An Ultrasonic guided wave tomographic imaging procedure is developed to examine critical areas of a civil structure, for example on bridges and pipelines. Specially developed permanently mounted piezoelectric sensors can be placed anywhere on a structure surrounding an area of concern. Wired and wireless approaches to data acquisition and analysis are possible. Each sensor sends information to every other sensor and an image is produced that shows structural changes of thickness, corrosion or defect growth within the contained area. Aspects of mode control and frequency for special sensitivities and a set of physically based features for special tomographic imaging makes effective imaging possible. Basic principles of wave propagation used to accomplish this will be presented along with a sample problem of defect growth in a welded plate.