Which log is primarily used to estimate porosity by measuring the travel time of acoustic waves?

Which log is primarily used to estimate porosity by measuring the travel time of acoustic waves?

• A. Sonic log
• B. Neutron log
• C. Density log
• D. Gamma ray log

Answer: (A)

The main idea is that porosity can be inferred from how long it takes an acoustic wave to travel through the rock. The sonic log records the interval transit time of a compressional wave over a known formation thickness. This travel time, Δt, is longer when there is more pore space filled with fluid, because the pore fluid and rock frame slow the wave. By using a porosity-velocity relationship (such as the time-average model), the observed travel time can be converted into porosity: φ = (Δt_log − Δt_m)/(Δt_f − Δt_m), where Δt_log is the measured interval transit time, Δt_m is the matrix travel time, and Δt_f is the fluid travel time. So, this log is specifically designed to translate acoustic travel-time measurements into porosity estimates. Neutron and density logs estimate porosity from hydrogen content and bulk density, respectively, while gamma ray logs help identify lithology, not porosity via acoustic travel time.

The main idea is that porosity can be inferred from how long it takes an acoustic wave to travel through the rock. The sonic log records the interval transit time of a compressional wave over a known formation thickness. This travel time, Δt, is longer when there is more pore space filled with fluid, because the pore fluid and rock frame slow the wave. By using a porosity-velocity relationship (such as the time-average model), the observed travel time can be converted into porosity: φ = (Δt_log − Δt_m)/(Δt_f − Δt_m), where Δt_log is the measured interval transit time, Δt_m is the matrix travel time, and Δt_f is the fluid travel time. So, this log is specifically designed to translate acoustic travel-time measurements into porosity estimates. Neutron and density logs estimate porosity from hydrogen content and bulk density, respectively, while gamma ray logs help identify lithology, not porosity via acoustic travel time.