Surface waves: application to lithostructural interpretation of near-surface layers in the meter and decameter range

The exact determination of lithologic and structural parameters of near-surface layers with thicknesses of a few meters and decameters is an important tool to improve the information of seismic reflection and refraction signals from deep and shallow targets. Surface waves are able to deliver reliable information about layer thicknesses, compressional and shear wave velocities and sometimes about densities. This information is obtained by analyzing the dispersion characteristics of the phase velocity and group slowness of Rayleigh surface waves. The data obtained from the dispersion analyses are used to determine the lithostructural parameters of the subsurface by applying an inversion algorithm. The inversion of the phase velocity and group slowness data sets shows that a simultaneous inversion yields the best result compared with those of the single inversion of group slowness or phase velocity data. Different field data sets were available: For a data set of a reflection seismic survey near Lyngby, Denmark, and a refraction seismic data set recorded in the frame of the LITASEIS project, the frequency content of the recorded data was not sufficient for a detailed analysis. However, the application of surface wave analysis gives hints to inhomogeneities in the top layers of the underground. Better results could be obtained by using a seismic source having a frequency content that covers a wider range, or by a frequency adjustable source. In the Hungarian field case, group slowness data could be derived because only a few seismic traces were recorded. The improved modified moving window analysis of the field data shows that the second mode was generated in the seismic wave field. In the Thueringen field case, Rayleigh surface wave data and results from the refraction seismic interpretation were available. Relative model distances serve as quality control factors of the different inversions compared with the result of refraction seismics; the lower the relative model distance, the better the inversion result. The surface wave inversion is a successful method to obtain P-, S-wave velocities and layer thicknesses of near-surface structures using only one component of the recorded seismic wave field; this also holds true for embedded low-velocity layers. (orig./HK)Available from TIB Hannover: RN 2688(50) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

A joint inversion algorithm to process geoelectric and surface wave seismic data. Part II: applications

Seismic and geoelectric methods are often used in the exploration of near-surface structures. Generally, these two methods give, independently of one other, a sufficiently exact model of the geological structure. However, sometimes the inversion of the seismic or geoelectric data fails. These failures can be avoided by combining various methods in one joint inversion which leads to much better parameter estimations of the near-surface underground than the independent inversions. In the companion paper (Part I: basic ideas), it was demonstrated theoretically that a joint inversion, using dispersive Rayleigh and Love waves in combination with the well-known methods of DC resistivity sounding, such as Schlumberger, radial dipole-dipole and pole-pole arrays, provides a better parameter estimation. Two applications are shown: a five layer structure in Borsod County, Hungary, and a three-layer structure in Thuringen, Germany. Layer thicknesses, wave velocities and resistivities are determined. Of course, the field data sets obtained from the 'real world' are not as complete and as good as the synthetic data sets in the theoretical Part I. In both applications, relative model distances, in percentages, serve as quality control factors for the different inversions; the lower the relative distance, the better the inversion result. In the Borsod field case, Love wave group slowness data and Schlumberger, radial dipole-dipole and pole-pole (i.e two-electrode) data sets are processed. The independent inversion performed using the Love wave data leads to a relative model distance of 155%. An independent Schlumberger inversion results in 41%, a joint geoelectric inversion of all data sets in 15%, a joint inversion of Love wave data and all geoelectric data sets in 15% and the robust joint inversion of Love wave data and the three geoelectric data sets in 10%. In the Thuringen field case, only Rayleigh wave group slowness data and Schlumberger data were available. The independent inversion using Rayleigh wave data results in a relative model distance of 19%. The independent inversion performed using Schlumberger data leads to 34%, the joint and robust joint inversion of Rayleigh wave and Schlumberger data gave results of 18% and 20%, respectively