Weathering and elevation (near-surface) corrections
Show that when the source is below the LVL, weathering and elevation corrections for a geophone at the source are given by
where and are the elevations of the source point and datum, is the depth of the source, and is the uphole time.
Corrections are necessary to eliminate the effect of changes in the elevation of the surface and in the thickness of and velocity in the LVL. The corrections in effect reduce the traveltimes to those that would be observed if the source and geophones were located on a reference datum, usually a horizontal plane below the base of the low-velocity layer. These corrections are called static corrections because they are the same for all reflections regardless of their arrival times.
In Figure 8.18a, the correction to the traveltime for a wave going from the source at down to the datum is
The correction for travel from the datum up to a geophone at the source point is
so the total correction for the traveltime for a geophone at the source point is
If the split spread in Figure 8.18a is used to find the dip, what correction must be applied to the dip moveout?
The dip moveout is obtained by subtracting traveltimes at sources and in Figure 8.18b. If traveltimes have not been corrected for weathering and elevation, the dip moveout must be corrected; this is the differential weathering correction . Using equation (8.18b) we have
Derive an expression to correct the traveltime for a geophone at in Figure 8.18b and being source points.
We use the first-break traveltimes and that correspond to the paths and . The velocity contrast at the base of the LVL is usually large enough that the paths and are so close to vertical that the distance is very small. Hence the sum is given by
where is the traveltime through the LVL at , and . Therefore the correction that effectively places the geophone at on the datum is
We must add to this the correction that locates the source on the datum, namely , given by equation (8.18a). Thus the total correction for a traveltime recorded at is
The weathering and elevation corrections given by equations (8.18a) to (8.18g) assume that the source is below the base of the LVL. What changes are required if the source is within the LVL?
When the source is within the LVL, the wave traveling down to the datum is in the LVL for the distance , hence equation (8.18a) is changed to
where . Equation (8.18b) is unchanged provided we use the value of in equation (8.18h). Equation (8.18c) becomes
Equation (8.18d) is unchanged provided we use equation (8.18h) for . Equation (8.18e) becomes
where , . Thus, equation (8.18e) becomes
Equations (8.18f,g) are unchanged except that we must use the values of and from equations (8.18j) and (8.18h).
|Previous section||Next section|
|Interpreting uphole surveys||Determining static corrections from first breaks|
|Previous chapter||Next chapter|
|Seismic equipment||Data processing|
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