11. Measurement and Identification of 3-D Coherent Noise Generated from Irregular Surface Carbonates

Abstract

One of the primary causes of poor seismic data quality is the masking of desired reflection signals by seismic waves scattered from topography or near-surface heterogeneities. For successful seismic acquisition and processing, one must be able to identify this scattered noise and quantify its strength relative to the desired signal so that appropriate measures can be taken to suppress it. An economical procedure has been developed that allows analysis of scattered noise. A field test consisting of a series of shots into a 3-D grid of receivers, whose dimensions depend on the range of noise velocities to be analyzed and the choice of temporal frequencies over which to make the analysis, makes it possible to identify events by measuring the direction and magnitude of their horizontal velocity. This 3-D grid also allows the formation of a series of nested areal arrays with graduated attenuation levels that can be used to quantify the signal-to-noise ratio .by attenuating the scattered noise by known amounts and observing the level at which desired signal appears. For any given shot, the seismic traces recorded by the grid can be analyzed and a graphic workstation can be used to produce radar-like displays of seismic events showing their arrival time, horizontal velocity, azimuth, temporal frequency bandwidth, and amplitude. With this information, one can design field arrays of the appropriate size and attenuation level to suppress the scattered noise. Often the required arrays are so large that they must be recorded in segments to prevent loss of desired signal due to necessary static and cross-line dip corrections. This array approach is most suitable for 2-D and wide-line profile acquisition schemes. Noise suppression levels of 20-30 dB have often been achieved, which has successfully revealed signal in many areas, such as Edwards Plateau in West Texas, karstic limestone regions of the former Yugoslavia, and Black Warrior Basin of Mississippi.

3D Seismic Survey Design, Second edition

“…This figure can be used to predict what to expect from a radar analysis inside the ground-roll cone of a cross-spread. In a radar analysis (Regone, 1997(Regone, , 2001, energy is measured as a function of slowness and azimuth. In the directions of the acquisition lines, an apparent slowness of 2/V will be found; in all other directions, the slowness tends toward 2√2/V.…”

Geophone array formation and semblance evaluation

“…Data acquisition and array forming of the present study follow the work of Regone (1997) in that we used data from a walkaway survey with a square receiver patch to form areal geophone arrays, but they differ in that we used smaller grid spacing in the receiver patch and shorter formed arrays. Whereas Regone (1997) used fixed subarrays of geophones at each node of his receiver patch, we used a bunched string of geophones at each node.…”

“…Rigdon and Hoover (1987) S/N ratios of data from different arrays in the frequency domain. Regone (1997) evaluated performance of Chebychevweighted areal geophone arrays based on S/N ratios by applying a series of array filters with known attenuation levels to raw data. He also used a beam-steering method to identify the directional properties of noise.…”

“…Whereas Regone (1997) used fixed subarrays of geophones at each node of his receiver patch, we used a bunched string of geophones at each node.…”

(t,x) domain, pattern‐based ground roll removal

“…Furthermore, as with Ocean Bottom Cable (OBC) applications, placement of arrays with adequate areal extent may be logistically impossible. Finally, traditional 2-D field arrays are ineffective at removing out-of-plane backscattered energy, so a 3-D array is required to suppress it (Regone, 1997), adding considerably to the acquisition cost.…”