Definition of  transverse wave propagation velocity by  MASW method

In engineering seismic exploration, using the simplest surface sources of elastic waves, more than 85 percent of the impact energy is spent on the formation of low-frequency and low-velocity surface waves. The following seismogram, obtained on the asphalt pavement of the embankment with an 8 kg sledge hammer pulse, illustrates the dominance of low-velocity surface waves in relation to refracted and reflected waves. Being the strongest hindrances for the method of reflected waves, Rayleigh surface waves at the same time contain very valuable information for the study of soil properties about the velocity of propagation of transverse waves.

The seismogram with the records of surface waves

The most advanced and rapidly developing at present in shallow seismics methods of processing based on multi-channel analysis of surface waves (MASW), gives quite a detailed one and two – dimensional model of shear wave velocities Vs distribution in the soil mass. A natural limitation of the MASW technique is a small depth of penetration of the surface wave energy into the soil massif thickness. An essential factor for increasing the depth of the method is the possibility of attracting passive low-frequency noise of transport and operating mechanisms for joint analysis with the records obtained in the active mode.
The main precondition for multichannel frequency analysis is the predominance of the energy components of transverse displacements during propagation of oscillations along the surface of the media and frequency dispersion of phase velocities – an increase of visible period and, accordingly, an increase of the penetration depth of surface wave energy into the thickness of the soil massif. The matrix of phase velocities of several Rayleigh surface wave modes characterizing strong frequency dispersion of phase velocities is shown below. The dispersion curves of the zero, first and second harmonics of the surface wave are used for calculation of the transverse wave velocities model.

Selection of dispersion curves of Rayleigh surface waves phase velocities and construction of a single-dimensional model of transverse wave propagation velocities

The above results of multichannel analysis of surface waves are obtained using RadExPro software complex (Deco-Geophysics, Moscow).

Assessment of soil physical properties on the basis of complex interpretation of longitudinal and transverse wave velocities

The sections shown in the figure characterize the typical ratio of representativeness of geotechnical methods, drilling and geophysics in assessing the physical properties of the soil massiif. Despite the relatively high density of drilling points and dynamic penetration testing, they do not provide detailed spatial configuration presentation of the boundaries of engineering-geological elements within a relatively homogeneous array of moraine loams. At the same time, the transverse wave velocities, which characterize, first of all, the properties of the soil skeleton, give a clear spatial picture of the distribution in the moraine loams of large-block fraction clusters (moraine ridges) with increased velocity of the transverse wave propagation.
In geotechnical surveys, several types of waves are usually used to assess the state and properties of the soil massif and its saturated fluids: longitudinal reflected, refracted and surface waves. The following figure shows the types of seismic sections in one of the profiles in the central part of Latvia. In the upper part of the section up to 35 m of moraine loams lies, then about 20 m of Plavinian dolomites, below- the upper devonian sediments – Amata-Gauja sandstones lie. At a depth of 160 m (both sections in the depth scale) on the CDP section a fairly well-correlated boundary follows, perhaps the bed of the Amata-Gauja sandstones.
These materials obtained with the use of 8 kg sledge hammers and a single 64-channel receiving arrangement of geophones with a step of 2 m. However, the complexity of the field work and data processing for these sections varies considerably. So, if during obtaining of CDP cross-section by the reflected waves, the interval between shot points was 2 m, to obtain velocity sections from refracted and surface waves were used only seismograms that were obtained with a step of shot points along the profile 16 m.

Calculation of the sections of soil physical properties on high mountain deposits of placer gold in Chucapaca (department Moqeguia, Peru)

Examples of determining of transverse waves propagation velocity according to MASW and prediction of physical and mechanical properties of soils using the method of refracted waves along one of the lines of the working of the gold-bearing field in the South of Peru are shown in the following figure.

Based on the longitudinal waves propagation velocity sections obtained from first arrivals of refracted waves and transverse wave velocities from MASW method the sections of the forecast density, Poisson’s ratio and elastic modulus are calculated along one of profile at the site of placer gold deposits working (Chucapaca, department Moquegua, Peru).