Unlike the cross-over distance interpretation, both the Time-Delay/Straight Ray Approximation (SIPT2 provided by Rimrock Geophysics) and Non-linear Refraction Travel Time 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 interpretations, the SIPT2 program determines the depth to the different "refractor layers" below each geophone. There are two limitations inherent with the Time-Delay methods. First, the model is still susceptible to not seeing thin layers and seismic inversions (i.e. low velocity zones, caused by fracturing or build-up of gas within the soil pores). Second, 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 GeoTomo LLD, initially assumes a homogeneous half space over a layer for it's 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 broke down in to smaller components. Hence, data resolution is increased.
The other unique interpretational element to the NRTT processing approach is that it looks at the travel time gradient, i.e. how first arrival times change with distance. By back fitting the observed travel time arrivals and gradients using a geophysical statistical method called the Tikinov Regularization Model, the model determines the error in the initial model and makes numerous iterations 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 methods can supposedly detect low velocity zone below the higher ones (i.e. inversions). The disadvantages of he NRTT method is that one 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 evaluation data is required to obtain an accurate model.
Non-linear travel time tomography graph with siesmic critical distance superimposed