A computer program in BASIC for construction of two-layer, seismic refraction forward models within which elevation and thickness of the upper layer change and velocities vary laterally within both layers [unknown]

W.P. Hasbrouck
1990 Antarctica A Keystone in a Changing World   unpublished
A computer program in BASIC for construction of two-layer, seismic refraction forward models within which elevation and thickness of the upper layer change and velocities vary laterally within both layers by Wilfred P. Hasbrouck1 Open-File Report 90-320 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards. Any use of trade names is for descriptive purposes only and does not imply endorsement by the USGS. Although the computer
more » ... in this report has been extensively tested, the USGS makes no guarantee whatever of correct results. 1990 'Golden, Colorado ABSTRACT This report presents a computer program in BASIC to construct two-layer, seismic-refraction forward models with elevation and thickness variations of the upper layer and lateral velocity changes within each layer. The program accomplishes this by dividing both the upper and lower layers into vertical zones with boundaries at each station location, constructing refracted ray paths within each zone, and summing the times to travel these paths from a source point to a detector position. A requirement is that rays refracted upward from the lower layer do not cross between zones within the upper layer. True scale plots of the model with ray paths drawn to each station are visually examined to see if this condition is met before computation of arrival times proceeds. The output includes traveltime curves (X/T plots) showing direct and refracted arrival times along both forward and reverse spreads, plots of delay times, and an option to produce elevation-corrected traveltime curves and delay-time plots. The program was developed for use by engineering geophysicists, and as such it uses units that though non-standard are more applicable in their field. For example; arrival times are in milliseconds (ms), distances are in meters (m), and velocities are expressed as m/ms. Option is provided for those who prefer to work in the English system, in which case, distances are in feet and velocities are entered as ft/ms. Because the computing procedure is completely analytic, it operates very quickly on a desktop computer. The program was written to run on the Hewlett-Packard 9845B computer and it uses the Hewlett-Packard BASIC resident on that machine. With the exception of its graphics sections, this program language is sufficiently transportable that it can be modified to operate on most desktop computers. In this report, after the conditions on the forward model are discussed, the computing scheme is developed, and a step-by-step description of the operation of the program is given. In the final section, examples of results produced by the program are presented. Two cases of practical importance are emphasized: seismic refraction surveys across landfills and stream beds. CONDITIONS ON THE FORWARD MODEL Shown on figure 1 is a sketch illustrating the conditions imposed upon the modeling procedure. The vertical dot-dash lines define the partitions at each station location, and the vertical, double solid lines separate the velocity zones within each layer. The source point (SP) is positioned far to the left side and the rays from it are shown traveling to the right along the top of the higher speed layer. The arrowed paths in the upper layer show selected ray paths to detectors at positions A, B, and C after being refracted from the lower layer.
doi:10.3133/ofr90320 fatcat:j6cfz5ft2bfd3jxu2ovjo3rat4