Measured and Modeled Gravity Anomalies above the Tunnel in Clays – Implication for Errors in Gravity Interpretation

We measured gravity anomaly above the tunnel excavated in stiff clays. Technical parameters of the tunnel were known. In order to obtain material parameters of clays the core hole was drilled in the study area. For calculation of gravity anomaly of the tunnel we used wet bulk densities of clays derived from laboratory measurements of drill core samples. But the fit between observed and calculated gravity data were poor in this case. If we use for interpretation densities calculated from laboratory measurements we interpret the roof of the tunnel in the depth of 18 m instead of correct 12 m. The reason is that the borehole clay samples can expand after removal from the depth and they do not retain natural moisture probably. Much better fit between observed and calculated gravity values we received when we used for modeling densities derived from gamma-gamma log. We also modeled gravity effect of the deformation zone which forms around the tunnel. Gravity effect of the deformation zone was negligible.