Geology Reference
In-Depth Information
V
p
(m/s)
Log Calibration
(depth to time)
2000
2570
4000
6000
Well Seismic Matching
(quantitative assessment of wavelet
shape and phase)
2580
V
p
Time Avg
Backus Avg
2590
Regional
Interpretation
Experience
VSP
displays
Well Ties
2600
Zero phasing
Horizon identification
Wavelet for seismic inversion/modelling
Evaluation of offset scaling
2610
2620
Figure 4.1
The well tie process.
Figure 4.2
Model showing effect of averaging
V
p
logs; grey
curve
time average over 7 m window,
red¼Backus average over 7 m window.
¼ V
p
model, blue curve
¼
average V
p
and V
s
can be determined from the har-
monic averages of the P wave modulus (
Μ
) and the
shear modulus (
) as follows.
(1) Determine the M modulus and shear modulus
from P wave velocity, S wave velocity and density
(V
p
, V
s
and
μ
4.2.2 Drift analysis and correction
Log calibration seeks to analyse and resolve the dif-
ferences in times derived from the sonic log and from
checkshots. The general workflow is described below
and with reference to
Fig. 4.3
.
(1) Integrate velocity log from the uppermost or
lowermost checkshot that ties the log.
(2) Calculate the drift (i.e. seismic time minus time
from integrated log).
(3) Evaluate quality of checkshots using drift points
and checkshot velocities.
(4) Repeat (2) if checkshots are de-selected
(5) Fit a curve to the drift points such that the
differences between the final time
ρ
μ ¼
V
s
2
ρ
¼
V
p
2
ρ
.
(2) Over an averaging length to be defined below,
calculate the arithmetic average of
) using
and M
ρ
and the
, e.g. M
bavg
1
n
1
harmonic average of
Μ
and
μ
¼
P
M
1
(3) Use these averaged parameters to calculate the
Backus averaged velocities using: V
p bavg
¼
q
M
bavg
ρ
avg
ρ
av
q
.
Various suggestions have been made for the averaging
length required in the Backus average. Liner and
Fei (
2007
), who provide a useful general background
to the theory of the method, propose a maximum value
of V
s_min
/3f, where V
s_min
is the minimum shear vel-
ocity after Backus averaging and f is the dominant
frequency in the wavelet (
Chapter 3
). In general,
Backus upscaling is important where there are strong
contrasts in velocity, such as in a sequence of interbed-
ded shales and limestones, but has a much less marked
effect where the velocity contrasts are small, as is often
the case for a sand/shale sequence (
Fig. 4.2
).
μ
bavg
and V
s
¼
bavg
depth relation
and the integrated calibrated velocity log are less
than about 2 ms.
(6) Apply the drift correction to the time
-
depth curve
from the integrated velocity log. The corrections
may also be applied to the velocity log to generate
the calibrated velocity log.
(7) Evaluate the effect of corrections on the velocity
log (large differences are not expected).
-
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