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Standard subsurface imaging using seismic refraction data produces two-dimensional (2D) images of the subsurface. Engineers and geologists (i.e., the end users) typically must integrate other subsurface data with results from a seismic investigation. Advancements in refraction data analysis have increased the ability to image geologic features and provide better 2D graphical presentation of refraction results. As refraction tomography increases the ability to meet project objectives and present 2D color images more representative of the subsurface, we begin to approach the end users needs. That is, the end product from a refraction survey can be either a 2D image or a full 3D model. Full 3D earth models can be used for many purposes after the geophysical survey is complete. Subsurface physical property data in 3D model space permits assessment of a site from a whole new perspective. For example: structural loading, seismic loading, or construction excavation requirements can be uniquely handled using 3D model results.This paper presents an advanced approach to refraction data processing, presentation, and visualization, using the "Geostructural Analysis Package" (GAP). GAP incorporates several numerical modeling processes: discrete element method, particle flow code, finite differencing, and the material point method. These four numerical modeling methods have been combined and optimized for seismic applications. GAP is an innovative tool that allows better data analysis and presentation that can be used to produce 3D volumetric models for further analysis. For example, mapping top-of-rock may be the objective of a geophysical investigation, but it is not the engineering purpose of the survey (e.g., construction of critical structures -a dam or a bridge foundation).3D model results from two case histories are presented to demonstrate the benefit of processing and presenting seismic refraction data using a new perspective -GAP modeling. GAP represents the newest advancement in subsurface modeling using refraction data.

Two-dimensional seismic refraction microtremor data were acquired, processed and interpreted for the Honolulu High-Capacity Transit Corridor Projectwest of Honolulu, Oahu, Hawaii. The objective was to image the lateral and vertical extent of soft-soil conditions and determine the depth-to-bedrock. Approximately 4.28-line km (2.66 miles) of two-dimensional (2D) refraction microtremor (ReMi) data was acquired along 12 lines. Data were acquired along the Farrington and the Kamehameha Highways. Line locations were selected to obtain additional subsurface information between and below geotechnical borings. Results indicate that thick softsoil conditions exist; and, that the basalt bedrock has considerable relief. The bedrock can be encountered as shallow as 1.5 m (5 ft), to as deep as 70 m (230 ft) in this area beneath the existing highways. An innovative application of 2D seismic testing successfully mapped the lateral and vertical variability of the soft-soils beneath areas with very high traffic volume, without interrupting vehicle flow.