Selecting optimum field methods
Assume that you wish to map objectives 3 to 5 km deep in an area with topography ranging from flat to gentle hills (gradients usually less than 3%). Dips at objective depths may be up to . The velocity at the base of the low-velocity layer is 2000 m/s, that at objective depths is 4000 m/s, and at the basement (8 km) probably about 6000 m/s. Five surface-source units and 96-channel recording equipment are available. Both ground roll and air waves may be problems, but the low-velocity layer (about 10 m thick with a velocity about 600 m/s) is probably fairly uniform. The area is fairly noisy and moderate effort will probably be required to achieve adequate data quality. Propose field methods and explain the bases for your proposals.
The following rules are based to some extent upon theory, but also, to a considerable degree, on experience.
- The maximum offset should roughly equal the shallowest depth of interest; this usually provides large enough NMO that primaries can be distinguished from multiples while avoiding problems such as large changes in the reflection coefficients and errors arising from approximations in the NMO equation (4.1c) on which CMP corrections depend.
- The minimum offset should not exceed the depth of the shallowest zone of interest for the reasons given in (1). However, noise produced by the source may dictate a larger value that this.
- The maximum array length must not exceed the minimum apparent wavelength [see equation (8.5a)]. The minimum apparent velocity usually occurs at the maximum offset, and the minimum for should be within the main lobe in Figure 8.6b(i).
- The minimum inline geophone spacing within a group should be less than the minimum of the noise, which is usually that of the lowest-velocity noise.
- Provided that rule (3) is not violated thereby, the geophone group interval should not be more than double the desired horizontal resolution (see problem 6.2) at the depth of interest.
Since the velocity varies from 2 km/s near surface to 4 km/s at depth, we shall base estimates on an average velocity of 3 km/s. Thus the reflection traveltimes at the objective depths range from 2 to 3 s. A basement reflection may occur at about 4 s.
To map at depths of 3–5 km, we would like to use a maximum offset of about 5 km. If we were to use a split-spread configuration to cover the distance, the group interval would have to be about 100 m unless we sacrifice some short-offset data, and 100-m group intervals might be too large for the expected 30-degree dips. Using an end-on configuration, the group interval would need to be only about 50 m, a much more conservative arrangement.
The maximum frequency for the deeper reflections will probably be no more than 40 Hz, in which case the minimum apparent wavelength [see (8.5a)] at the surface will be . To avoid spatial aliasing we must sample at least twice per cycle (see problem 9.25), so 50 m is the maximum group interval.
Because the area is expected to be moderately difficult in terms of signal/noise ratio, we shall probably require high multiplicity (problems 5.12 and 8.3), probably 24 fold, perhaps 48 fold. To start, we will probably use a 100 m source-point interval, twice the geophone-group interval, changing our recording pattern depending on the initial results.
Assuming the ground roll has a broad spectrum from 10 to 40 Hz, the corresponding wavelengths will be 20 to 80 m. We will want the inline geophone group length to equal the geophone-group interval so that the gathers effectively represent a group having the spread length which will permit maximum attenuation in stacking (this is the stack-array concept). Thus we should space geophones within the group at intervals no larger than 10 m, probably 3–4 m, which will require 10–12 geophones per group.
The ground roll with a velocity of 800 m/s will arrive at 3000 m offset at about 3.8 s and at 5000 m offset at 6.2 s, so we should be able to record reflections on most traces before the ground roll arrives.
We might use a split spread with a near-offset of 1200 m and a far-offset of 5000 m, 80-m geophone-group intervals and 80-m groups. An end-on spread from 0 to 5000 m with 50-m geophone-group interval is more conservative, and a split-spread does not offer much advantage over an end-on spread.
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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