High-Resolution Mapping of Surface Morphologies with Focused Ultrasonic Transducers

Using a rotating sensor to take acoustic measurements of geological formations from a fluid-filled borehole is common in the oilfield industry. The deployment of ultrasonic devices enables characterizations of the borehole diameter (caliper) and detailed well bore shape, as well as geological or geomechanical interpretations of formation features such as fractures, rock fabric, stress orientations, failure mechanics and borehole damage. A common mode of operation adopts the ultrasonic pulse-echo technique to achieve a sufficiently high spatial resolution. High-resolution imaging is desirable to improve the ability to discern and characterize morphological features in the borehole surface, which can play a significant role for the production of the reservoir. This paper presents experimental high-resolution investigations using ultrasonic pulse-echo measurements with focused transducers on a range of water-immersed target shapes in a laboratory setup. The targets are composed of combinations of nylon, aluminum, and steel. To facilitate a comparison with numerical modelling, simplified benchmark geometries have been machined into the otherwise planar targets. Here, we report results on notches, cylindrical, and spherical defects. The scanned targets thus represent borehole features, such as borehole breakouts-a form of localized damage of the formation. Fluid-coupled measurements of echo amplitudes and pulse-echo round trip times for 1D or 2D scans were obtained. These scans at nominally vertical incidence were repeated at various sensor standoffs to characterize the response and resolution of the focused transducers. To support the understanding of the spatial sensitivity, numerical acoustic wave field simulations were performed using a k-space pseudo-spectral model of linear elastic wave propagation, available as an open source third party toolbox (named k-Wave®) for MATLAB®. We determine the spatial sensitivity of focused transducers and find a good agreement between the experimental results and the model predictions for these simplified benchmark geometries. We identify the arrival of late echoes after multiple reflections inside of concave recesses and examine how these features can lead to misidentifications of the apparent standoff from the surface without suitable processing.