Multi-position analysis of surface wave energy attenuation

In traditional methods of shallow-depth engineering seismic exploration, when elastic oscillations are excited by shock surface sources or explosions of superimposed and low-level charges, the vast majority of energy is spent on the generation of low-frequency low-speed surface waves. During the propagation of these waves in the subsurface soil with varying physical properties  the reflections occur from local inhomogeneities, so that at records local anomalies of high energy surface waves can appear due to the convergence effect.  Such anomalies may be as indicative sign of hard reflective surfaces, in particular vertical zones of fracture and discontinuity of the soil mass and the bedrock.  The method of surface waves  energy analyzing is based on determining the frequency-dependent absorption coefficient, which, in turn, allows  to estimate the attenuation of different harmonics and frequency components of their spectra at different depths of oscillation penetration  into the soil thickness depending on the wavelength.

Field seismogram of surface waves (lower pool of the  Plavinas HPP dam)

The parameter of attenuation of the surface wave energy is calculated according to the relationship of the spectral components in pairs of  seismic record traces equidistant from a common central point (midpoint) on the profile. When the relationship calculating  an attenuation of wave energy due to the wave front geometric spreading in according to the law of inverse square root of the distance between the traces takes into account. With a high multiplicity of overlapping observations, the number of tracks belonging to each midpoint of the records from different sources is large, thereby a powerful statistical effect is achieving that minimizes, in general, the fluctuations of spectra due to differences in surface conditions of geophone installation.

Working window of the program of trace-to-trace relationship calculation of surface wave spectra

The result of the calculation is a two-dimensional convolution (quasi-section) of the surface wave energy absorption coefficient array along the depth and distance. The depth to which the calculated value of the absorption coefficient relates is determined by the depth of  energy maximum of  corresponding frequency component, which, in turn, depends on its phase velocity. Therefore, joint interpretation of multichannel analysis of surface waves (MASW) and calculation of frequency-dependent absorption coefficient is most effective.   The relationship of the absorption coefficient with P-wave velocities is less significant, but  you can see some high-quality matching characteristics of the investigated section, based on two independent methods.

The section of the longitudinal wave velocities and of surface waves energy dissipation on the profile downstream  the Plavinas HPP dam

Section of longitudinal wave velocities and attenuation of surface wave energy at the site of  a local  earth’s surface subsidence on the mine field over underground salt mines

Following  results of calculation of longitudinal wave  velocity sections  and energy of surface wave dissipation based on the materials submitted by the division of active seismoacoustic of Mining Institute of UB RAS (Perm).

The positive values of the absorption coefficient (blue) correspond to the areas with low longitudinal wave velocities at the earth’s surface subsidence sites. The anomalous areas with negative values of the absorption coefficient (red colors) correspond to places where the “feeding” of  surface wave energy takes place  due to reflection and scattering at the potential boundaries of zones in the rock massif with vertical fracturing and crushing.

Longitudinal wave velocities sections according to inverted VSP and attenuation of surface wave energy data

On these sections zones of surface wave energy increasing  due to reflection on possible sites of vertical fracture and crushing of rock massif on borders of the low-velocity zone of the earth’s surface accelerated subsidence on the mine field over the abandoned salt developments are shown.