An empirical expression used for calculating porosity from interval transit time determined from acoustic logs:

where t = observed interval transit time, t_f = transit time in the pore fluid, and t_ma = transit time in the rock matrix. This relation works well in clean consolidated 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.