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Calibration of this method requires knowing the appropriate normal value of each parameter. Where the subscripts n and log refer to the normal and measured values of density, resistivity, or sonic delta-t P p is the actual pore pressure, and P hyd is the normal hydrostatic pore pressure. In the ratio method, pore pressure is calculated using the assumption that, for sonic delta-t, density, and resistivity, respectively, the pore pressure is the product of the normal pressure multiplied (or divided by) the ratio of the measured value to the normal value for the same depth. The only unique assumption required by equivalent depth methods is that effective stress is a linear function of depth.

Where P a,z and S a,z are the pore pressure and the stress at z, the depth of interest and a, the depth along the normal compaction trend at which the measured parameter is the same as it is at the depth of interest. The effective stress at depth Z is equal to the effective stress at depth A, and thus, the pore pressure at depth Z is simply P z = P a + ( S z– S a). The normal compaction trend (NCT) is a straight line in log-linear space that has been fitted to the decrease in slowness as a function of depth where sediments are normally compacting. 1-Illustration of the equivalent depth method using sonic ΔT. Because the value of the measured physical property is a unique function of effective stress, the pore pressure at any depth where the measured value is not on the NCT can be computed fromįig. When the log of a measured value is plotted as a function of depth, NCTs can be displayed as straight lines fitted to the data over the normally compacted interval. This method first assumes that there is a depth section over which the pore pressure is hydrostatic, and the sediments are normally compacted because of the systematic increase in effective stress with depth. One example of analysis using a trend line is the equivalent depth method illustrated in Fig. Methods to determine pore pressure Equivalent depth methods

1.5 Sonic method with depth-dependent normal compaction trendline (NCT).1.4 Resistivity method with depth-dependent normal compaction trendline (NCT).

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