Inverted vertical seismic profiling

Inverted vertical seismic profiling (VSP) in wells is performed using a borehole pulse source and ground-based geophone receiving lines. Simultaneously receiving the signals can be run in adjacent wells on a multi-channel piezo-hydrophone  probe. As a signal source, an electric spark source consisting of a current pulse generator and a multi-electrode emitter placed in a sealed container filled with salt water is used. The accumulated electric energy is of the order of 1.0 to 1.5 kJ. The step of moving the emitter along the borehole from the bottom up to wellhead is 1-5 m.

The profiles of the receiving points are located along lines arranged in any order relative to the well with the emitter. The profiles can be laid on the dotted lines. The step of geophones along profiles is 2-4 m. Maximum removal of geophones from the wellhead is determined by the energy of the emitter and the properties of the upper loose part of  soil massif and can reach 150-180 m with the above source parameters.

Scheme of inverted VSP performing

Processing of seismic records and the calculation of the velocity sections shown below  is made using VSP data  obtained by division of  active seismic of Mining Institute UB RAS (Perm)

Diagram of  projections of the seismic rays on the earth’s surface during performing inverted VSP from two wells

 

Analysis of the amplitudes and the spectral content of the recorded signals of inverted VSP

Tests have shown that the energy of the acoustic pulse emitted by the electric spark source was quite high at distances up to 120-150 m from the well. Spectrum of the registered signal in close proximity to the well has a steady single mode shape with a maximum at a frequency of about 300-350 Hz. As the receiving channel is removed from the well, additional modes appear in the spectrum of signals, the maximum frequency is recorded in the range of 80-100 Hz. The strong influence of interference is manifested on the records at a sufficiently large distance, more than 60-80 m from the wellhead.

Program of adaptive selection of ray velocities by time-distance curves of the first breaks of  direct wave

For data processing  a specialized  software is used. The basis of algorithm of ray velocities calculation is the principle of minimal travel time along the ray between source and receiver.  The selection of ray velocities is performed by an iterative algorithm until the specified minimum difference between the observed  time and calculated theoretical time-distance curve is reached  for a thin-layered model with an arbitrary velocity distribution in the layers over the laterals. The use of the program is possible for mutual schemes of observation – direct and inverted VSP.

Main menu of the VSP processing program

The program’s working windows displaying the observed hodographs, the beam pattern and adaptively selected velocity graph (in green)

Working windows of the program with display in the central window of the observed (blue color) and adaptively selected (red color) vertical hodographs

 

Calculation of anomalies of the first break time field  on a series of profiles on a surface 

The inverted VSP data processing program of provides calculation of various kinematic and dynamic parameters of the recorded waves. In particular, the horizontal sections of the field of  first break times are calculated for different levels of the source location in the well. Anomalies of this field are  direct and  most obvious sign of longitudinal wave propagation velocity local inhomogeneities   presence in the soil massif.

Calculation of horizontal velocity sections

When processing the obtained data  on the set of observation profiles, a spatial 3-dimensional sample of coordinated velocity values bound to different segments of seismic rays is collected on the surface. For geological interpretation of the data on this sample, horizontal and vertical velocity sections are calculated, representing local inhomogeneities of the soil massif and bedrocks.

Calculation of  velocity sections along arbitrary set of vertical planes

Vertical section of velocity can be made along arbitrarily specified combinations of vertical planes, the projection of which on the earth’s surface have the form of a broken line. In some cases, it is advisable to construct a section along the inclined plane, crossing the spatial sampling of coordinated velocity values at an arbitrarily given angle.

The scheme of projections of rays on the day surface and the projections of the vertical planes of the velocity       cross section (dashed line in pink)

Vertical velocity  section example

Three-dimensional representations of velocity heterogeneity of the soil mass

The inverse VSP data processing program prepares files for a visual three-dimensional representation of the model of the distribution of elastic wave velocities in space. For this, a set of sections is formed from a coordinated spatial velocity sample along a series of parallel vertical sections, in which the averaged velocity values ​​are located at the grid nodes with a given step in depth and along one of the coordinate axes. Then, with fixed XY coordinates, the velocity values ​​distributed over the depth are converted into an analog of the seismic trace, supplied with the corresponding trace header, and the file prepared in this format is exported to the RadExPro software package. By means of visualization of this complex, a cube of seismic velocities is created and visual interactive data interpretation procedures are performed.

Three-dimensional display of the longitudinal wave velocity distribution in the space under study.

The relief of the surface of the rock mass with a velocity of elastic waves of more than 1600 m/s with the display of the diagonal zone of decompaction and fracture of the overlying interval