John B. Hickman, Kentucky Geological Survey, University of Kentucky, Lexington, KY 40506-0107
Geoscientists use reflection seismic data to determine the geometry of the geology of the subsurface. Unfortunately, a seismic section displays in what is essentially an "apples and oranges" set of dimensions. The horizontal (X, Y) dimensions are in distance units (meters, miles, etc.), while depth (Z) units are in time (seconds). To make it even more complicated, the correlation between these terrestrial and temporal units changes depending on lithology, porosity, depth, etc. This not only changes the depth to subsurface targets, but also can change their geometry (dips of anticline limbs, fault planes, etc). This is not a problem for larger, well-funded petroleum exploration companies since the calculations involved in this time-to-depth correlation are included in all high-end seismic interpretation software packages. This software is often financially out of reach for researchers in academia or for smaller independent exploration companies, however.
The computations and data that make up seismic velocity models are relatively simple. With the online availability of scanned images of geophysical well logs in many areas, most geoscientists have all the software they need already on their desktop computer. KGS researchers were able to create a procedure using common spreadsheet and GIS software packages to manipulate stratigraphic well tops and sonic log data into a 3D seismic velocity model. An added advantage of this procedure is that it can be used in reverse, generating expected depths in time of target horizons from nearby well data, even if no sonic log was recorded from that particular well.
AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York