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(A) Sonic logs measure the velocity of sound waves as they travel through a rock. Note again the influence of gas on the sonic-log properties of the sandstone (SS) in the figure. Shales (SH) generally exhibit a large sonic transit time because it takes a long time for sonic waves to travel through them (i.e., they have a low sonic velocity). (B) The neutron log measures concentrations of hydrogen (water and hydrocarbons) in pores, expressed in units of percent porosity, and is also an indicator of gas content.
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