A system for measuring a porosity of a gravel pack in a wellbore includes a transmitting acoustic transducer attached to a sand screen and configured to transmit an acoustic signal. The system further includes multiple acoustic transducers attached to the sand screen and configured to receive the acoustic signal through the gravel pack. The system also includes a control device configured to receive electrical signals from the multiple acoustic transducers and determine the porosity of the gravel pack around the sand screen based on at least the electrical signals. The electrical signals are generated from the acoustic signal received by the multiple acoustic transducers.