A system for measuring a gap width in a structure , wherein the gap is between a first portion and a second portion of the structure , the system comprising: a first vessel , the first vessel comprising: one or more sidewalls and an endwall ; a liquid port configured to connect to a source to fill the first vessel with a liquid ; and an open portion configured to be placed against the structure ; a second vessel surrounding the first vessel , the second vessel comprising a vacuum port configured to connect to a vacuum system to pull a vacuum in a space between an outer surface of the first vessel and an inner surface of the second vessel when the open portion of the first vessel and an open portion of the second vessel are adjacent to the structure ; a pair of electrodes disposed within the first vessel ; a power supply connected to the pair of electrodes , wherein the power supply is configured to provide an electrical pulse to create an underwater plasma between the pair of electrodes to generate a compression wave; a first ultrasonic detector configured to detect generation of the compression wave; a second ultrasonic detector , wherein, in use, the second ultrasonic detector : is positioned in a pitch-catch orientation with the pair of electrodes and the structure ; and is configured to detect the compression wave subsequent to the compression wave passing through the first portion of the structure , the second portion of the structure , and the gap ; a synchronization circuit configured to synchronize the electrical pulse and the first and second ultrasonic detectors ; and a computing system configured to determine the gap width based on detection of the compression wave by the first and second ultrasonic transducers. a first vessel , the first vessel comprising: one or more sidewalls and an endwall ; a liquid port configured to connect to a source to fill the first vessel with a liquid ; and an open portion configured to be placed against the structure ; one or more sidewalls and an endwall ; a liquid port configured to connect to a source to fill the first vessel with a liquid ; and an open portion configured to be placed against the structure ; a second vessel surrounding the first vessel , the second vessel comprising a vacuum port configured to connect to a vacuum system to pull a vacuum in a space between an outer surface of the first vessel and an inner surface of the second vessel when the open portion of the first vessel and an open portion of the second vessel are adjacent to the structure ; a pair of electrodes disposed within the first vessel ; a power supply connected to the pair of electrodes , wherein the power supply is configured to provide an electrical pulse to create an underwater plasma between the pair of electrodes to generate a compression wave; a first ultrasonic detector configured to detect generation of the compression wave; a second ultrasonic detector , wherein, in use, the second ultrasonic detector : is positioned in a pitch-catch orientation with the pair of electrodes and the structure ; and is configured to detect the compression wave subsequent to the compression wave passing through the first portion of the structure , the second portion of the structure , and the gap ; a synchronization circuit configured to synchronize the electrical pulse and the first and second ultrasonic detectors ; and a computing system configured to determine the gap width based on detection of the compression wave by the first and second ultrasonic transducers. <EOS>