TIME-DELAY / STRAIGHT RAY APPROXIMATE AND

NON-LINEAR REFRACTION TRAVEL TIME TOMOGRAPHY

METHODS OF INVESTIGATION

Unlike the cross-over distance interpretation, both the Time-Delay/Straight Ray Approximation (SIPT2 provided by Rimrock Geohysics) and Nonlinear Refraction Traveltime Tomography (NRTT) approaches will give depth to bedrock below each geophone. The Time-Delay method relies on ray-tracing methods (using a straight line approximation) to fit an initial model to observed first-arrivals. After many interations, the SIPT2 program determines the depth to different "refractor layers" below each geophone. There are two limitations inherent with the Time-Delay methods: 1) the model is still susceptable to not seeing thin layers and seismic inversions (i.e. low velocity zones, caused by fracturing or build-up of gas within soil pores), and 2) The SIPT2 model assumes that the velocities do not change laterally within a "velocity layer".

The NRTT approach, developed by Dr. Jie Zhang formerly of M.I.T. and currently with GeoTomo LLD, initially assumes a homogenious half space over a layer for its initial model and then uses first arrival times to create a realistic model. Because the number of shot points are significantly increased, the amount of duplicate "ray paths" and hence redundancy in the data is significantly increased. The increased amount of data and its redundancy results in the model inversion elements being broken down into smaller components. Hence, data resolution is increased.

The other unique interpretational element to the NRTT processing approach is that it looks at the traveltime gradient, i.e. how first arrival times change with distance. By backfitting the observed traveltime arrivals and gradients using a geophysical statistical method called the Tikinov Regularzation Model, the model determines the error in the initial model and makes numerous iterizations to minimize it and make the model more accurate. The presumed advantages of this technique are: 1) Prior knowledge of the subsurface geology is not required in order to produce an accurate interpretation, 2) lateral and vertical resolution is far superior to other seismic methods, and 3) The method can supposedly detect low velocity zones below higher ones (i.e. inversions). The disadvantages of the NRTT method is that more time, and hence money, is required in the field to collect data (i.e. the number of shot locations is 2 to 3 times greater than conventional methods), and 2) Reasonably accurate elevation data is required to obtain and accurate model.