NDT Corporation conducted geophysical cross-hole and vertical seismic profiling (VSP) measurements in a series of boreholes along the proposed tunnel alignment between the MacArthur Causeway on Watson Island and the Port of Miami on Dodge Island. The VSP exploration program was conducted to study the condition of the bedrock formations through which the tunnel will be constructed. The seismic technique is the most useful exploration technique for engineering design in that the measured compressional and shear wave velocity values of the rock can be used to estimate its mechanical properties. The best resolution and definition of conditions for the various geologic layers are from seismic measurements in or proximate to the layer(s) of interest, through the use of boreholes.
VSP data were obtained using a 24 channel seismograph, and a down hole sensor array consisting of a string of 8 triaxial geophones spaced 10 feet apart and a “seis-gun” energy source. Each sensor has a pneumatic bladder that is inflated to clamp the triaxle sensor to the borehole casing. The measured first-arrival travel times were inverted with tomographic software to obtain 2D images of the velocity pattern. Tomographic inversion determines what velocity pattern gives the best fit to measured travel times. The inversion begins with an initial estimate of the velocity pattern, and then adjusts the velocity pattern to provide the best fit. The software provides options for using site information to improve the reliability of the results.
The results of the VSP tomography were shown as 2D compressional and shear wave velocity contour cross sections. The results indicate several locations where the velocity values differ from the typical layering. The differences are primarily in layers 2 and 3 and are an indication of locations where the limestone of layer 2 thickens or thins and locations where there is more sandy material and fewer limestone inclusions or areas with more limestone inclusions and less sand in layer 3. The results correlate with the geologic data produced from the drilling blow counts (N counts).
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