Trapped water bottom multiple and peg-leg multiple...

Details Trapped water bottom multiple and peg-leg multiple... Trapped water bottom multiple and peg-leg multiple...

2002-05-22

2004-01-13

Moskowitz, Nelson (Department: 3663)

Communications, electrical: acoustic wave systems and devices

Seismic prospecting

Offshore prospecting

C367S022000, C367S026000, C181S112000, C702S013000, C702S014000, C702S017000

Reexamination Certificate

active

06678207

ABSTRACT:

BACKGROUND OF THE INVENTION
Marine seismic exploration usually involves acquiring seismic data using a seismic acquisition system whose source initiates a down-going seismic wavefield. A portion of the down-going wavefield travels into the underlying earth where it illuminates subsea geologic formations. As it illuminates the interfaces or boundaries between the formations, part of the wavefield is returned (or reflected) back through the earth (propagating in the up-going direction). Part of the reflected wavefield is received by the seismic acquisition system, converted into electrical signals, and recorded for subsequent processing. An analysis of these recorded signals makes it possible to estimate the structure, position, and lithology of subsea geologic formations, thereby completing an important step in the exploration process.
FIG. 1
shows a simplified example of a typical marine seismic acquisition system. A first ship
1
tows a seismic source
2
several feet below the surface
3
of the ocean. The seismic source
2
is activated to produce a down-going wavefield
4
d
that is at least partially reflected by a subsea formation boundary
5
or subsea impedance discontinuity. The up-going wavefield
4
u
then travels toward the sensors
6
and is detected.
The sensors
6
used in marine seismic exploration include pressure sensors and velocity (also referred to as “particle velocity”) sensors. Typically, the pressure sensors are hydrophones and the velocity sensors are geophones. The hydrophones measure a scalar pressure and are not sensitive to the propagation direction of the wavefield. The geophones, which may be vertical geophones, provide a vector response measurement whose polarity depends on whether the direction of propagation of the wavefield is up-going or down-going. The amplitude of the geophone response is also related to an angle of the propagation relative to the sensitive direction of the geophone. If a wavefield is recorded by a hydrophone and a geophone with similar electronic impulse responses, then a polarity comparison between the hydrophone and geophone measurement determines whether the wavefield is propagating in the up-going or down-going direction. Hydrophones and geophones disposed at the seafloor are typically used in pairs when collecting seismic data. A combination of this two component or “dual sensor” data (pressure and particle velocity) has been useful to cancel down-going multiples from a combined pressure and vertical velocity data signal. The importance of this aspect of the sensor pairing will be discussed in detail below.
In one type of marine seismic surveying, the sensors
6
are located at regular intervals in ocean bottom cables (OBC)
7
that are arranged on the seafloor
9
. When necessary, a second ship
8
is used to move the OBC
7
to a new position on the seafloor
9
. Several miles of OBC
7
are typically deployed along the seafloor
9
, and several OBCs are typically deployed in parallel arrangements. OBC
7
arrangements are particularly well suited for use in certain zones (such as zones cluttered with platforms or where the water is very shallow) where the use of ship-towed hydrophone arrays (not shown) (which are located proximate the ocean surface
3
and are typically referred to as “streamers”) is not practical.
The collection of data with OBC during seismic data acquisition is complicated by secondary wavefields, also known as “multiples.” Multiples comprise trapped water bottom multiples, source side peg-leg multiples, and receiver side peg-leg multiples. Multiples can mask the seismic data of interest, and they amplify and attenuate certain frequencies, thereby complicating the analysis of the recorded signals. The “multiple problem” is caused by, among other factors, the air-water interface at the surface of the ocean or water column. The following discussion provides a more detailed description while implicitly assuming one dimensional geometry.
When the seismic source is fired, the direct arriving down-going wavefield impacts the seafloor. A portion of the down-going wavefield travels into the subsurface and provides the primary seismic data by reflecting off of subsurface formations. Another portion of the same down-going wavefield is reflected back into the water column. This up-going wavefield travels back to the ocean surface and is reflected back in a down-going direction. A down-going wavefield reflected off of the ocean surface may be referred to as a “ghost.” A ghost subsequently impacts the seafloor where, as for the direct arriving down-going wavefield, a portion travels into the subsurface and a portion is reflected back into the water column to generate subsequent ghosts. Hence, some portion of a ghost is reflected back into the water column and remains trapped in the water column, forming the trapped water bottom multiple (or subsequent multiple ghost arrivals), while the remainder propagates into the subsurface, leading to the formation of delayed and scaled copies of the primary seismic data (referred to as source side peg-leg multiples) as the delayed down-going wavefield reflects off of the subsurface formations.
Up-going wavefields from the subsurface result from the portion of the direct arrival that initially travels into the subsurface (the primary reflection) and the subsequent source side peg-leg multiples that pass through the seafloor. The up-going wavefields will be recorded at the seafloor. However, after being recorded, the up-going wavefields continue upward and subsequently impact the air-water interface and are reflected in a down-going direction. As a result, the primary and source side peg-leg multiples form down-going ghosts. The water trapped portions of these wavefields are called the receiver side peg-leg multiples, and the portion of these wavefields that travel into the subsurface are ignored for the purposes of this discussion because they contain higher order subsurface reflections than are relevant for the analysis presented herein.
FIG. 2
shows two-dimensional examples of wavefields that are produced by a source
10
and are detected by a sensor pair
11
. The source
10
is typically located proximate the ocean surface
12
. A direct arrival
18
is a wavefield that travels directly from the source
10
to the sensor pair
11
. A receiver-side peg-leg
13
, which may also be referred to as a “receiver side multiple,” is produced when the wavefield is first reflected by a subsurface formation
16
and then by the ocean surface
12
before being detected by the sensor pair
11
. A source side peg-leg
15
, which may also be referred to as a “source side multiple,” is produced when the wavefield reflects off of the seafloor
14
, off of the ocean surface
12
, and then off of a subsurface formation
16
before being detected by the sensor pair
11
. These wavefields differ from a primary wavefield
17
that reflects off of the target formation
16
and is then detected by the sensor pair
11
before experiencing any additional reflections. The water trapped multiple
19
is first reflected off the seafloor and then off the ocean surface before being detected by the sensor pair
11
. For all of these multiples, there may be many reverberations in the water column, but no more than one two-way travel path in the subsurface (for the water trapped multiple
19
, there is no travel path in the subsurface). Detection and proper processing of the primary wavefield
17
is an important objective in seismic exploration. The primary wavefield
17
may be corrupted by the multiples that may also be detected by the sensor pair
11
.
The elimination of multiples can be an important part of obtaining good OBC data because, unlike a towed streamer where surface multiples produce notches in the frequency spectrum that lie beyond the usable bandwidth of the seismic energy, multiples in the OBC data produce notches within the usable bandwidth. The effectiveness of the removal of the multiples is dependent upon how well the pressure and velocity data are matched, so that up-goin

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