In Situ Shear Modulus Measurements in a Fractured High-Porosity Chalk Mass

Abstract

This study explores in situ small-strain shear modulus in low-density structured chalk, a key input parameter in empirical and numerical models. A range of in situ testing procedures, supported by detailed core logging, have highlighted the difficulties and opportunities in characterizing the in situ shear modulus of the stiff fractured chalk mass. Over 1,000 seismic traces obtained from tightly controlled PS logging, borehole geophysical, and seismic cone penetration testing were assessed for data quality. Interpretation using the automated cross-correlation technique demonstrated robustness while more time consuming and subjective approaches were essential for lower-quality data. Where comparable measurements were taken, the results tended to be relatively consistent between measurement techniques. The spacing and nature of fractures in the mass were shown to influence the results. The in situ shear modulus from seismic and pressuremeter tests tended to increase steadily from relatively low values at ground level. Sharp increases were seen at the water table, where the fractures became partly closed and water-filled, with a weak tendency to increase with depth or burial stress thereafter. While laboratory shear modulus significantly exceeded the in situ values in the shallower layers, the results are shown to converge with depth as the fracture frequency reduces. The new in situ shear modulus profile offers important insights and input parameters for chalk structure interaction models. Based on the results, guidance is offered for obtaining high-quality measurements in structured chalk masses for engineering applications.