Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 10—Evaluation by Geophysical and Nondestructive TestingFigure 145.—A rotating sonar transducer mounted on a sled can be pulledthrough a conduit to measure and record changes in dimensions along theconduit. This can be very useful when determining the size for a proposedslipliner. Here, the measured corrosion loss of the crown of a steel pipe isshown at many locations on one image. The bottom of the conduit wassubmerged. The sonar device can work in water or air, but requiresdifferent settings for each. Thus, both above water and underwater sonarmeasurements cannot be made at the same time, although a mechanicaldevice on the sonar sled can simultaneously provide information for thebottom of the pipe. Figure courtesy Sonex Corporation.position the virtual observation point anywhere in space. Features that are notvisible from the camera’s view angle can become readily apparent (Britzman andHansen, 2002, p. 3).10.6 Ultrasonic pulse-echo and ultrasonic pulse-velocityThese methods measure the velocity and frequency content of acoustic pulses ofenergy through metallic (i.e., CMP) and nonmetallic (e.g., concrete) materials.Piezoelectric transducers are passed through the structure using a “smart pig” deviceor are attached to the structure to transmit and receive the pulses.These methods generally use a source with known impulse characteristics, so that atransfer function can be computed between the input and the measured output(receiver) signal. By examining typical and nontypical (anomalous) velocity andfrequency information, correlations may be established between sound and poorconcrete conditions, and the corresponding acoustic signatures.255