3.3 Land-data geometries

Land-data geometries vary more widely than marine ones because receiver locations are not constrained to be attached to a towed streamer. There are several possible templates that can be adopted when a land survey is designed.

One important template is cross-swath geometries. The receiver lines are crooked because of obstacles in the terrain. The sources are moved in a direction approximately orthogonal to the receiver lines; hence the name cross swath. When the source moves outside the swath covered by the receiver array, the farthest receiver line is rolled over to the side of the new shots. This acquisition configuration assures that the receiver arrays of individual shot gathers are well sampled along the receiver-line direction, but they are aliased in the orthogonal direction [5].

Conversely, common receiver gathers are well sampled along the cross-line direction, but poorly sampled along the in-line directions. Only the midpoint axes are well sampled along both directions. The offsets of a cross-swath geometry have a preferential direction aligned along the receiver lines.

Button-patch acquisition has been developed with the aim of sampling the data azimuths over the whole range between 0and 360. Wide-azimuth data are necessary to gather information on how seismic properties, such as velocity and reflectivity, vary with propagation direction.

This detailed information can be useful when the subsurface is anisotropic.

The acquisition is called «button patch» because the receivers are deployed in a checkered pattern (buttons). The ensemble of several buttons makes up a patch that is rolled along to cover contiguous areas. The receiver locations (dots) and source location (asterisk) from a shot gather recorded in Wyoming with the button-patch technique.

For the streamer geometry, only the positive half of the in-line offset component is plotted, because reciprocal traces (a trace pair with the respective source and receiver locations exchanged) provide equivalent sampling for prestack imaging operators. This comparison is only qualitative, because other factors such as propagation velocity, target depth, and signal frequency should be considered, when the sampling criteria of seismic reflections are determined [1].

However, the difference in offset coverage and trace density among the four geometries is quite dramatic. The azimuthal coverage increases as we move from streamer geometry, to parallel swath, to cross swath, and finally to the button-patch geometry.

3.3.1 Wide-azimuth geometries

Wide-azimuth geometries have the obvious advantage that data with different azimuths can provide useful information on the anisotropy of the subsurface, as well on the heterogeneity of the velocity field. On the other hand, if we consider the number of recorded traces as fixed, the drawback of wide-azimuth geometries is that they sample the offset plane less densely, and thus they are more subject to aliasing problems of the offset axes. This is usually not a problem if no imaging operator is applied in the prestack domain, and only trace to trace transformations, such as NMO or stacking, are applied to prestack data. However, if prestack imaging is necessary because of structural complexity (DMO) and/or strong velocity variations, the sparse sampling of the offset axes may cause artifacts in the imaging results [4].

This trade-off between the width of the azimuth coverage and data density can be avoided by either recording more traces, or by improving the methods used to prestack image wide-azimuth data.

Considerations of subsurface illumination may further complicate the trade-off between narrow and wide azimuth data. Since, under complex overburden, target illumination varies with the data azimuth, a wide-azimuth survey has «illumination holes» more uniformly distributed in the subsurface. Therefore, it has a better chance of providing at least partial information on all the geological structures than a narrow-azimuth survey has. For marine exploration, wide-azimuth OBC surveys can be used to address this problem. A practical and economical solution is to acquire two conventional narrow-azimuth data sets with the sailing directions orthogonal to each other.

To facilitate the subsequent data processing, often the order of the traces is changed according to the traces’ geometry. Sorting and binning are two common operations that reorder the traces.

Sorting is a reordering of the traces according to specific trace coordinates, called sorting in dexes. Typical sorting operations are the reordering of the data in shot gathers or receiver gathers. These sorting operations are conceptually simple because all the traces sharing the same source (receiver) location have the exact same source (receiver) coordinates. On the other hand, common-midpoint sorting is complicated by the fact that, because of irregularities in the data acquisition, traces do not share exactly the same midpoint coordinates. Therefore, common-midpoint sorting requires a prior common-midpoint binning [6].

The data are binned by superimposing a regular grid onto the midpoint plane; then all the traces that fall within each grid cell, or bin, are assigned effective midpoint-coordinates equal to the central point of the cell. The nominal bin size, that is, the length of each side of the cells, is determined by the acquisition parameters and can vary between the in-line direction and the cross-line direction. The nominal fold is equal to the number of traces that would fall in each grid if there were no irregularities in the geometry.