May
2015
Oilfield Technology
|
5
setting the potential for some links in the chain to fall behind or
leap ahead. Economic forces tend to keep the balance of technology
in check; equipment manufactures assume additional risk when
developing new technologies that threaten the traditional balance.
This tends to restrain the progress of new technology development
but it does not eliminate all progress, as has been witnessed over
the previous 20 years of development.
Seismicsources
Seismic energy source development has received much attention
recently, driven primarily by the growing interest in broadband.
The way dynamite has been used as a seismic source has evolved
dramatically. Twenty years ago, it was not uncommon to place 50
or even 60 pounds of dynamite in a shot hole 50 ‑ 100 ft in depth.
Today it would be difficult to find a dynamite survey with charges
that big or shot holes that deep, yet the data quality has improved
dramatically. This fact seems to contradict the fundamental
assertion of the ‘bigger hammer’ theory – i.e. hitting the ground
harder does not necessarily yield a better result. In the case of
dynamite, a large dynamite charge generates more energy, but that
energy is expended on signal and noise at disproportionate rates.
There is an optimised charge size and charge depth in most areas
that produces the highest signal to noise level. The same is true of
the vibrator as a source.
The essential components of the
current vibrator have been dramatically
improved in the last 10 years. The modern
commercial vibrator delivers signal with
better phase and amplitude control
and lower distortion over a broader
bandwidth. Small vibrators are shown in
many areas to produce adequate energy
to image targets as deep as 10 000 ft.
Again, the ‘bigger hammer’ theory does
not seem to apply. We achieve better
seismic results because the new vibrators
have better control over the energy that
goes into the ground. More benefit is
derived simply by understanding the
behaviour of the engineering aspects
of the vibrator than by using bigger
vibrators, more vibrators, longer sweeps
or more sweeps. Many experts still rely
on simple equations that relate factors
such as the number of vibrators and the
maximum force to provide guidelines
for equalising vibrator source effort
with different shooting configurations.
These simple equations include incorrect
assumptions such as perfect coupling
with an elastic earth. The engineering
advances in vibrator control provide the
opportunity to optimise vibrator shooting
parameters and configurations for each
project with a detailed understanding of
the performance characteristics of the
sources.
Learning from the dynamite
experience, one should expect the optimal
solution will not always mean that the
biggest vibrator is best, or that more
source effort will produce a better result.
For instance it has been shown that mutual admittance interaction
effects can reduce ground roll significantly (Brune et. al., 2013).
Understanding the properties of the energy that goes into the ground
is more important than simply generating more energy.
Figure 5.
The seismic data shownare low frequency filter panels fromVSPdataat 2700 ftdepth. The
vibroseis sweepwas 1 ‑ 11Hz. The frequency panels show the direct arrival energy andprogress by 1Hz
steps left to right. The leftpanel is the input followedby 1Hz slices.
Figure 4.
Small (2600 lb) vibrator at work in the SouthGeorgiaRiftBasin,
USA.
Figure 6.
The seismic data shownare frequency filter panels fromVSPdataat 2700 ft. depth. The
vibroseis sweepwas 5 ‑ 200Hz. The frequency panels show the direct arrival energy andprogress by
10Hz steps left to right. The leftpanel is the input followedby 10Hz slices. The last visible energy is at
180 ‑ 190Hz illustrating successful input of high frequency energywitha vibroseis source. This data is the
result of one vibroseis and one sweep.
