SEISMOACOUSTIC INVESTIGATIONS IN GEOTECHNICAL WELLS

Seismic acoustic sounding according to ASTM d 4428/D standards

Measurements are carried out in three wells drilled at a distance of 3 m from each other along the same line. Wells are cased with polyethylene pipes with a check valve at the bottom of the casing and filled with water. The space between the case and borehole walls is tamponed by  clay, or cement, or filled with water. During the measurement, the pulse emitter and receiver are lowered into adjacent wells to the same depth, provided by the technical specifications, a series of pulse excitations  and recording of the stacking signal are performed, and then the emitter and receiver are moved to new positions. The first time the emitter and receiver are lowered to the depth of the water level in the wells, then moved down the trunk at intervals of 1.0-1.5 m, as provided by the specification. In accordance with the standard ASTM D 4428/D 4428M-07 in single channel mode, there are two or three series of measurements of longitudinal and transverse waves  travel times: with the location of the emitter in the well 1, and receivers in the borehole 2 at a distance of 3 m and in well 3 at a distance of 6 m from the borehole 1.  of the Acoustic pulse start synchronizes with the beginning of the recording by the starting piezoelectric transducer, placed in one container with emitter and connected to the data logger via a communication line.

The figure shows the records obtained from the piezoceramic emitter with a pulse frequency of 4 kHz with an accumulation of 20 actions in each series of measurements. Measurements were made on the basis of 3 m (left) and 6 m (right). The first breaks of a direct longitudinal wave on the most part of the investigated depth interval are traced on the records. In the thickness of dolomites at a depth of more than 18 m signal-to-noise ratio reaches values of 20-30. In the upper part of the section, the conditions for signal passing deteriorate dramatically. Also the arrivals of the shear wave are not enough confident in many of the intervals.

Seismic acoustic sounding in a multi-channel mode

The receiving of oscillations is performed by a multichannel (8 to 24 channels) piezo-hydrophone    probe into the observation well. The requirements for the arrangement of observation wells in this case remain the same as for measurements according to ASTM D,-4428/D standards.If it is impossible to use polyethylene pipes for casing and in case of partial replacement with steel ones in the upper collapsing intervals of the ground array, the observations are carried out in the intervals of open well trunk to the level of filling them with water. A more powerful electric spark type emitter (sparker) with discharge energy of the order of 1-1.5 kJ moves with a given step along the trunk of the neighboring well, as a rule, for the entire length of the studied interval. If necessary to study the geological section interval, exceeding the length of the receiving probe, the probe is moved along the trunk of the observation well to a new position, and a series of pulse excitations with the movement of the source along the well is repeated. 

Seismoacoustic sounding and calculation of tomographic sections of velocities

 Processing of the seismic records shown below and calculation of tomographic sections are executed on the materials submitted by department of active seismoacoustics of the Mining Institute of UB RAS (Perm).

 When using an electric spark source with a accumulated charge energy of about 1000 J, it is possible to reliably register signals in wells at distances up to 100 meters and more. At the same time, due to the attenuation of the high-frequency components of the signal in the soils, the central band of the operating frequencies of emitted signal spectrum of the 1000-1600 Hz is narrowed to 100-400 Hz.

Examples of tomographic sections of P-wave velocities

Summary tomographic section of velocities obtained by radiation in borehole 17A and reception of signals in borehole 17B at a distance of 20 m and in borehole 17ig at a distance of 96 m.

Velocity section obtained during the sounding between the open wellbores in the interval of depths from 19 to 69 m. The section represents the superposition of two tomographic images obtained by the counter transmission between boreholes 16A and 16ig.
Acoustic sounding is performed with mutual replacement of the emitters and receivers arrangement in two bores. Observations are made using different energies – 600 J and 1000 J, and, respectively, at different central frequencies of emitted pulses. Nevertheless, the high convergence of tomographic images in counter-transmission is obtained, both in absolute values of velocities and in the location of local anomalies of velocities. This makes it possible to conclude that the low-velocity anomalous zones revealed in the section adequately reflect the features of the structure and physical properties of soils and rocks at the site of sounding.