A defect monitoring system for monitoring a defect at a location under test within an area on a wall of a structure under test, the system comprising - an ultrasound transmitter or a plurality of transmitters (12, 30) connectable onto said wall; - an array of ultrasound receivers (12, 14, 31, 32) connectable onto said wall at an edge of the area; - a signal processing system coupled to the ultrasound receivers (12, 14, 31, 32), configured to - receive measured signals from the ultrasound receivers (12, 14, 31, 32), detected from ultrasound that emerges laterally from the area in response to ultrasound transmitted by the ultrasound transmitter or plurality of transmitters (12, 30); characterized in that the signal processing system is configured to - compute a first estimated signal at the location under test within the area, based on first measured signals detected by first ultrasound receivers in the array, using a model of ultrasound propagation through the wall to compute the first estimated signal at said location under test from the first measured signals; - determine a normalization factor for the first estimated signal by computing a reference signal at said location under test according to said model, from a sum of the first estimated signal and a second estimated signal at the location under test, the second estimated signal being based on second measured signals detected by second ultrasound receivers in the array from ultrasound that emerges laterally from the area, the first ultrasound receivers and the second receivers being located on opposite sides of the area, and/or computing the reference signal at said location under test according to said model from third measured signals detected by the first ultrasound receivers in the array from ultrasound that enters laterally into the area. - an ultrasound transmitter or a plurality of transmitters (12, 30) connectable onto said wall; - an array of ultrasound receivers (12, 14, 31, 32) connectable onto said wall at an edge of the area; - a signal processing system coupled to the ultrasound receivers (12, 14, 31, 32), configured to - receive measured signals from the ultrasound receivers (12, 14, 31, 32), detected from ultrasound that emerges laterally from the area in response to ultrasound transmitted by the ultrasound transmitter or plurality of transmitters (12, 30); - compute a first estimated signal at the location under test within the area, based on first measured signals detected by first ultrasound receivers in the array, using a model of ultrasound propagation through the wall to compute the first estimated signal at said location under test from the first measured signals; - determine a normalization factor for the first estimated signal by computing a reference signal at said location under test according to said model, from a sum of the first estimated signal and a second estimated signal at the location under test, the second estimated signal being based on second measured signals detected by second ultrasound receivers in the array from ultrasound that emerges laterally from the area, the first ultrasound receivers and the second receivers being located on opposite sides of the area, and/or computing the reference signal at said location under test according to said model from third measured signals detected by the first ultrasound receivers in the array from ultrasound that enters laterally into the area.