Overpressure is one of the major difficulties we have to face when exploring the deep earth. Geophysical remote sensing methods, especially seismic and electrical methods, are widely employed to predict overpressure for de-risking the drilling of deep boreholes in the earth’s crust. However, there are major uncertainties in relating the measured geophysical parameters to the physical properties of crustal rocks, needed for reliable overpressure estimation. We address this knowledge gap through dedicated laboratory measurements of elastic velocity, electrical conductivity and porosity with increasing pore pressure and the analyses and interpretation of the experimental results. We find a linear increase in the porosity, and an exponential increase and decrease in the electrical conductivity and compressional wave velocity, respectively with increasing pore pressure. We also find an exponential reduction in the cementation exponent and effective pore aspect ratio inverted from the pore pressure induced electrical conductivity and compressional wave velocity, respectively. We further demonstrate that the changing cementation exponent and effective pore aspect ratio by increasing pore pressure affects more significantly the electrical and elastic rock properties than does by dilating porosity. The results not only strengthen our understanding of the influencing mechanism of overpressure on the physical properties of crustal rocks, but also provide new insights for the more reliable detection of overpressure zones from geophysical remote sensing.
Pore pressure coefficient for soil and rock and its relation to compressional wave velocity
