GB2368911A

GB2368911A - Computing a stacked seismic line by interpolation between known stacks

Info

Publication number: GB2368911A
Application number: GB0027372A
Authority: GB
Inventors: Francois Daube; Henrik Bernth
Original assignee: Geco Prakla UK Ltd
Current assignee: Westerngeco Ltd
Priority date: 2000-11-09
Filing date: 2000-11-09
Publication date: 2002-05-15
Also published as: AU2002212548B2; GB0027372D0; WO2002039144A1; CA2428541A1; ATE275272T1; DE60105309D1; AU1254802A; NO20032066D0; EP1334375A1; CN100354655C; NO20032066L; EA200300546A1; US20040022126A1; US6996028B2; EP1334375B1; CN1473275A

Abstract

A number of seismic stacks are precomputed for known velocity fields Vi. The velocity fields Vi are chosen to span the range of velocities of interest. The stacks are then arranged in the 3D memory of a graphics computer, using time and position as first dimensions and the index of the velocity field as the last dimension. In such 3D space, any velocity field V to be used for stacking appears as a surface within a volume. Projecting the seismic stacks onto that surface provides the seismic line stacked for the velocity field of interest V.

Description

PROCESSING SEISMIC DATA The present invention relates to a method of processing seismic data, and provides a technique for computing a stacked line by interpolation between known stacks Seismic data are collected using an array of seismic sources and seismic receivers. The data may be collected on land using, for example, explosive charges as sources and geophones as receivers; or the data may be collected at sea using, for example, airguns as sources and hydrophones as receivers After the raw seismic data have been acquired, the reflected signals (known as traces) received by each of the receivers as a result of the process of actuation of a seismic energy source are processed to form a subsurface Image. The processing includes the steps of accounting for the separation (known as offset) between source and receivers and summing related traces together to increase

signal/noise ratio (a process known as stacking).

Figure 1 of the accompanying drawings schematically illustrates an idealized source and receiver arrangement arranged along a line First second and third source 1, 2, 3 respectively cooperate with first, second and third receivers 4,5, 6 respectively. The sources and receivers are arranged about a common mid point (CMP) 7. Seismic energy produced from the actuation of the first source 2 is reflected from the partial reflectors 9 and received by each of the receivers 4,5 and 6. The travel time of the energy from a source to a receiver increases with increasing distance (offset between source and receiver). The travel time is also a function of the depth of the reflectors and of the velocity of propagation of the signal within the subsurface formations Figure 2 of the accompanying drawings illustrates the travel time for the situation shown in figure 1, as the offset increases. The round trip travel time with respect to offset for each of the reflectors defines a curve. In this simplified

situation the curve can accurately defined by t2 (offset) = (off < ) / (veloc/y) (zerooffset) where t is the round trip travel time, offset is the distance between source and receiver and velocity is the speed of propagation of seismic signals within the subsurface formations.

During the processing of the seismic survey data, the traces are assigned to their respective common mid-points such that the geology beneath the line of sources and receivers can be probed at a plurality of positions. A velocity analysis is then performed for each common mid point and indeed for each reflector 9. This is achieved by specifying a range of hyperbolas, as defined in the above equation, related to a range of velocities and computing the reflection amplitude along all specified hyperbolas. The seismic traces for a plurality of offsets are then converted in accordance with the hyperbolas to equivalent traces having zero offset and the traces are then summed (stacked). The resulting amplitudes at zero offset are examined to determine which hyperbola gives the best result for each of the reflectors of each common mid point. Figure 3 shows a typical example of velocity analysis at point 1, where the velocity field selected by the user varies between a range of known velocities.

Once a velocity field has been analyzed for a common mid point, the seismic data related to the common mid point are then corrected to zero offset according to the previous equation and then stacked for that particular common midpoint. The stacked trace has an Improved signal-noise ratio compared to the traces recorded at the receivers. That process, repeated at each of the common mid points of the lines produces a stacked seismic line that gives an indication of the geology of the line. The quality of the stacked line is directly related to the quality of the velocity field used for stacking. Stacking a line is a CPU intensive process that necessitates the use of large and powerful machines to be done in real time.

The present invention replaces the method of conventional stacking and replaces it by a technique based on interpolation that can be performed very quickly on modern graphics computers.

Assume that we choose a plurality of n velocity fields Vi that span the range of range of velocities of Interest, as shown in Figure 3. Assume also that we choose to work in a 3D space whose dimensions are the time (t), the CMP index and the index of the velocity field, as illustrated in Figure 4. In that 3D space (t/CMP/i), any velocity field V (CMP, t) will be represented by a surface S (as long as its values lie between the V1 and Vn). This is illustrated in Figure 4.

Finally, a number of seismic stacks are precomputed for the known velocity fields Vi. The stacks are then arranged in the 3D memory of a graphics computer, using time and CMP and the index of the velocity field as the three dimensions. Thus, we have defined a 3D volume of seismic data which can be projected onto surface S. That projection is the stacked line for the seismic velocity field of interest V (x, t). The method described does not yield a true stack but only one that is computed by interpolation between stacks precomputed for n velocity fields Vi. The projection operation can be done very quickly on a modern graphics computer, eg a Sun or Silicon Graphics workstation, or a high end PC, having sufficient memory to store the cube of data, and a 3D graphics card with texture mapping that supports the OpenGL language from Silicon Graphics, to perform the interpolation.

Claims

CLAIMS 1. A seismic data processing method in which a number of seismic stacks are precomputed for known velocity fields Vi which are chosen to span the range of velocities of interest, and the stacks are then arranged in the 3D memory of a graphics computer, using time and position as first dimensions and the index of the velocity field as the last dimension, to provide a seismic line stacked for a velocity field of interest V.
2. A seismic data processing method substantially as herein described with reference to the accompanying drawings.