Seismic Inversion for Near Surface Applications and the Derivation of Geomechanical Properties

Seismic inversion methods broadly fall in to two categories; conversion of seismic event amplitudes in to reflectivity or the analysis primarily of seismic event arrival times (and waveform shape) to derive a velocity model. These are generically referred to as Acoustic Impedance (AI) inversion and Full Waveform Inversion (FWI) respectively, the former typically working from processed seismic reflectivity data and the latter being derived during the processing phase. Both procedures have application in the characterisation of the rock properties of shallow stratigraphic sections, indeed FWI is specifically designed (and limited to) no deeper than approximately 1500m below the mudline (though this depth is dependent on seismic acquisition parameters; notably cable length, water column height and subsurface velocity). This paper will review several different approaches to AI inversion, which can be calibrated to derive rock mechanical properties, and discuss their application to the near surface. The paper will also demonstrate how FWI can yield a high resolution image of near surface velocity which improves the seismic image and thus enhances AI inversion results. Case studies will be used to demonstrate the procedures and contrast the advantages and disadvantages of different methods.