Vertical seismoacoustic profiling in geotechnical boreholes

          Vertical seismoacoustic profiling (VSP) in shallow geotechnical wells is used to solve various problems – to obtain static corrections for the zone of low velocities in traditional seismic surveys for oil and gas, to tie reflections in areas of shallow CDP seismic, to obtain detailed sections of elastic modules and predictive estimates of strength and deformation properties of soils.
Usually, a multi-channel (8-24 channels) piezoelectric sensor is used to receive vibrations. The work is carried out with a pulse impact source (sledgehammer). At a low level of groundwater, the well is cased with 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 to the surface of the earth, protected from collapse and used for repeated observations. The distance of the source from the mouth is 3-6 m. The estimated recording parameters can be as follows: sampling rate 100-200 μs, number of readings 1000-2000, recording duration 100-400 ms. Traditionally, the distance between hydrophone channels in the probe is 1 m. If more detailed data is required, for example, in 0.25 m increments, 4 series of excitations are carried out with the probe moving 25 cm each time up the wellbore. After that, the probe extends to its full length from the initial position +1m and the following 4 series of excitations and registrations are performed. The processing program compiles a summary VSP seismogram with a step between receivers of 25 cm. With such a depth interval on the vertical hodograph, it is possible to obtain a velocity section of elastic waves, approaching the level of detail to the sections of the cone penetrometer (CPT).

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

The longitudinal wave velocity is determined by the first arrivals of the direct (downward) wave. To determine the shear wave velocity in a section represented by a thick layer of loose Quaternary and moraine soils in water-saturated wells with a sufficiently high accuracy, it is possible to use the velocity of the hydrowave recorded by piezohydrophone pressure sensors by a simple method without using three-component probes.

 

The VSP section was obtained by recording oscillations in the well using an 8-channel probe with piezoelectric pressure sensors. The step of the receivers is 1 m, the distance of the SP from the wellhead is 5 m. Along with the first arrivals of the direct longitudinal wave, a low-frequency low-speed hydrowave is recorded.

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

Non-longitudinal vertical seismic profiling

A system of a non-longitudinal VSP, made along a linear profile using several different shot points, can be converted into an areal system of a non-longitudinal VSP with the location of profiles along several spatially spaced lines of the location of the shot points. The data acquisition and processing program used in this case generates spatially coordinate discrete values ​​of elastic wave velocities along curvilinear seismic rays. On the basis of such a sample, it is possible to obtain vertical and horizontal sections of velocities on arbitrarily given planes, reflecting the spatial distribution of elastic wave velocities in the soil mass.

Scheme of execution of non-longitudinal VSP

An example of contouring, according to 3D VSP data, a zone of deconsolidation in karst rocks and boundaries of a dense dolomite layer on vertical sections in vertical planes remote from the well at a distance of 8, 10 and 12 m (from left to right)

Velocity sections were calculated based on data from non-longitudinal VSP presented by the Department of Active Seismic Acoustics of the Mining Institute, Ural Branch of the Russian Academy of Sciences (Perm).

        Removal of sources from the wellhead up to 64 m. The number of accumulations in the excitation pulse is 3. Shot points are located along three parallel lines, one of which runs along the wellbore, the second is 15 m away from the wellhead, and the third is 30 m away. The distance between the shot points  4 m on each line. Record sampling rate 200 µs and number of samples 2000, record length 400 ms. Using this observation system, it is possible to obtain the distribution of elastic wave velocities in the near-wellbore space, which has the shape of an inverted pyramid with a base on the observation surface. As the well approaches, the depth of the area under study increases, but its transverse dimensions in the horizontal plane decrease.