1. Field of the Invention
The invention relates to apparatus and methods for seismic data processing and is applicable particularly to objectively evaluating the effectiveness of the application of one or more selected processing sequences in the interpretation of seismic and other geophysical data. Concomitantly, this invention allows for the optimal selection of parameters for use in the associated processes.
2. Description of the Related Art
In the area of the seismic method applied to hydrocarbon reservoir exploration and development, a source at or near the surface of the earth radiates an acoustic wavefield. Propagating through the earth, the wavefield encounters subsurface earth strata whose layer boundaries offer acoustic impedance contrasts. The wavefield is reflected therefrom back to the surface where the reflected wavefield is detected by an array of seismic sensors. The sensors or receivers provide electrical signals that are representative of the mechanical disturbances due to the propagating seismic wavefield. Sensors in common use include geophones that measure particle velocity, hydrophones that measure pressure of compressional waves or accelerometers that measure accelerating forces.
During the conduct of a seismic survey, an acoustic source is positioned relative to an array of spaced-apart receivers but separated therefrom by a preselected gap or offset such as 25 or 50 meters or a multiple thereof. The receivers within the array may include hundreds or even thousands of units that may be distributed over many kilometres along a line or over a region to be surveyed. Following each wavefield-radiation episode, the source and receiver are repositioned at new survey observation stations.
Seismic signals as detected by the respective receivers are transmitted to a central station over suitable communication channels which may be electrical, optical, acoustic or ethereal. The signals from each of the individual receivers or receiver groups are stored in a multi-channel data-recording unit for archival storage. The stored data are delivered to a data-processing center where the data accumulated during a survey are quantized and processed, preferably by computer because of the huge volume of seismic data that may total many terabytes.
The elapsed two-way reflected travel times of the wavefield radiated from the source to the respective subsurface strata and back to the receivers at the surface are a measure of the depths of those strata provided the acoustic propagation velocity characteristic of the intervening rock layers is known. Thus, a detailed 2-dimensional (2-D) image of the topography of the subsurface beneath a single line of survey can be determined from seismic reflection times. Use of multiple lines of survey provide areal 3-D coverage.
The results of a seismic survey may be displayed as a cross section of the earth in the time-space domain as a series of time scale traces, one trace per receiver channel ordered according to geographic position along a line or region of survey. Prominent reflected events can be followed along the cross section, indicating the structural attitude of the underlying bedding planes. Various combinations of source-receiver configurations may be used to enhance the quality of a display such as common receiver gathers, common source gathers, common offset gathers or common depth point gathers, all of which are well known.
A seismic wavelet begins to propagate at the instant of energy release from the source. If the wavelet were indeed able to retain its identity as a sharp pulse throughout its entire propagation path the time, indicated by the onset of a reflected pulse as recorded at a receiver, could be measured exactly. But during propagation through the earth, due to earth filtering and other causes, the pulse rapidly degenerates into a complex wavelet, often with a number of side lobes. Random noise contaminates the wavelet. Complex subsurface geology further complicates the reflected waveform such that the true onset of the reflected wavefield is obscured.
Seismic data as received are complicated by the cumulative effects of the impulse response of the source, the earth, the receivers and the ancillary electronics. Because of that complexity, a simple display of raw seismic traces is difficult and often impossible to interpret. For that reason, the data must be processed by use of exotic methods requiring massive, expensive computing resources.
Because of the sometimes great expense involved, preparatory to a full-blown processing campaign, one must decide on the cost-effectiveness of one or more of a series of proposed data processing steps to be used in determining a usable subsurface image of the geological structures. Having chosen a preferred process, one must determine the optimal parameter values to be used in the selected processing step.
Heretofore it has been customary to compare, by inspection, the results displayed on a seismic cross section before and after a particular processing step has been applied to the data. In addition, having selected a processing step, the interpreter must incrementally perturb the parameters in order to decide the optimal values of the associated parameters. That method of quasi-visual quality-evaluation is not only highly subjective but is decidedly uneconomical in the face of massive volumes of data.
There is a need for a method that quantifies the effectiveness of a chosen processing step or sequence of processing steps to achieve a set of seismic traces in which each trace is a close representation of the subsurface vertical acoustic response below the geographical location. In addition, there is also a need to obtain the optimum selection of seismic processing parameters applicable to the selected processing steps.