COPCrosshole GPR data and raypath configuration for constant offset profile (COP). Depths are relative to original ground surface. Small triangle a 2.25 m indicates approximate level of water table. Absence of data below 5.25 m is due to signal attenuation by conductive bay mud that underlies sandy fill. T_{x} = transmitter, R_{x} = receiver. Click on figure for larger image (36K). 
MOGCrosshole GPR data and raypath configuration for a multiple offset gather (MOG) at Treasure Island. Transmitter (T_{x}) was fixed at a given position for each of 27 data files (only two shown here). Depth scale on the data files is relative to original ground surface, and indicates the position of the receiver (R_{x}) as it was lowered down hole. Small circles in the boreholes at right depict individual antenna positions. Click on figure for larger image (57K). 
TOMOGRAPHIC MANIPULATION 

Substantial processing of crosshole GPR data is required before interpretable images are produced. Tomographic analysis utilizes the path length and precise measurements of oneway travel time to determine the velocity structure of the intervening materials. The positions of the transmitter and receiver antennas in the boreholes are well known, and therefore the raypath distance between the two is accurately calculated for each shot. The objective is to collect traveltime data along as many ray paths and along as many different angles as possible. The analysis first divides the single plane connecting the boreholes into a grid of cells, or pixels, and calculates the number of raypaths that intersect each cell. The result is a matrix of simultaneous velocity equations with a nonunique solution. The analysis then diverges from an initial estimated model of velocity structure to find a ‘best fit’ velocity with the observed data. The more raypaths or "hits" for each cell, the better definition of transmission properties. Fewer raypaths provide a less certain solution. For this reason, data quality is often low in the corners and edges of tomographic images, and we use only the high quality data from the center. 
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contact: Robert Kayen
last modified 2019