An empirical expression used for calculating [[porosity]] from [[interval transit time]] determined from [[acoustic log|acoustic logs]]:

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where t = observed interval transit time, t<sub>f</sub> = transit time in the pore fluid, and t<sub>ma</sub> = transit time in the rock [[matrix]]. This relation works well in [[clean]] [[consolidated]] [[formation|formations]] with uniformly distributed pores. In vuggy formations, the [[sonic log]] may not reflect the [[secondary porosity]]; and in [[unconsolidated]] formations, this relationship may overestimate porosity. In such cases, the formula may be empirically modified to give better values.