designed for offshore oil and gas explorations and is based on the modern high-technology concept mentioned briefly above.
SUMMARY OF THE INVENTION
More specifically the invention relates to a system for marine seismic exploration of structures under the sea bottom, comprising a transmitter means with an array of acoustic transducers for synchronously and coherently generating one or more monochromatic acoustic waves in the water, and a receiver means for detecting acoustic waves reflected from these structures. What is novel and specific to the system according to the invention in the first place, consists therein that the receiver means comprises a radar system adapted to illuminate that area of the sea surface in which the reflected acoustic waves appear, in order to therein establish a number of interferograms each corresponding to the acoustic wave field from a scattering point in the structure given by the position of the point and the frequency of said field, and that said receiver means is adapted to determine the degree of coincidence between said interferograms and acoustic perturbation patterns affecting the electromagnetic reflection coefficient of the sea surface, thereby to obtain a recording of the seismic properties of said structures.
As will appear from the following description, much of the theory and explanations given apply both to electromagnetic waves (the radar part) and acoustic waves (the acoustic part) as these are incorporated in the new system described here.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention shall be explained more closely in the following with reference to the drawings, of which:
FIG. 1 illustrates the basic concept of the system according to the invention;
FIG. 2 schematically shows an acoustic source arrangement for providing a narrow multifrequency beam in the water, to be used in the system according to the invention;
FIG. 3 schematically shows
REFERENCES:
patent: 4146870 (1979-03-01), Ruehle
patent: 4583095 (1986-04-01), Peterson
Gjessing Dag K. T.
Hjelmstad Jens F.
Tonning Andreas
Lobo Ian J.
Tarcza Thomas H.
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