In reality, receiver cables are not straight and parallel to one another as illustrated in Figure 7.1-2. Instead, receiver cables are subject to a certain amount of sideways drift, called feathering, from the ideal cable lines. Cable feathering is caused by cross currents. The cable shapes associated with a selected set of shot points from a marine 3-D survey are shown in Figure 7.1-3. The angle between the actual cable position and the shot-line direction (the boat track) is called the feathering angle. This angle is not always constant, even along the same cable associated with a single shot.
Figure 7.1-4 shows two types of source-cable combination — a single-source and dual-cable, and dual-source and dual-cable. As a result of cable feathering, midpoints associated with each of the source-cable combination depart from a straight subsurface line as illustrated in Figure 7.1-2. Instead, midpoints are scattered within the hatched areas. For a typical 10-degree feathering angle and a 2400-m-long cable, the midpoint associated with the far receiver is offset more than 200 m off the shot line. This is four lines off at a 50-m line spacing. Note that the direction and strength of cross-currents determines the crossline width of the midpoint distribution. In the ideal case of zero feathering, midpoint distribution would have zero crossline width. The single-source, dual-cable configuration simultaneously yields two bin lines, whereas the dual-source, dual-cable configuration yields four bin lines. The latter is conducted by alternate pops produced by the two sources.
Figure 7.1-4 Sketches of (a) a dual-cable (RC1, RC2) and single-source (S) marine recording geometry, (b) a dual-cable (RC1, RC2) and dual-source (S1, S2) marine recording geometry. Crosscurrents cause the marine cable to feather and drift sideways from the shot-line direction. This causes the midpoints to spread in the crossline direction over subsurface strips as denoted by the hatched areas. When data are sorted into common-cell gathers, each cell contains midpoints associated with more than one source line. Circles with arrows on the cable trajectories represent compasses used for cable locationing. (Courtesy Western Geophysical).
Because of significant variations in cable shape during recording, in reality, midpoint distribution in the crossline direction can be quite irregular. We must know exactly where each of the receivers is located along the cable, as well as the shot-point location. Navigation data collected on the survey vessel normally include the boat location, source location, and cable compass readings. There are 8 to 12 digital compasses along a typical marine cable. Readings from these devices allow computation of the (x, y) coordinates of the cable compasses. Cable shape then is computed based on a curve-fitting procedure that rejects any anomalous measurements. Navigation data are analyzed during processing, and quality control is carried out to derive the final shot-receiver locations.
- Migration aperture
- Spatial sampling
- Other considerations
- Marine acquisition geometry
- 3-D binning
- Crossline smearing
- Strike versus dip shooting
- Land acquisition geometry