Julian Ivanov

Multichannel analysis of surface waves to map bedrock

In many geologic settings, topographic variations and discontinuities in the surface of bedrock can influence the transport and eventual fate of contaminants introduced at or near the ground surface. Determining the nature and location of anomalous bedrock can be an essential component of hydrologic characterization. Preliminary analysis of the hydrologic characteristics of a site in Olathe, Kansas, U.S., based primarily on borehole data alone, suggested that a cluster of fractures and/or an unmapped buried stream channel may influence fluid movement along the drill‐ defined bedrock surface.

Comparing shear-wave velocity profiles inverted from multichannel surface wave with borehole measurements

Recent field tests illustrate the accuracy and consistency of calculating near-surface shear (S)-wave velocities using multichannel analysis of surface waves (MASW). S-wave velocity profiles (S-wave velocity vs. depth) derived from MASW compared favorably to direct borehole measurements at sites in Kansas, British Columbia, and Wyoming. Effects of changing the total number of recording channels, sampling interval, source offset, and receiver spacing on the inverted S-wave velocity were studied at a test site in Lawrence, Kansas. On the average, the difference between MASW calculated V s and borehole measured V s in eight wells along the Fraser River in Vancouver, Canada was less than 15%. One of the eight wells was a blind test well with the calculated overall difference between MASW and borehole measurements less than 9%. No systematic differences were observed in derived V s values from any of the eight test sites. Surface wave analysis performed on surface data from Wyoming provided S-wave velocities in near-surface materials. Velocity profiles from MASW were confirmed by measurements based on suspension log analysis. ⓒ 2002 Elsevier Science Ltd. All rights reserved.

Combined use of active and passive surface waves

With a surface wave method to estimate shear-wave velocity (Vs) from dispersion curve(s) of known mode(s), accurate modal identification is obviously a crucial issue. In this regard, the dispersion imaging method is an essential processing tool as it can unfold the multi-modal nature of surface waves through direct wavetield transformations. When a combined dispersion curve of an extended frequency range is prepared from analyses of both passive and active surface waves attempting to increase the maximum depth of Vs estimation, the modal nature of the passive curve (as well as the active one) must be assessed although it has usually been considered the fundamental mode. We report two experimental survey cases performed at the same soil site, but at two different times, employing the passive and active versions of the multichannel analysis of surface waves (MASW) method for an increased investigation depth. In the earlier survey, the modal nature of the imaged dispersion trends from the passive ( 20 Hz) data sets was identified as the fundamental mode, whereas it was confidently re-identified as the first higher mode from the later survey. Modal inspection with the dispersion image created by combining passive and active image data sets was the key to this confident analysis. The modal nature of the passive curve was identified from its context with active curves, whose confident analysis therefore had to come first. An active data set acquired with a small (<1.0 m) receiver spacing and an impact point located close to the receivers appears important for this purpose. (Less)

Multichannel analysis of surface waves (MASW) - Active and passive methods

The conventional seismic approaches for near-surface investigation have usually been either high-resolution reflection or refraction surveys that deal with a depth range of a few tens to hundreds meters. Seismic signals from these surveys consist of wavelets with frequencies higher than 50 Hz. The multichannel analysis of surface waves (MASW) method deals with surface waves in the lower frequencies (e.g., 1–30 Hz) and uses a much shallower depth range of investigation (e.g., a few to a few tens of meters).

Delineating a shallow fault zone and dipping bedrock strata using multichannal analysis of surface waves with a land streamer

The multichannel analysis of surface waves (MASW) seismic method was used to delineate a fault zone and gently dipping sedimentary bedrock at a site overlain by several meters of regolith. Seismic data were collected rapidly and inexpensively using a towed 30-channel land streamer and a rubberband-accelerated weight-drop seismic source. Data processed using the MASW method imaged the subsurface to a depth of about 20 m and allowed detection of the overburden, gross bedding features, and fault zone. The fault zone was characterized by a lower shear-wave velocity ( Vs ) than the competent bedrock, consistent with a large-scale fault, secondary fractures, and in-situ weathering. The MASW 2D Vs section was further interpreted to identify dipping beds consistent with local geologic mapping. Mapping of shallow-fault zones and dipping sedimentary rock substantially extends the applications of the MASW method.

Joint analysis of refractions with surface waves: An inverse solution to the refraction-traveltime problem

We describe a possible solution to the inverse refraction-traveltime problem (IRTP) that reduces the range of possible solutions (nonuniqueness). This approach uses a reference model, derived from surface-wave shear-wave velocity estimates, as a constraint. The application of the joint analysis of refractions with surface waves (JARS) method provided a more realistic solution than the conventional refraction/tomography methods, which did not benefit from a reference model derived from real data. This confirmed our conclusion that the proposed method is an advancement in the IRTP analysis. The unique basic principles of the JARS method might be applicable to other inverse geophysical problems.

Utilization of high-frequency Rayleigh waves in near-surface geophysics

Rayleigh waves, surface waves that travel along a “free” surface such as the earth-air or the earth-water interface, are usually characterized by relatively low velocity, low frequency, and high amplitude. Rayleigh waves are the result of interfering P and SV waves. Particle motion of the fundamental mode of Rayleigh waves in a homogeneous medium moving from left to right is elliptical in a counter-clockwise (retrograde) direction along the free surface. As depth increases, the particle motion becomes prograded and is still elliptical when reaching sufficient depth. The motion is constrained to a vertical plane consistent with the direction of wave propagation.

Resolution of High-frequency Rayleigh-wave Data

It is essential to understand the potential resolving power of surface wave data in interpreting inverted vertical (layered 1D) and horizontal (‘pseudo-2D’) shear (S)-wave velocity models. The model resolution matrix indicates that the S-wave velocity model of a layered earth model can be perfectly resolved in a least-squares sense if error-free data are inverted. However, we show that errors in real data introduce a smear matrix in the least-squares solution, reducing the resolution of an S-wave velocity model. A field test shows how accurate dispersion curves can resolve a low velocity layer of about 25% contrast at 3.5m depth below a stiff overburden. The horizontal resolution of surface wave surveys is limited to the aperture (length) of the recording array. In phase velocity measurements, the layered model estimate represents an average over this length. Any lateral discontinuities introduce a systematic error into the inversion, leading to acquisition geometry-dependent estimated models. The multich...

Mapping Poisson's Ratio of Unconsolidated Materials from a Joint Analysis of Surface‐Wave and Refraction Events

Seismically, σ can be determined if P(Vp) and S-wave (Vs) velocities are known. This would indicate that two separate (Pand S-wave) surveys should be performed in order to obtain the separate maps for Vp and Vs. Running both types of survey for one project will be expensive in terms of equipment, data processing, and overall time. In addition, S-wave survey is generally known as being much more difficult to acquire good quality data than the P-wave survey.

Underwater MASW to evaluate stiffness of water-bottom sediments

Stiffness measurements are often necessary for geotechnical characterization of an underwater site. Seismically, these measurements can be made through the dispersion analysis of the Rayleigh-type surface waves. Successful terrestrial application of this method has been reported by many investigators using spectral analysis of surface waves (SASW) and more recently using multichannel analysis of surface waves (MASW). The MASW method was originally developed as a land survey method to investigate the near-surface materials for their elastic properties: for example, the shear-wave velocity (VS), by recording and analyzing Rayleigh-type surface waves using a vertical (impulsive) seismic source and receivers. The acquired data are first analyzed for dispersion characteristics and, from these the shear-wave velocity is estimated using an inversion technique.