Marine walkaway vertical seismic profiling

Walkaway vertical seismic profiling is conducted in a body of water. A marine seismic cable is positioned substantially vertically in a body of water. The cable has a number of hydrophones spaced along its length. A seismic source is moved along a line of traverse past the seismic cable and is periodically activated to emit acoustic waves. The response of the hydrophones to reflection of the acoustic waves from subsurface formations is then recorded for processing and analysis.

BACKGROUND OF THE INVENTION 
1. Field of Invention: 
The present invention relates to seismic surveying, namely marine walkaway 
vertical seismic profiling. 
2. Description of Prior Art: 
Walkaway vertical seismic profiling, or walkaway VSP, is a known technique 
of seismic surveying. It involves taking seismic data readings with a 
hydrophone at each one of various depths in a well borehole as a seismic 
source emits seismic waves at a number of shot-points along a seismic line 
of profile. Although the data obtained were useful, the technique has not 
been widely used. It could only be performed at locations where a well 
borehole had already been drilled. Additionally, VSP data acquisition was 
expensive since it required the seismic source be moved to each shot point 
along the entire line of profile several times to emit waves, once for 
each different hydrophone depth of interest in the well borehole. 
SUMMARY OF INVENTION 
Briefly, the present invention provides a new and improved method and 
apparatus for marine walkaway vertical seismic profiling. A marine seismic 
cable having plural hydrophones is located in a substantially vertical 
position in a body of water. A seismic source is then moved by being towed 
behind a marine vehicle along a traverse or line of profile past the 
seismic cable. As the source moves along the line of profile, it 
periodically emits seismic waves. The hydrophones in the cable sense the 
response of subsurface formations beneath the body of water to the emitted 
seismic waves. The responses or seismic data sensed by the hydrophones are 
sent by a transmitter attached to the cable. The transmitter is part of a 
digital telemetry system, the receiver of which is on the marine vessel. 
Once received, the seismic data are recorded so that they can be processed 
and analyzed.

DESCRIPTION OF PREFERRED EMBODIMENT 
In the drawings, the letter C designates generally a marine seismic cable 
for walkaway vertical seismic profiling according to the present 
invention. The cable C is deployed in a substantially vertical position in 
a body of water 10, at least between an anchor mechanism A on a floor 12 
of the body of water and a buoyant sphere 14 mounted with the cable C. The 
anchor A preferably is in the form of a suitable number of connected heavy 
chain links attached to the seismic cable C at a lower end 16. The chain 
links permit the cable C to be towed in the water to a new survey site 
without tangling with objects on the marine floor 12. 
The buoyant sphere 14 is filled with a gas (usually air) and is of a size 
(two and one-half to three feet in diameter, for example) to displace 
sufficient water to maintain an adequate lifting force on the seismic 
cable C between an intermediate portion 18 where the sphere 14 is located 
and the lower end 16 to maintain a lower portion 20 in a substantially 
vertical position. The cable C serves a dual function, both as a mooring 
member and as a conductor of electrical signals. The cable C is not oil 
filled nor insulation jacketed. 
The cable C is formed of a suitable number of stranded electrical 
conductors, based on the number of signals to be acquired, insulated with 
a coating of a suitable synthetic resin, such as an ethylene propylene 
copolymer. The conductors are formed in twisted pairs and coated with a 
synthetic resin such as polyurethane for mechanical protection. 
Applied over the core of conductors is a jacket of synthetic fiber braid 
which serves to produce low stretch in the cable C and to provide strength 
thereto. Generally a nominal breaking strength of over 10,000 pounds is 
desirable. Applied over the braid is a thin jacket of a suitable polyester 
to which are attached a suitable number of rows of fringe-type fairing. 
The fairing eliminates low frequency cable strumming noise caused by the 
cable being moored vertically in the currents of the body of water 10. 
A number of hydrophones H are mounted within the cable C in the lower 
portion 20 at spaced positions (100 feet, for example) along its length. 
Each of the hydrophones H is attached to a conductor pair in the cable C 
by bringing the conductor pair through the fiber braid and polyester 
jacket at the desired location. With suitable seals, the conductor pair is 
connected to the hydrophone. The hydrophones H are then installed in 
open-ended cylindrical sleeves which protect the hydrophones H from damage 
when the cable C is being deployed, retrieved or stored. The cylindrical 
sleeves with the hydrophones H are then secured to the cable C by tape or 
some other suitable attaching mechanism. 
The hydrophones H are thus in a substantially vertical array and spaced 
from each other. The hydrophones H sense the response of subsurface earth 
formations beneath the marine floor 12 to seismic waves emitted from a 
seismic source 22, such as an air gun, towed behind a seismic exploration 
vessel 24. The hydrophones H convert the sensed responses to electrical 
signals in the form of seismic data. 
Each of the hydrophones H is individually connected by an electrical 
conductor extending within the cable C to provide input data to a 
transmitter portion 26 of a conventional digital seismic telemetry system 
mounted with a surface buoy 28. The data so provided are then sent by a 
transmitting antenna 30 mounted on the surface buoy 28 to a receiving 
antenna 32 on the seismic exploration vessel 24 for provision to a 
receiver portion of the digital telemetry system for recording. The 
received seismic data is then recorded in the known manner for processing 
and analysis. The data may also be plotted and inspected as the survey is 
being performed, if desired. 
In addition to the buoyant sphere 14, a number of smaller buoyancy floats 
34 are mounted with the seismic cable C at spaced positions above the 
hydrophones H and below the surface buoy 28 to assist in controlling the 
buoyancy of the cable C. As can be seen in the drawings, the surface buoy 
28 may be moved by wind and wave action at the surface from a location 
directly above the vertical cable C. However, the buoyancy sphere 14 and 
the anchor mechanism A maintain at least the lower portion 20 of the cable 
C containing the spaced hydrophones H in a substantially vertical position 
in the body of water 10. 
In conducting marine walkaway vertical seismic profiling according to the 
present invention, the cable C is deployed in the body of water 10 with 
the anchor mechanism A on the marine floor 12 beneath a mid-point, 
indicated at 50, of a seismic line of profile which begins at a shot point 
52 and ends at a shot point 54. The buoyancy sphere 14, with the aid of 
the buoyancy floats 34, maintains at least the lower portion 20 of the 
cable C in a substantially vertical position in the body of water 10. 
The seismic vessel 24 then begins towing the seismic source 22 along the 
seismic line of profile at a substantially constant speed. The source 22 
is then activated to emit seismic waves at periodic intervals 
corresponding to a suitable shot point spacing 56. An example shot point 
spacing could be, for example, 25 meters. The hydrophones H in the 
vertical seismic cable portion 20 sense the response of the subsurface 
formations to the emitted seismic waves and form data signals which are 
sent by the digital seismic telemetry system from the transmitting antenna 
30 to the receiving antenna 32 on the vessel 24 for recording for 
subsequent processing and analysis. If desired, many parallel seismic 
lines of profiles can be recorded, to obtain three-dimensional imaging of 
the subsurface. 
FIG. 2 in the drawings shows a group of adjacent shot records obtained 
according to the present invention after the data have been subjected to 
convention bandpass filtering and automatic gain control. It is to be 
noted that the records exhibit the typical crisscross patterns of upgoing 
and downgoing waves of vertical seismic profiling records. There are 
certain anomalous noise bursts in the data of FIG. 2 which are indicated 
in circled portions designated 60 which were caused by an interfering 
seismic boat within the area where marine walkaway vertical seismic 
profiling according to the present invention was being performed. 
FIG. 3 in the drawings is a hydrophone gather of a lowermost hydrophone in 
a cable C according to the present invention at a depth of approximately 
1100 feet in a body of water. The gather of FIG. 3 is composed of 
approximately 400 traces, one from each shot point at a spacing of 25 
meters along a seismic line of profile of ten kilometers traversed by the 
seismic exploration vessel 24. 
FIG. 4 shows the result of processing the hydrophone gather of FIG. 3 by a 
conventional migration process appropriate for walkaway VSP profiling. 
Migrated sections of the type of FIG. 4 were obtained for each of twelve 
hydrophones H in a vertical seismic cable C according to the present 
invention. The twelve migrated sections were then stacked. FIG. 5 shows 
the stacked final image of the twelve migrated hydrophone gathers of the 
type of FIG. 4. The stack of data in this manner has the effect of 
cancelling interfering waves and of increasing the signal-to-noise ratio 
in the data. As can be seen, FIG. 5 provides an accurate, high-quality 
image of the subsurface earth features beneath a body of water. 
The foregoing disclosure and description of the invention are illustrative 
and explanatory thereof, and various changes in the size, shape and 
materials, as well as in the details of the illustrated construction may 
be made without departing from the spirit of the invention.