Vertical seismoacoustic profiling in geotechnical wells

          Vertical seismoacoustic profiling (VSP) in shallow geotechnical wells is used to solve various problems – to obtain static corrections for the zone of low velocities during conventional seismic exploration for oil and gas, to bind reflections on sections of shallowl-depth seismic CDP exploration, to obtain detailed sections of elastic modules and forecast estimates of strength and deformation properties of soils.
Usually for reception of oscillations piezo-hydrophone multichannel (8-24 channel) piezo-probe is used. Works are carried out with a pulse shock source (sledge hammer). At low groundwater level, the well is cased by a column of polyethylene pipes with a diameter of 100-105 mm with a check valve at the bottom of the column. In this case, the well can be filled with water up to the earth’s surface, protected from collapses and used for repeated observations. Removal of the source from the wellhead 3-6 m. The estimated records parameters can be following: sample rate 100-200 µs, and the number of samples 1000-2000, the recording duration 100-400 ms. Traditionally, the distance between the hydrophone channels in the probe is 1 m. If it is needed more detailed data, for example with a step size of 0.25 m 4 series of excitations are performed with the movement of the probe each time for 25 cm up the well bore. After that, the probe moves up to its full length from the initial position of + 1m and the following 4 series of excitations and registrations are performed. The treatment program compiles a summary VSP seismogram with increments between receivers 25 cm. With this rate of the times at the vertical hodograph it is possible to obtain an elastic waves velocity section, approaching on detail level to the charts of cone penetration testing (CPT).

VSP sections with a step between seismic routes of 25 cm, obtained in a well with a depth of 35 m at the location of the source at the wellhead(left) and at a distance of 6 m from the wellhead (right)

        Longitudinal wave velocity is determined by the first breaks of the direct (downgoing) wave. To determine the transverse wave velocity in conditions of section, presents a thick layer of loose Quaternary and moraine soils in water-filled wells with high enough precision, it is possible to use the hydrowave velocity recorded by piezo-hydrophone pressure sensors by a simple method without using th three-component probes.

VSP section obtained at registration of oscillations in the well using 8-channel probe with a piezoelectric pressure sensors. The step of receivers is 1 m, the shot point removal from the wellhead is 5 m. Along with the first breaks of the direct longitudinal wave, a low-frequency low-velocity hydro wave is registered

Graphs of physical and mechanical parameters of soils obtained on the basis of VSP

Non-longitudinal vertical seismic profiling

The system of non-longitudinal VSP performed on a linear profile using several different points of excitation can be transformed into an area system of non-longitudinal VSP with the location of profiles along several spatially spaced lines of excitation points location. Used in this case program of data collection and processing forms the spatial coordinating sampling values of elastic waves velocities along the curved seismic rays. On the basis of such sample vertical and horizontal sections of velocities can be obtained on arbitrarily set planes, reflecting spatial distribution of elastic waves velocities in soil array.

Scheme of non-longitudinal VSP performing

  Example of contouring according to 3D VSP data of the decompaction zone in the karsted rocks and boundaries of dense dolomite layer on vertical sections on the vertical planes removed from a well on distance of 8, 10 and 12 m (from left to right)

The calculation of velocity sections is made on materials not-longitudinal VSP, submitted by the division of the active seismoacoustic of the Mining Institute of UB RAS (Perm).

          Sources removal from the wellhead up to 64 m. Number of accumulations in the excitation pulse – 3. The shot points are located along three parallel lines, one of which passes through the bore, the second removed from the wellhead at 15 m, and the third – by 30 m. The spacing between shot points 4 m along each line. The recording sample rate 200 µs and number of samples 2000, the recording length 400 ms.
With the help of this observation system it is possible to obtain the velocity distribution of elastic waves in the near-well space, which has the form of an inverted pyramid with a base on the observation surface. As the well approaches, the depth of the studied area of space increases, but its transverse dimensions decrease in the horizontal plane.