Prerequisites (Knowledge/Experience/Education required)
The course is designed to be followed by anyone with a broad geoscience background: no specific detailed foreknowledge is required, although a familiarity with geophysical terminology (e.g., frequency, Fourier Transform) will be useful.
This two-day short course introduces the most important theoretical and practical aspects of the multichannel analysis of surface waves (MASW) method by a principle from the research group that originally developed the methodology at the Kansas Geological Survey. Each student will be exposed to the most current approaches for using Rayleigh, Scholte, and Love surface waves for the estimations of 1D shear-wave velocity (Vs) vertical profiles to depths of a few tens of meters, extendable to 2D and 3D volumes. Various practical topics related to MASW data acquisition and analysis will be covered, including the use of active and passive sources, optimum source distance and spread-size determination, dispersion-curve imaging using conventional and high-resolution methods (e.g., HRLRT), fundamental- and higher- mode evaluations, inversion of fundamental- and multi- mode Rayleigh-, Scholte, and Love-wave dispersion curves with the incorporation of 2D compressional-wave velocity (Vp) and density a-priori information, construction of 2D Vs images with topography and variable depth, etc. Advance topics will include dispersion-curve imaging and the FK transform, stitching/blending dispersion-curve images for optimizing horizontal resolution and maximum investigation depth, muting surface-wave components, challenging dispersion-curve patterns, surface-wave modeling and inversion theory, impacts of a-priori density trends on inversion results, and Rayleigh-wave attenuation measurements and inversion for Qs (+-Qp). The course will examine case studies, such as, mapping bedrock, mapping fault zones, 3D characteristics of seismic properties, studying levees, studying voids, etc., as means to help evaluate and appreciate the range of applications currently employing the MASW method. Use of 2D (or 3D) Vs estimates for enhancing static corrections for both Vs and Vp reflection seismic data processing will be discussed. Attendees will be provided software with a temporary license to use during the short course to experience and evaluate the MASW analysis in class and options for practice with, once back at their office. The goal of the short course is to develop an understanding of and skill sets with the method sufficient that participants can confidently incorporate the MASW method in their work.
- Recognize surface-wave energy on raw seismic data and approximately estimate overall surface-wave velocity and approximate depth of investigation.
- Obtain a dispersion-curve image from a raw seismic shot record, optimize velocity and frequency ranges, evaluate the applicability of the surface-wave method, and quickly estimate approximate velocities and depths.
- Discriminate fundamental-mode energy on dispersion images.
- Choose optimum spread-size and source-offset field parameters for optimum dispersion-curve visualization and defend the choice.
- Interpret and pick fundamental- and higher-mode dispersion curves.
- Construct a 2D shear-wave velocity (Vs) initial depth model from multiple dispersion curves with topography and variable depth and make initial evaluations of the investigated site.
- Assemble 2D compressional-wave velocity (Vp), Poisson’s ratio, and density a-priori information for the inversion.
- Select appropriate inversion parameters.
- Evaluate the possibilities for increasing both the minimum and maximum depth of investigation as well as horizontal and vertical resolution of the final Vs estimates.
- Experiment with more than one set of spread-size/source-offsets for dispersion-curve imaging and evaluate the possible combined use of the resulting images (or only selected parts of those images).
- Compare inverted-model calculated dispersion curves to the energy trends on the original dispersion-curve images and experiment with forward modeling.
- Design optimal field survey parameters for MASW analysis given equipment availability and goals of the investigation.
- Argue about using Rayleigh or Love surface waves for obtaining Vs estimates (both acquiring and analyzing).
- Illustrate the possibility of using proportionally more sources with a software tool instead of using more equipment (geophones and seismographs).