Timed shooting with a dynamic delay

Details Timed shooting with a dynamic delay Timed shooting with a dynamic delay

2001-03-13

2002-11-19

Toatley, Jr., Gregory J. (Department: 2838)

Communications, electrical: acoustic wave systems and devices

Seismic prospecting

Offshore prospecting

C181S107000

Reexamination Certificate

active

06483774

ABSTRACT:

FIELD OF THE INVENTION
This method is directed to marine seismic data acquisition systems and, in one aspect, such systems with a streamer, streamers, and/or one or more seismic sources towed by one or more powered vehicles. This invention is related to the field of Common Mid-Point (CMP) marine seismic data acquisition.
BACKGROUND OF THE INVENTION
In the field of marine seismic data acquisition, seismic signal sources, e.g. air guns, are towed behind a vessel, which may also tow a set of seismic sensors. The sensors are normally contained in streamers that are comprised of seismic signal receivers, e.g. hydrophones, which are sensitive to reflections and refractions from sound impulses emitted by seismic sources.
It has long been known that for high quality data, multiple samplings of the same subsurface areas are required. The addition of multiple signals from the same subsurface reflection point results in an increase in the information in signals along with a cancellation of the noise. This process is commonly known as the Common Mid-Point method.
FIG. 1A
shows schematically a marine seismic survey system. A ship
14
tows a seismic source
5
of any known type, for instance air guns. Activating or firing a seismic source initiates acoustic energy which propagates from the source through the water and the earth. Also towed by ship
15
in this example are one or more sets of seismic sensors
13
or seismic receivers usually referred to generally as streamers. Acoustic energy propagated through the earth from subterranean features and may be detected by seismic sensors. The seismic sensors in the streamers may be single sensors or groups of sensors. The same vessel often tows streamers and sources, but this varies with survey acquisition logistics. Streamer
13
contains, as an example for the illustration,
15
sensors or groups of sensors; sensor
11
is closest to source
5
while sensor
19
is furthest away. Modem streamers may contain hundreds of sensors or sensor groups. Modem seismic acquisition systems may contain many streamers and several sources all in operation to acquire data in an area.
FIG. 1B
shows how seismic energy propagates from the seismic source
5
to the streamer
13
sensors. Each time a seismic impulse is generated by a source, there is a corresponding midpoint (or Common Mid-Point, CMP) for every seismic sensor in a streamer. Seismic energy generated by source
5
travels to midpoint or CMP
1
and is reflected back to seismic sensor
11
, the nearest sensor to source
5
of streamer
13
. Seismic energy also reflects off of CMP
2
and travels to seismic sensor
19
, the furthest sensor from source
5
of streamer
13
. Of course, there are midpoints or CMPs corresponding to each seismic source impulse with each of the streamer
13
sensors. For example, the streamer shown in
FIG. 1B
contains 15 seismic sensors, and so each seismic impulse generated by source
5
would produce 15 CMPs from CMP
1
to CMP
2
inclusive.
FIG. 1C
shows how the addition of another seismic source
7
towed, for example by ship
18
, will produce CMPs from a source following the acquisition system in the direction of survey as demonstrated in
FIGS. 1A and 1B
. Here, the seismic energy travels from seismic source
7
to CMP
4
and back to the seismic sensor
19
nearest the source
7
. Seismic energy travels from source
7
to CMP
3
and then the seismic sensor
11
furthest from source
7
. Again, there are CMPs for each seismic sensor and seismic source pair, each time a source generates a seismic impulse.
As is known in the art, the number of times the same subsurface mid-point areas are sampled is commonly referred to as the “fold” of the survey. A uniform distribution of fold over a survey area is desirable. A method or system that will lead to efficient acquisition of seismic surveys having a uniform distribution of fold is desirable. Uniformity in fold distribution results in a more even and predictable subsurface resolution, and therefore, seismic surveys with a more uniform fold distribution are desirable and operational changes that result in a haphazard or unpredictable fold distribution are not desirable.
Another way to improve subsurface resolution is to increase the areal resolution of a seismic survey by moving sampling bins closer together by decreasing the size of the subsurface sampling areas known as midpoint or CMP bins. Subsurface sampling areas (e.g. CMP spacing) need to be made as small as operational and economic constraints will allow.
The recording systems used for marine seismic data acquisition normally allow firing of shots based on distance, the Distance Shooting method, or based on time, the Timed Shooting method. For geophysical reasons (for instance, fold uniformity) the preferred option is to fire shots based on distance. However, when currents in the seismic survey area are too strong the Distance Shooting method may be problematic to implement and up to half of the expected shots may be missed. Under strong current conditions the timing method may be preferred. Recording systems are set up to handle either the ‘distance shooting method’ or the ‘timed shooting method,’ but not both at the same time. Recording systems must normally not be acquiring data for a switch between one system and the other to be made. This results in an interruption of the survey acquisition program and can be an inefficient use of resources.
Marine seismic operations are often large, complex and expensive. There are recognized needs for improving the efficiency of seismic field operations. A system and method that will increase survey efficiency and streamline operational logistics is highly desirable. The present invention addresses these recognized needs.
Accordingly, there is a need for a method and system to acquire seismic data efficiently without having to interrupt field operations to change from one acquisition method, for example based on timed shooting, to another method, for example based on distance or position shooting. The prior art does not disclose systems or methods to improve the efficiency of the use of acquisition equipment as embodied in the present invention.
SUMMARY OF THE INVENTION
The present invention, in certain embodiments, discloses a method for acquiring marine seismic data using a seismic acquisition system where shot points are determined and shot records recorded. A time at which the source vessel will be substantially at a shot point position is predicted and the seismic source is activated at the shot point. The shot is recorded and the next location for recording a shot is determined. If the source vessel has passed the determined next shot point position, the seismic source is activated and a shot is recorded. If the source vessel has not reached the next shot point, a time delay is implemented until the shot point is reached. The method allows for efficient acquisition of seismic data under varying field conditions with minimal operational interruptions by acquiring data using a distance based shot point determination method unless a time based acquisition method becomes more appropriate due to field conditions or other operational considerations.


REFERENCES:
patent: 4300653 (1981-11-01), Cao et al.
patent: 4511999 (1985-04-01), Bowden et al.
patent: 5650981 (1997-07-01), Jacobsen et al.
patent: 5724241 (1998-03-01), Wood et al.
patent: 6002640 (1999-12-01), Harmon
patent: 6327537 (2001-12-01), Ikelle

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