Observed and calculated gravity anomalies above a tunnel driven in clays – implication for errors in gravity interpretation
The aim of this work was to evaluate the gravity effect of a deformation zone, which forms around subsurface cavities and to assess which method of density determination of soils in the surveyed area is optimal. We measured and modelled a gravity anomaly above a tunnel excavated in stiff clays. Technical parameters of the tunnel were known. The shape of the deformation zone around the tunnel was determined by geotechnical modelling. Material parameters of clays necessary for the geotechnical model were obtained from a measurement of borehole samples in a soil mechanics laboratory. For calculation of the gravity anomaly of the tunnel we used densities of clays derived from laboratory measurements of borehole samples. However, the fit between observed and calculated gravity data was not accurate in this case. The reason was probably that the volume and water content in the borehole clay samples changed after their removal from the depth. A better fit was obtained when we used densities derived from a gamma-gamma well log. We found that the gravity effect of the deformation zone that forms around the tunnel in stiff clays is negligible (0.001 mGal). In our case, a concrete tunnel lining has a bigger influence on the amplitude of the anomaly than the deformation zone (0.008 mGal). When we neglect the lining in the interpretation, we obtain an error of 20% in the estimate of the tunnel depth.