Selecting survey parameters
Assume that you wish to survey a km area where Eocene and Cretaceous anticlinal structures with their long axes north-south are expected, the minimum size of an economically viable structure being 2 km across. The maximum dip expected is and reflectors are listed in Table 8.15a “The Recent” reflection will be useful in making static corrections. A noise test gave a prestack . Propose the line spacing and orientation of a reconnaissance survey.
Static corrections are corrections that are independent of traveltime; these include corrections for variations in the surface elevation and the weathered layer (see problem 8.18).
A noise test is discussed in problem 8.11.
A reconnaissance will mainly use east-west lines plus a few north-south lines to tie the survey together. Any prior knowledge of the area will help in locating the lines. This includes examination of the land surface to see if surface features may relate to deeper structure. We do not expect to locate all possible structures on the first reconnaissance so east-west lines will be spaced 10 km or more apart and the north-south lines about double this. We shall plan on about seven east-west and three to four north-south lines and we should run the east-west lines first so that we can use their interpretation to locate the north-south lines. We will then select a couple of portions of the area for more detailed surveying where we can infill with east-west lines 2 to 3 km apart plus additional north-south lines to tie the area together. We must keep an open mind about the prior knowledge that the anticlines are oriented north-south and we may alter line orientations as interpretation unfolds. We may wish to shoot additional lines more-or-less perpendicular to the strike of faults that may affect structures.
What multiplicity is required to give
From problem 8.14a we know that S/N varies as for random noise; so to increase S/N from 0.5 to 3.0 requires a multiplicity of .
What spread geometry should be used, that is, what are the required near- and far-offsets, group spacing to avoid aliasing for 15 to 40 Hz, and minimum number of channels?
Because the objectives are Eocene to Cretaceous, we will want maximum offsets of 5000 m and, since we expect to use the Recent reflection to make static corrections, we will also need short offset data. Hence, end-on spreads should extend from near the source to 5000 m. We should use 96 channels and 50-m group intervals although 48 channels and 100-m group intervals might suffice. We probably should use geophone spacing within a group no larger than 5 m and have the same group length and group interval.
How long will the survey require, assuming production of 270 km/month can be achieved for 24-fold multiplicity?
The reconnaissance of seven east-west and three to four north-south lines, each being 75 km long, amounts to about 800 km and therefore will take about 3 months.
Answer part (d) assuming 210 km/month production for 48-fold multiplicity.
Increasing the multiplicity from 24 to 48 will increase the survey cost but will be worthwhile considering the signal/noise improvement expected. The reduction in production rate will increase the time by about 3 weeks. Shortening the group interval might achieve the same improvement in S/N.
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Also in this chapter
- Effect of too many groups connected to the cable
- Reflection-point smear for dipping reflectors
- Stacking charts
- Attenuation of air waves
- Maximum array length for given apparent velocity
- Response of a linear array
- Directivities of linear arrays and linear sources
- Tapered arrays
- Directivity of marine arrays
- Response of a triangular array
- Noise tests
- Selecting optimum field methods
- Optimizing field layouts
- Determining vibroseis parameters
- Selecting survey parameters
- Effect of signal/noise ratio on event picking
- Interpreting uphole surveys
- Weathering and elevation (near-surface) corrections
- Determining static corrections from first breaks
- Determining reflector location
- Blondeau weathering corrections