Borehole Geophysics and Petrophysics

Downhole seismic systems
The GSC operates both sidewell-locking geophones and hydrophone arrays for measurement of compressional and shear wave velocities in overburden materials. These units have been constructed in-house using commercial hydrophone and geophone elements. Surface seismic sources commonly used are the . buffalo gun. for P waves and the polarized horizontal hammer on a weighted plate for S waves. Recording is done on an multichannel engineering seismograph. A depth capability of 300 m is possible for both wave velocities.
All systems are operated in a cased borehole, preferably one which is not open or screened at the bottom.

P wave velocity log
The downhole P wave velocity log is derived using either a 12- or 24-channel hydrophone array. This array is moved incrementally either up or down the borehole; a surface source (commonly a 12-gauge Buffalo gun fired in a shallow hole) is placed close to the borehole (3 to 6 m to one side, at 1 m depth).
The spacing between hydrophones is fixed at 0.5 meters; hence incremental vertical moves of the array in the order of 1 m between source records will yield considerable redundancy of hydrophone locations. Travel-times between source and receivers are individually picked for each shot record. The data redundancy is used to obtain best estimates of interval velocities over short vertical intervals (Hunter et al., 1998). For this compilation plots of P wave velocities are given at intervals of 0.5 meters downhole. Usually 3 pt (over 1 m vertically) or 5 pt (over 2 m vertically) velocity fit results are shown.
Compressional (P) wave velocities are strongly affected by the presence or absence of pore-water. Low velocities are exhibited above the water table and in areas of the borehole where gas exists in the pore space. Most normally consolidated water-saturated soils have velocities close to that of water (1480 m/s). Overconsolidation of water-saturated soils ( with resulting reduction of porosity) is indicated by somewhat higher velocities (e.g. a compacted coarse-grained basal till can yield velocities of 2500-3500 m/s. Lithification to rock, or ice-bonding of soils, results in velocities which may range between 2500-5500 m/s. Empirical relationships between soil porosity and P wave velocities have been developed.
S wave velocity log
The downhole S wave velocity log is derived using a single- or 3-pod well-locking geophone array. Each pod consists of 3 orthogonal 14 Hz geophones which can be locked against the side of the borehole with a motor-driven bow spring. The orientation of the single- or multi-pod array can be done from ground surface down to a maximum depth of 100 m. Commonly the array is moved vertically in increments of 1 meter. The seismic source is placed close to the borehole on ground surface; commonly a steel I-beam or wooden plank loaded by the front wheels of a light truck is struck horizontally to obtain polarized shear wave energy.
The first arrival data from all three components is examined using commercial picking and display routines. Least squares fits of the data are computed and plotted; commonly a 3-pt fit is displayed.
Shear wave velocities can be used to indicate the presence or absence of soft soils and resonant boundaries for earthquake hazards assessment and can be used to estimate liquefaction potential of non-cohesive soils. The values can also be used to estimate ultimate strength of cohesive soils, and to identify the presence of stress anisotropy associated with natural or man-made slopes. Emperical relationships between shear wave velocity and soil porosity have been developed.

 

posted by Geology on 04:13

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