Over the past 2 decades seismic inversion, ostensibly the process of deriving rock properties from seismic measurements, has evolved significantly. The early methods of recursive inversion converted seismic traces to well log traces providing a measurement of the "pseudo acoustic impedance". The acoustic impedance could also be expressed as "pseudo-acoustic velocity" by assuming a simple relationship between velocity, density and acoustic impedance. In any event though, the inverted property was still acoustic impedance.

While the property of acoustic impedance is more of a geophysical measurement than a geologic rock property, it did yield some indication of actual rock types. Most importantly, it demonstrated that valuable physical information was present in seismic data which was being overlooked by conventional wiggle traces.

The resolution of recursive inversion was limited to the bandwidth of the seismic data (hence the name bandlimited inversion). By using spike detection algorithms to convert the seismic trace to a high frequency sparse reflectivity series prior to inversion, sparse spike inversion algorithms could achieve high resolution. The "blocky" lithologic boundaries created by sparse-spike methods more accurately modeled actual geologic conditions although the output physical quantity was still "pseudo-acoustic impedance".

Recently, model-based inversion schemes have evolved which essentially rely on the fact that the forward model of a "good" inversion should very closely match the actual seismic data. Using iterative forward modeling schemes, these methods perturb an initial acoustic impedance model until it's forward model matches the seismic traces. These methods have the advantage of allowing some degree of control over the starting point and hence the resulting inversion. In any event, model based inversions still derive acoustic impedance.

AVO techniques have demonstrated that measurement of the conversion of compressional energy to shear energy at interfaces can yield information about the fluids and lithology present. More recently, advances in pre-stack imaging and analysis have resulted in significantly improved pre-stack signal quality with better preservation of lithologic information.

We present LithSeis, a pre- tack inversion technique which combines inversion and A VO technology with anisotropic petrophysics. LithSeis uses pre-stack seismic data as well as sonic, density and gamma ray logs to directly derive clastic rock properties including sand/shale content, gas saturation, water volume, and effective porosity. The new technique demonstrates that significantly more information is contained in the seismic wavefield than simply acoustic impedance and that we can reliably quantify rock properties from seismic data.

We demonstrate the technique on North American and international seismic data. Ongoing research will soon yield oil content and carbonate mineralogy venturing closer to the ultimate goal of deriving complete rock properties from seismic data.



About the Author(s)

Rick Wallace graduated from The University of Calgary in 1982 with a B.Sc. in Electrical Engineering. He began his career in geophysics as a Special Project Geophysicist with Western Geophysical in Calgary. In 1983 he joined Veritas as Group Leader of Modeling where he worked closely in the testing and development of Maximum Likelihood Deconvolution methods. In 1985, with Veritas Software, Rick developed the first version of Vista, which at that time was the first PC-based seismic processing system.

From 1985 to 1988, Rick worked in the field of Project Management, specifically in developing Risk Analysis applications which were subsequently used on the Syncrude Capacity Addition Project and the Northwest LRT system in Calgary.

In 1988, Rick re-joined Veritas where he founded and managed the Analytical Services department. This high technology group provided pioneering efforts in AVO and inversion, working closely with Hampson-Russell Software in testing and implementing the STRATA inversion algorithm. In 1989, Rick developed the Meridian Network Match Filtering technique for spectral grid balancing of seismic data.

In 1991, Veritas promoted Rick to Manager of Research and Software Services with the specific goal of overseeing the completion and deployment of their SAGE 3D seismic processing system which was successfully completed in late 1992.

In 1993, Rick founded Ulterra Geoscience Ltd., a high technology group specializing in inversion, AVO and migration services. In 1994, with assistance from the NRC, Ulterra developed the Common Scatter Point migration algorithm which set new standards for the speed and quality of prestack 3D migration. Ulterra currently employs 10 people, including one in Houston, and continues to be a leader in high technology processing services and software development.

During his career, Rick has authored and presented numerous papers on 3D inversion, AVO, grid balancing and, most recently pre-stack inversion. Rick served for many years on the CSEG Convention Committee and in 1995, Rick was presented with the CSEG Best Paper Award for his joint luncheon presentation with Doug Uffen on the Swan Hills 3D Inversion.

Roger Young graduated in 1980 with a BS in Physics from Clarkson College of Technology, and in 1992 with an MS in Petroleum Engineering from the University of Houston. From 1980 to 1986 he was General Field Engineer with Schlumberger Wireline Services, then from 1986 to 1989 he was Systems Integration Engineer for the Grumman Aerospace Neutral Particle Beam "Star Wars" Project, and in 1989 he joined Union Texas Petroleum and is currently Senior Staff Petrophysicist.



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