The Crosshole Seismic (CS) system and method determine shear and compressional wave velocity versus depth profiles. From these measurements, parameters, such as Poisson’s ratios and moduli, can be easily determined. In addition, the material damping can be determined from CS tests. These dynamic soil and rock properties are often utilized for earthquake design analyses necessary for certain structures, liquefaction potential studies, site development, and dynamic machine foundation design. The most complete version of this downhole system, as manufactured by Olson Instruments, is comprised of a borehole source capable of generating shear and compressional waves and a pair of matching three component triaxial geophone receivers. These instruments are lowered to the same depth in boreholes set at ~ 10 ft (3 m) apart in a line. The instruments are coupled to the side of the grouted borehole inclinometer casing, allowing for the detection of shear and compressional waves as they pass between the receivers.
The Downhole Seismic (DS) investigations are similar to CS investigations, but require only one borehole to provide shear and compressional velocity wave profiles. The DS method uses a hammer source at the surface to impact a wood plank and generate shear and compressional waves. This is typically accomplished by coupling a plank to the ground near the borehole and then impacting the plank in the vertical and horizontal directions. The energy from these impacts is then received by a pair of matching three component geophone receivers, which have been lowered downhole and are spaced 5 to 10 ft (1.5 to 3 m) apart.
■ Real-time waveform display while testing
■ Thin layers, which are often invisible to surface methods, can be detected with CS/DS investigations
■ Acquisition and processing software are easy to use, yielding fast and accurate results
■ CS method is the most accurate method for determining material properties of rock and soil sites
■ Accuracy and resolution for the CS test method are constant for all test depths, whereas the accuracy and resolution for
the DS surface method decreases with depth
■ Sources and receivers can be oriented with inclinometer casing dummy probes
■ P-SV source used in CS tests can impact in the up, down, and radial directions
■ Correlation between CS and Spectral Analysis of Surface Waves (SASW) tests on soil sites showed that the values from both tests typically compare within a 10-15% difference
The CS investigation requires drilling of two or more (ideally three) boreholes cased with PVC or slope inclinometer casing
for deeper borings up to 328 ft (100 m), and grouted in accordance with ASTM standards to ensure good transmission of wave energy. The boreholes are typically 4-6 inches in diameter cased with 2.32 to 3 inch (59 to 76 mm) I.D. casing. The testing is simplified if inclinometer casing is used rather than normal PVC pipe. Typical distances between adjacent in-line boreholes are on the order of 10 ft (3 m). The testing is performed by lowering both the source and receiver(s) to an investigation depth, firing the source, and recording the energy with the receivers.
The DS investigation requires drilling a single borehole with similar specifications as listed above, except that only a single grouted 2 inch (50 mm) to 3 inch (76 mm) I.D. PVC casing is needed. The testing is performed by lowering the receiver(s) to an investigation depth, impacting the coupled surface plank, and recording the energy with the receivers.