Free Consultation with Delta Geophysics
You will get more out of a
commercial consultation with us if you first familiarize yourself with the
ideas in the following public literature, which also establishes our claim for
world-class expertise.
Porosity. This 1985
paper, the first of the following to be cleared for publication by Amoco,
was specially selected by GEOPHYSICS for inclusion in its Golden Anniversary
issue. It explains how to model the seismic properties of rocks with both
equant and fracture porosity, in a way that is consistent with Biot theory.
Fluids, Lithology. This discussion extended over 13
years, beginning with a TLE Round Table Discussion entitled "Poisson was not a
Geophysicist!", and ranging widely over many topics in seismic rock
physics, with emphasis on the (minor) role played by Poisson's Ratio in the
analysis.
Pore Pressure. This paper describes a unique " global algorithm" for predicting
subsurface pore pressure using seismic data. It commonly predicts the
occurrence of "subsurface fluid compartments", i.e. extended volumes
with a local hydrostatic gradient, and an elevated head. But, all pore pressure prediction
algorithms are imperfect, so it is important to apply several independent algorithms,
which preferably rely on different input datasets, e.g. Vp/Vs.
Understanding Anisotropy.
This set of
Lecture Notes supported the 2002 SEG/EAGE Distinguished Instructor Short
Course; here is the Introduction. The Notes
themselves (2nd Edition) are for sale through the SEG, as are recordings of
the lectures, in VHS and DVD formats.
Polar Anisotropy. This 1986 paper established the modern era in anisotropy studies. It is the single
most-cited paper in the history of Geophysics; if you Google 'Thomsen
parameter', you will find over 300,000 hits. It is essential reading for any
study of seismic anisotropy. (There is a bizarre
phenomenon for SV-waves in strongly anisotropic media, first discovered
over 100 years ago, but not really understood until 2002.This new understanding
may yet turn out to have useful implications.)
P-wave AVO.
One consequence of this advance in understanding is a profound
conclusion regarding Amplitude Variation with Offset. It was realized early on that the polar-anisotropic term in
AVO was potentially as large as the isotropic terms. But this insight was ignored for 20
years, with an entire sub-industry being built on isotropic AVO analysis
(neglecting the anisotropic term), since there was no way to estimate it in
field data. However, 20 years later an
algorithm has been discovered to do just that; it is the subject of a UH patent
application.
Azimuthal Anisotropy
(P-waves). Azimuthal AVO ("AVOAz") was
discovered by Thomsen in 1981, and analyzed in an internal Amoco report. With
the recent expansion of wide-azimuth seismic surveys, the effect is seen to be
ubiquitous; this 2006 paper offers the best confirmation
that the observed effect corresponds to real fractures in the subsurface. In
1995, the effect of fractures on seismic wave
propagation was explained theoretically (significantly revising earlier
theory), and successfully predicted (not fitted) experimental data which
had been obtained separately.
Azimuthal Anisotropy
(S-waves).
Azimuthal anisotropy causes two modes of shear waves to propagate at
near-vertical incidence, rather than one. Here is the first
explanation of (the now well-known) "Alford Rotation" solution to
this phenomenon of shear-wave splitting, and the
first report of the phenomenon in exploration field data. If the azimuth of
the symmetry axis varies with depth, then a layer-stripping
procedure is required.
Converted Waves used to be esoteric phenomena of
interest only to academics. But, with the establishment of the feasibility of
4-Component Ocean Bottom Seismic surveying, they became an essential part of
the seismic toolkit. The industry learning curve has been rapid, paid for by
the ability of converted waves to image inside gas chimneys, gas clouds,
etc. This 1999 paper was named "EAGE Best
Presentation" for establishing the concepts of "C-waves",
"registration", "gamma-effective", "vector
infidelity", and "diodic velocity". Since then, the list of
seismic problems which can be profitably addressed with 4C OBS data has
expanded considerably, to also include:
This list continues to grow, but one essential learning of
the past decade is that inclusion of anisotropy (both polar and azimuthal) in
the analysis is crucial for most C-wave processing.
Fluid dependence. For over 50 years, the standard theory
for understanding the fluid dependence of seismic velocities has been that due
to Biot and Gassmann. But the
experimental support for that theory is very thin. Now it develops that the
theory itself is not quite right; we
have been doing it a bit wrong all
these years! This 2010 refinement to B/G
challenges the experimental community to a program of experiments for
determining the extra parameter required.
Shale resource.
With the recent realization that shale can be a reservoir rock, people have
begun to understand the importance of anisotropic
rock physics. An important advance was the 2012 realization that the fluid dependence of seismic anisotropy is
complicated, but there is an unexpected
simplicity as regards the anisotropic parameter h. Another 2012 paper clarified the
(indirect!) relation between log data and Young’s
modulus for anisotropic shales, and challenges the experimental community
to a program of experiments to estimate brittleness from data that we can
actually measure in the field.
Electromagnetic exploration.
CSEM has now been shown to be a viable adjunct to seismic exploration, since it
provides a near-direct detection of subsurface hydrocarbons.But there might be
a better way to do it, based on the deep
connections between seismics and EM. Since, at low frequencies, EM
velocities are comparable to seismic velocities, seismic-style processing based on the moveout
of the data, is an alternative to conventional amplitude inversion. See
more discussion here.
These ideas have helped to establish the modern paradigm of exploration
geophysics. If this public material makes you think that these ideas might help
you with your particular problem, contact us.
Just as importantly, they establish the current base for
further progress in exploration geophysics. If you think that your particular
problem is not fully addressed by these ideas, contact us anyway;
the way to make progress is through confronting particular problems, finding general solutions
through the failure of the old ideas.