Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2003-10-10
2004-12-21
McElheny, Jr., Donald (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06834235
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This present invention relates to a method of processing seismic data, in particular to a method of processing multi-component marine seismic data in order to estimate properties of the seafloor and sensor calibration filters. It also relates to an apparatus for processing seismic data.
2. Description of the Related Art
FIG. 1
is a schematic illustration of one marine seismic surveying arrangement. In this arrangement, a seismic source
1
is towed through a water layer (in this case the sea) by a survey vessel
2
, and is caused to emit discrete pulses of seismic energy. The surveying arrangement includes a seismic sensor
3
, generally known as a “receiver”, for detecting seismic energy emitted by the source
1
. In
FIG. 1
the receiver
3
is disposed on the sea-bed. (A practical seismic surveying arrangement will generally include an array of more than one receiver; for example, in an Ocean Bottom Cable survey a plurality of receivers are attached to a support cable and the cable is then deployed on the sea-bed. However, the principles of a marine seismic surveying arrangement will be explained with reference to only one receiver, for ease of explanation.)
Seismic energy may travel from the source
1
to the receiver
3
along many paths. For example, seismic energy may travel direct from the source
1
to the receiver
3
, and this path is shown as path
4
in FIG.
1
. Path
4
is known as the “direct path”, and seismic energy that travels along the direct path
4
is known as the “direct wave”.
Another path of seismic energy from the source
1
to the receiver
3
involves a single reflection at a reflector
7
disposed within the earth, and this is shown as path
5
in FIG.
1
. (This path will also involve refraction at the sea-floor and at interfaces between different layers within the earth, but this has been omitted for clarity.) This path is known as the “primary path”, and seismic energy received at the receiver
3
along this path is known as the “primary reflection”. Only one reflector is shown in
FIG. 1
, but typical seismic data will contain primary reflections from many different reflectors within the earth.
Not all downwardly-propagating seismic energy that is incident on the sea-bed will pass into the earth's interior, and a proportion will be reflected upwards back into the sea. Furthermore, the source
1
may emit some upwardly-propagating seismic energy which will reach the receiver after undergoing reflection at the sea-surface. These effects give rise to seismic energy paths, for example such as paths
6
a
and
6
b
in
FIG. 1
, that involve more than pass through the water. These paths are known as “water layer multiple” paths.
The existence of many paths from the source
1
to the receiver
3
in a seismic surveying arrangement of the general type shown in
FIG. 1
complicates analysis of seismic data acquired by the receiver
3
. When seismic data acquired by the receiver
3
are analysed, it is necessary to distinguish events arising from a primary reflection, events arising from the direct wave and events arising from a water-layer multiple. In deep water there is generally a significant time delay between an event arising from the direct wave and an event arising from a water-layer multiple, but in shallow water an event arising from a water-layer multiple may occur very shortly after an event arising from the direct wave.
A further factor that complicates the analysis of seismic data acquired by the receiver
3
is that the properties of the earth are not uniform. In particular, there is frequently a layer
8
at or near the surface whose properties may well be significantly different from the properties of the underlying geological structure
9
(hereinafter referred to as the “basement”). This can occur if, for example, there is a layer at or near the earth's surface that is less consolidated than the basement. In particular, the velocity of seismic energy may be significantly lower in the surface or near-surface layer
8
than in the basement
9
, and such a surface or near-surface layer is thus generally known as a “low-velocity layer” (or LVL). This difference in velocity will produce a shift in the travel time of seismic energy compared to the travel time that would be recorded if the surface or near-surface layer and the basement had identical seismic properties, and these shifts in travel time are generally known as “static shifts”, or just “statics”.
The static shift generated by a surface or near-surface low-velocity layer
8
depends on the thickness of the layer, and on the velocity of propagation of seismic energy through the layer. Lateral variations usually occur in both the thickness of a low-velocity layer
5
and the propagation velocity through the layer, so that the static shift observed at a seismic receiver at one location is likely to be different from the static shift observed at a receiver at another location. To a first approximation, the entire data set recorded at one receiver will be advanced or delayed by a static time shift relative to data recorded at another receiver.
The receiver
3
may measure only a single component of the received seismic energy. Recently, however, it has become common for the receiver
3
to record more than one component of the received seismic energy; for example, the receiver may record the x-, y- and z-components of the particle velocity and the pressure (which is a scalar quantity). interest in acquisition of multi-component seabed seismic data has increased significantly. Since multi-component seabed recordings record shear waves (S-waves), as well as P-waves, it is possible to image through sequences that are opaque to P-waves (e.g. gas-clouds). Moreover, since shear waves reveal the presence of anisotropy more clearly than P-waves, multi-component recordings can yield additional information about the physical properties of the subsurface or about the presence and orientation of small-scale fractures for instance.
Multi-component seismic data can be processed to give information about various seismic parameters, or can be separated into an up-going wavefield and a down-going wavefield. One problem encountered in processing multi-component seismic data is that incorrect sensor calibration can lead to one component of the recorded data being recorded less accurately than the other components. For example, where the receivers are mounted on a support cable, the component of particle velocity in the in-line direction (parallel to the cable) may be recorded more accurately than the component of particle velocity in the cross-line direction (perpendicular to the cable). This problem is known as “vector infidelity”.
There have been a number of proposals for filters that allow decomposition of multi-component seabed seismic data, for example by Amundsen, L. and Reitan, A., in “Decomposition of multi-component sea-floor data into up-going and down-going P and S-waves”,
Geophysics
, Vol. 60, No. 2, 563-572 (1995), by Wapenaar, C.P.A et al in “Decomposition of multi-component seismic data into primary P- and S-wave responses”,
Geophys. Prosp
., Vol. 38, 633-661 (1990), and by Amundsen, L et al, in “Multiple attenuation and P/S splitting of multi-component OBC data at a heterogeneous sea floor”,
Wave Motion
, vol 32, 67-78 (2000) and in “Decomposition of multi-component sea-floor data into up-going and down-going P- and S-waves”,
Geophysics
, Vol. 60, No. 2, 563-572 (2000). However, these filters rely on the assumption that the data input to these schemes are a good vector representation of the actual seismic signal acquired at the receiver, and they also require knowledge of the elastic properties of the seafloor, For this reason, the issues of wavefield decomposition, statics estimation and vector fidelity are intrinsically coupled.
Knowledge of the properties of the surface layer
8
is required in a number of processing steps for multi-component seabed seismic data. These include wavefield separation, statics estimatio
Muijs Remco
Robertsson Johan
McElheny Jr. Donald
Moser Patterson & Sheridan
WesternGeco L.L.C.
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