Acoustics – Geophysical or subsurface exploration – Seismic wave generation
Patent
1999-05-04
2000-08-22
Oda, Christine K.
Acoustics
Geophysical or subsurface exploration
Seismic wave generation
181121, 367 75, G01V 104
Patent
active
061057126
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns seismic wave generation in earth and is particularly adapted for generation of seismic intersection waves in earth sediments in a marine environment.
Exact determination of the extent of hydrocarbon-reservoirs and the structure in and around such a reservoir, is highly significant for increasing the rate of exploitation. To provide such information, it is necessary to have a method for transferring strong seismic intersection and pressure waves down into the earth. These waves will propagate downwards through the sediments to be reflected and transmitted in different directions due to changes in the physical properties of the rock types. Reflections are registered by one or more receivers situated in the earth or in the water.
It is relatively easy to transfer intersection and pressure waves downwards in formations below the mainland. However, in formations located under the sea there are completely different problems involved in transferring sufficiently strong intersection waves downwards into the formations during a certain period, for collecting exact information about the formation properties.
Conventional marine seismic investigations generate pressure waves (compression waves) in the sea, for example by means of a pneumatic cannon.
The conventional methods give a structural image of the earth and an estimate of the velocity of the compression wave for the different layers. However, no good quantitative indication for the fluid content is achieved by conventional techniques. This is because the velocity of the compression wave does not define a rock type uniquely. The relationship between the velocities of a compression wave and an intersection wave is a much better indicator of rock type.
If the impact velocity is known, the fluid contents and elastic parameters in the rock type can be determined by analysing the change in the reflection coefficient with varying distance between the source and the receiver.
In some formations, such as broken reservoirs, measurement of a compression wave will not show substantial parts of the reservoir, because gas above the reservoir will suppress the amplitudes and in addition lowers the velocity of the compression wave. A measurement of an intersection wave will not, however, be affected by this gas to the same degree, so that an opportunity of better mapping of the reservoir is given.
In addition to the conventional techniques a number of solutions have been proposed, involving transfer of intersection waves in order to overcome the weakness of the first mentioned methods.
Intersection waves can be generated indirectly in a marine environment by utilizing the converted energy of the intersection waves, which emerges when a compression wave hits a formation border with an angle of incidence that is not zero degrees. The generated intersection and compression waves can be registered by geophones placed in a well hole or on the surface. A problem with this method is that it is difficult to know where the wave conversion has taken place.
U.S. Pat. No. 4,867,096 describes a device for generating primary-elliptical polarized intersection waves. This device is meant to be suitable on a soft land soil or in marine conditions. The marine embodiment of the device comprises, in addition to the source of the intersection wave, a "rod" having a conical end comprising a transmission element to the rock. It is, however, clear from the description and drawing that said rod only to a limited degree is able to penetrate into the formation. Among other things this is related to what force is used to press or hit the rod down into the formation, and dimensioning and construction of the rod. For many subsea applications, the equipment described in U.S. Pat. No. 4,867,096 will be insufficient to give a good, durable anchoring of the transmission element to the seabed, which is a condition for transmitting strong intersection waves.
U.S. Pat. No. 5,128,900 and U.S. Pat. No. 5,128,906 describe corresponding devices for the generation of elliptic polarized
REFERENCES:
patent: 4310066 (1982-01-01), Won
patent: 4867096 (1989-09-01), Cole
patent: 5128906 (1992-07-01), Cole
Lieng Jon Tore
Tjaaland Egil
Jolly Anthony
Leiv Eiriksson Nyfotek AS
Oda Christine K.
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