Ultrasonic leaky-lamb wave imaging through a highly contrasting layer

This article presents an innovative and effective ultrasonic technique to image through a steel layer with immersion transducers. The purpose is to detect scatterers and external interfaces and evaluate the acoustic properties of materials bonded to the steel layer. The technique is based on the lowest-order antisymmetric leaky lamb wave propagating in the steel layer and is implemented through a pitch-catch transmitter-receiver arrangement. The main application presented herein is for a measurement that takes place in an oilfield well to evaluate the quality of the cement sheath that fills the annular region between steel casing and a rock formation. Current ultrasonic imaging techniques for such an application are based on the pulse-echo mode with a single trans-receiver. Because of the large acoustic impedance contrast between the steel and its surrounding media: fluid on the side of the transducers and cement, or fluid/mud on the other, pulse-echo techniques have proven to be limited to probing only the region adjacent to the casing-cement interface. Using field data acquired with an experimental device, we show that the proposed leaky-lamb wave-based imaging technique provides reflection echoes that allow for probing of the entire cement sheath and in particular of the imaging of the external cement-rock interface geometry.