Sea Technology

SEP 2017

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www.sea-technology.com September 2017 / st 11 be the bigger questions moving forward in applying seismic inversion for site-survey and geotechnical engineering ap- plications. What Is Seismic Inversion for, and How Does It Work? In the simplest sense, seismic inversion is used to re- duce uncertainties in the spatial mapping of rock or sedi- ment properties by linking key, high-resolution properties from the (typically) sparse network of isolated boreholes with the lower-resolution but spatially continuous (3D) or semi-continuous (2D) seismic reflection imaging response. Sometimes both borehole and seismic information are ex- plicitly included together as part of the inversion process; other times just the seismic data are included in the inver- sion, while the borehole data are used as an independent constraint, or as seeds, for a secondary geostatistical inter- polation. The physics behind such a procedure is compli- cated, not least because of the disparity in scales between the boreholes (centimeter resolution) and seismic reflection data (deci- to decameter resolution), but have been used with great success at reservoir level (or scale) to map a va- riety of properties such as porosity, permeability, lithology, pore fluids and hydrocarbon saturation. As discussed later, seismic inversion alone can provide valuable planning and early design information for near-surface applications even before borehole data become available. Generally, seismic inversion is cast as an optimization procedure, where a model of the subsurface is iteratively updated to minimize the difference between a synthetic data set and some combination of recorded seismic data and boreholes. In practice, how such a procedure is imple- mented can vary hugely depending on the nature of the data being inverted (e.g., just seismic or seismic and boreholes), the complexity of the inversion (e.g., acoustic impedance or anisotropic elastic full waveform), and the approach taken to optimize the inversion (e.g., Gauss-Newton, genetic algo- rithm, simulated annealing). Versions of the simplest seismic inversion techniques (e.g., impedance inversion) are available as processes within most commonly available seismic interpretation soft- ware packages. However, we advise caution when applying black box algorithms that have often been written with ex- ploration data in mind and therefore may not perform op- timally when given higher frequency site-survey data that is (typically) more noise contaminated. The more advanced inversion techniques (e.g., full-waveform inversion), in con- trast, are considerably more involved and can only be found within dedicated industry or academic research groups. We do, however, anticipate more robust inversion software tailored to the site characterization application to become available in the next few years. (Top) Flow chart illustrating a generic site investigation work- flow, together with a summary indication of how seismic inversion could contribute to the different phases. (Bottom) Example seismic inversion results using chirp data from the U.S. West Coast. Q-factor estimates have been determined for a number of different acoustic facies, providing insights into the mechanical behavior (i.e., cohesive versus granular) of the subsurface sediments (note the log 10 axis for Q-factor).

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