There are a number of common procedures that invoice the two-dimensional filtering of seismic sections (of "field records"). A few examples of these procedures are migration, running mixes, and dip filtering. A drawback of these procedures is that they often give a mixed or otherwise degraded appearance to the output. This is largely due to the effect of the filtering on the random background noise of the unfiltered seismic data.
For example, typical random background noise associated with a seismic record has energy that covers a wide range of dip angles. This combination of dips gives the background a salt-and-pepper appearance. In the case where a dip filter is applied, energy at certain dip angles is removed. This causes the remaining noise, when displayed, to exhibit segments of coherent energy at the remaining angles. The effect of this filtering on the random noise is quite evident. Because certain (usually high-angle) noise components are missing, a display of a dip filtered seismic record reveals segments of coherent noise energy at dips within the passband of the dip filter. These segments of coherent noise can interfere with signal detection on dip-filtered seismic data, thus hampering interpretation and reducing the ability to define structural detail. They also can give dip-filtered sections a mixed or wormy appearance, which may lead to the false conclusion that the signal has been greatly mixed when, in fact, the problem is related to distortion in the background noise field.
There are a number of conventional techniques that attempt to correct some of the background-noise effects of dip filtering or other filtering processes. One subgroup of these techniques will be referred to herein as "incomplete filtering procedures." An example of an incomplete filtering procedure is the addition of some of the original unfiltered section on top of the filtered section. This reduces some of the background mixed appearance at the expense of incomplete filtering of the undesired coherent noise. An alternate way to produce the same result in the context of dip-filtering, is to design the dip filter so that the dip-reject area never drops below some low threshold.
Another incomplete filtering technique is the use of a running trace mix as a dip filter. In typical implementations, the number of traces and their weights are selected to put a null in wave number space (k-space) which preferentially reduces the coherent noise. The disadvantage of this kind of dip filter is that components of coherent and random noise can pass through the filter. Thus, although this technique produces a less mixed appearance, the coherent noise is not completely filtered.
A second subgroup of known background restoration procedures can be employed for the special dip-filtering case in which the coherent noise occupies only a narrow range of dips. If a sharp dip filter that removes only the offending dip is designed for this case, most random noise energy at higher and lower dips can pass through, thus preserving the background noise appearance. However, such procedure is not generally applicable to data associated with noise outside this special category.
In addition to incomplete filtering and narrow-band dip-filtering approaches, there are other known background restoration procedures. One such procedure is the addition of computer-generated white noise to the filtered seismic data. This can sometimes help reduce extreme filtering problems, but has the disadvantage that too much noise is needed to overcome the filtering problems. Another procedure used with migrated sections is the preservation of the high-angle (over 45.degree.) components that are usually thrown away prior to migration. The preservation of the high-angle noise can help reduce some of the mixed appearance typical in migration. However, a drawback of the procedure is that the high-angle components will be migrated, even though they do not represent real data.
It has not been known until the present invention how to restore the appearance of the random background noise portion of a set of completely filtered seismic data while also completely filtering the coherent energy portion (including coherent noise and coherent signal components) of the data. Two-dimensional filtered seismic data processed in accordance with the inventive method has better cosmetic acceptability, greater reflector continuity and more accurate amplitude relationships than two-dimensional filtered data not so processed.
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