IDENTIFICATION OF THE ZONES OF SUBVERTICAL FRACTURE

Analysis of surface wave energy attenuation anomalies

Seismic records used on this page are acquired by the field team of the Department of Active Seismjacoustics of  Mining Institute of Ural branch RAS (Perm).

The most intensive Rayleigh surface waves registered on records of shallow-depth seismic survey often detect significant variability of amplitude and frequency along the observation profile. One of the main causes of this phenomenon may be lateral inhomogeneities of the subsurface part of the soil mass, including the zones of subvertical fracture, which represent the boundaries for waves propagating along the earth’s surface.

In places of manifestation of such boundaries there are areas of “feeding” the energy of the surface wave due to its reflection.

Correlation of anomalies of reduced longitudinal wave velocities and surface wave intensity

The most sensitive parameter for assessing the degree of attenuation or, conversely, the increase in the energy of the surface wave is the ratio of frequency spectra of many pairs of seismic traces registered with multiple accumulation from many sources and belonging to one middle point on a profile. The ratio of harmonic amplitudes of spectra at different frequencies allows us to relate the calculated values of the ratio to different depths of the section – the lower the frequency, the deeper the penetration of the wave into the soil array thickness.

The following figure shows a clear correlation of the narrow zone of the reduced velocity of the longitudinal waves and the increase in the energy of the surface wave due to its reflection from the vertical boundary of inhomogeneity.

Crosshole seismoacoustic sounding

In contrast to the low-frequency surface waves, high resolution, and very effective method of identifying zones of vertical fractures is crosshole seismoacoustic sounding. A dense network of observations with a small step displacements of sources and receivers of the pulses on the trunks of borehole allows  to display not only the layer-by-layer inhomogeneity of the geological section along a depth, but also to identify sub-vertical zones of decompression, which may be associated with fracture and vertical channels of filtration of surface waters.