Determining static corrections from first breaks
Figure 8.19a shows the first arrivals (first breaks) at geophone stations 100 m apart from sources 25 m deep at each end of the spread. The geophone group at each end is not recorded because of hole noise. The uphole time is on the third trace from the right. Elevations for each group are given at the top. Weathering velocity is 500 m/s. The valley midway between the sources produces a change in the firstbreak slope, as if two refractors were involved, which is not the case. How can we be sure?
The source instant or time break is the sharp deflection on the third trace from the right. Each time division is 10 ms.
Hole noise is caused by reverberations within the shothole and by material ejected from the shothole and falling back to the earth when an explosive charge is detonated.
We correct first-break readings for elevation by taking as the reference datum the elevation of the left-hand source point and adding or subtracting 2 ms (=1 m/500 m/s) for each meter below or above the datum. The corrected times and are given in Table 8.19a where and are first-break times for sources at and at offset from source , All times are in milliseconds, distances in meters.
The plots of the corrected times in Figure 8.19b give straight lines whose slopes have an average value of 2390 m/s for whereas the plots of the uncorrected times suggest a 3-layer situation with a low-velocity layer in between two higher-velocity layers. The corrected times fit a straight line in each case which is strong evidence that there is only one high-velocity layer.
Determine the weathering thickness at the two sourcepoints from the uphole times.
To find at and from , we have
The uphole times are 0.025 s and 0.049 s at and , m at both and , m/s m/s; thus m at and 24 m at .
What correction should be applied to reflection times at the two source-points for a datum of 1125 m?
Applying equation (8.18c), we have
|Times are in milliseconds and distances in meters.|
Calculate the weathering thickness and the time correction for each geophone station.
In Table 8.19b the second column is the sum of the uncorrected times at geophone from Table 8.19a. To get we require the quantity ; we now get . Next, (except for the source points where is obtained from the uphole times [part (b)]. The weathering correction for a geophone group is given by equation (8.18g), that is,
where , being given by equation (8.18e), namely,
This correction is equivalent to placing the geophone group at C on the datum. To locate the sources on the datum also, we must add to the time from the source down to the datum, that is, ; this amounts to 0 and 4 ms at sources and, respectively. The column headed gives the corrections for the arrival times at the given offset for source while that headed gives the corrections for source .
Plot corrected reflection arrival times in an - plot and determine the depth, dip, and average velocity to the reflector giving the reflection at 0.30 and 0.21 s in Figure 8.19a.
|Distances are in meters, traveltimes in seconds, corrections in milliseconds. Underlined values are doubtful.|
Tables 8.19c and 8.19d list the offsets and their squares, the uncorrected times and the correction for each (from Table 8.18a), the corrected times and their squares. Table 8.19c is for source , Table 8.19d is for source . The data are plotted in Figure 8.19c. Drawing the best-fit straight lines, we measure the intercepts on the -axis and the slopes. Assuming that the dip is small so that the factor in equation (4.3a) can be neglected, the reciprocals of the slopes give the velocities squared. The measured results are:
|Previous section||Next section|
|Weathering and elevation (near-surface) corrections||Determining reflector location|
|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