12 Well seismic surveying and acoustic logging • Multiple Analysis of Surface Waves (MASW) which, by analyzing the Rayleigh or Love wave phase velocity in the frequency domain (scatter diagram), enables the calculation of the evolution of the shear wave velocity (VS) within the first tens of meters of subsurface. This method is increasingly used in geotechnology in combination with the seismic refraction method to determine the shear modulus. The vertical resolution of all surface geophysical methods decreases as a function of the depth investigated. To obtain a precise model of the deep subsoil’s seismic parameters (propagation velocities of P waves (VP) and S waves (VS), and density), geophysicists use borehole data such as those provided by the well seismic and acoustic logging methods, in particular to carry out the tying and calibration at depth of surface measurements. In addition, processing provides both a model for the propagation velocities of waves (P and S waves) and also for density, such as the examples presented at the end of this introduction. The examples presented in Figure 1 are extracted from 3D seismic data. Figure 1(a) is a near-surface example (Mari and Porel, 2007). The P velocity distribution was obtained by seismic refraction (tomography) for the very near surface (up to 30 m deep) and by seismic reflection (acoustic inversion) for the deep seismic horizons (20 to 120 m). This first example is the subject of the case study in Chapter 5. It should be noted that a similar approach could be made by combining the MASW method and the S-wave seismic method. Figures 1(b), 1(c) and 1(d) are derived from the processing of a seismic reflection survey carried out to map horizons down to 1,500 m deep (Mari and Yven, 2014). The distribution of velocity (VP and VS) and density were obtained by elastic inversion. The examples presented in this introduction already make it possible to highlight the fact that surface and well seismic methods combined with acoustic methods can be used successfully to estimate mechanical modules (Poisson’s ratio, shear modulus and Young’s modulus...). The objective of this book is to illustrate that the processes applied in deep geophysical exploration, combining different seismic and logging methods, can be applied to certain geotechnical and hydrogeological surveys, and site characterization in the context of seismic hazard studies. This book, which is composed of five chapters, aims to present some of these approaches and their applications for near surface surveys (<150 m): • The first chapter provides an overview of the state-of-the-art technology in the geotechnical field regarding borehole measurements of subsoil shear wave velocity. It highlights the benefits of combining different methods: VSP-type well survey measurement with SH waves, generally called downhole, transmission between boreholes generally called crosshole, and dipole type acoustic logging (PSSL). • The second chapter is devoted to the well seismic method. It describes the implementation procedure, the means of acquisition (sources and sensors) used in the civil engineering field, the different types of waves that make up the well seismic recordings (volume waves and guided modes) and the processing sequences. For more information, see “Well seismic surveying” by J.L. Mari and F. Coppens, 2003, Editions Technip.
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