Geology Reference
In-Depth Information
Well to seismic ties
Chapter
4
4.1 Introduction
There are several possible objectives in performing
well ties:
copies. In today
s digital world the individual shot data
are seldom included in well data packages, so the
interpreter is usually reliant on the final contractor
compilation of depth
'
time data presented in the final
well report or well seismic report. Understanding the
datum of the data is critical for land data. Marine data
are invariably referenced to mean sea level.
Determining a continuous function of time and
depth requires the integration of checkshot and log
data. Given that velocity is dispersive (i.e. it is depend-
ent on the frequency), thought should be applied to
how these datasets are integrated. Effectively this
means that log based velocities should be upscaled
to the seismic scale prior to performing the well tie.
-
zero phasing: checking whether data are zero
phase, and helping to adjust the phase if required,
horizon identification: relating stratigraphic
markers in the well to loops on the seismic section,
wavelet extraction for seismic inversion or
modelling,
offset scaling: checking whether the seismic data
have been
processed to have the
correct AVO behaviour, and adjusting amplitudes
if necessary.
'
true amplitude
'
Achieving these objectives requires integration of
regional interpretation experience with well tie analy-
sis, linking surface seismic to synthetic seismograms
and vertical seismic profiles (VSPs), as shown in
Fig. 4.1
. One way of looking at the well tie is that it is
the interpreter
4.2.1 Velocities and scale
Different velocity averaging methods are appropriate
for different wavelength and layer spacing situations
(Marion et al.,
1994
). When the ratio of wavelength (
λ
)
s chance to conduct an experiment to
test the connection between the geology and the seis-
mic data. In practice, there are numerous issues to
consider and an analytical approach is useful. There
are some situations where the tie and the phase of the
seismic data are not in doubt; in other cases there is
significant uncertainty. Estimates of well tie and wave-
let accuracy frame the context for assessing the quality
of calibration in an interpretation and amongst other
benefits they provide insight into the feasibility of a
good quality trace inversion (
Chapter 9
).
'
to layer spacing (d)is
1, as it tends to be at logging
frequencies, then an appropriate averaging method is
to sum the travel times through each layer. This is
usually referred to as the time average and for regularly
spaced data such as sonic logs this equates to a simple
arithmetic average of the slowness over a sliding
window. When the ratio of wavelength (
≤
λ
) to layer
spacing (d)is
1 (e.g. at seismic wavelengths and
with typical rock layering) the situation becomes com-
plicated by the fact that the layering gives rise to anisot-
ropy. In this case the average or
'
velocity is dependent on the nature of the layering
and the path that the acoustic energy takes through
the rock (e.g. Backus,
1962
; Mavko et al.,
1998
).
The Backus average (Backus,
1962
) is an appropri-
ate way to describe average P and S wave velocities
through finely layered media. Essentially this method
averages moduli rather than velocity. For the purposes
of routine well ties, where vertical acoustic propagation
and isotropy are usually assumed, an appropriate
'
effective medium
4.2 Log calibration
-
depth to time
An important element in any well tie technique is the
conversion of depth to time. Typically this will involve
the use of checkshot or VSP depth and time data.
These data are derived from the direct arrivals of shots
from seismic sources suspended over the side of the
drilling rig. In the past it was not uncommon for
interpreters to re-interpret
38
these data from paper
