# Contactless Ultrasonic Wavefield Imaging to Visualize Near-Surface Damage in Concrete Elements
## 1 Introduction  
The term ultrasonic wavefield imaging implies visualizing ultrasonic wave motion over a region of interest in a test medium. During the last decade, ultrasonic wavefield imaging technology has proven to be an effective nondestructive ultrasonic inspection tool for various engineering materials [1; 2; 3; 4]. A source at a fixed position generates ultrasonic waves in a test medium, and a contactless scanning receiver, typically a laser Doppler vibrometer (LDV), collects the corresponding ultrasonic wave responses across a predefined spatial sampling grid on the surface [2; 5; 6]. A reciprocal hardware configuration for fixed sensing and scanning ultrasound generation can also be used [7; 8]. Using the scanning measurement data sets, video-like ultrasonic wavefield images are generated, where the interaction between propagating ultrasonic waves and inhomogeneities within a test medium can be observed and intuitively understood. Ultrasonic wavefield imaging technology usually incorporates an additional data processing method to extract damage-induced scattered ultrasonic wavefields, thereby visualizing possible damage in a test medium [5; 9; 10; 11].  
Although ultrasonic wavefield imaging technology has been shown to be effective for inspecting a range of engineering materials, it is rarely applied to infrastructure materials such as concrete [12]. This is in part because of restricted use of an LDV for contactless measurements in concrete owing to optically rough surfaces and variable material properties. The naturally occurring strong heterogeneity of concrete causes ultrasound attenuation and dispersion [13; 14], and lowers signal-to-noise ratio (SNR) of data obtained with contactless measurements. In addition, low laser light reflectivity at the concrete surfaces makes it difficult to achieve high measurement sensitivity. To improve measurement sensitivity for LDV measurements, special surface treatments (e.g., retro-reflective tapes) are necessary [12; 14; 15].  
Air-coupled ultrasonic transducers, on the other hand, do not require special surface treatment and can serve an alternative solution to perform contactless measurements needed for ultrasonicwavefield imaging [15; 16]. However, the spatial resolution of the acquired ultrasonic wavefield data can be undesirably low if the active sensing area of the air-coupled transducer is not small enough. Recently, the authors developed low-cost contactless ultrasonic wavefield imaging hardware based on a multi-channel MEMS ultrasonic microphone array especially for concrete testing applications [17; 18]. Those results demonstrated that the developed wavefield imaging hardware enables rapid collection of high SNR ultrasonic wavefield data from concrete elements.  
In this paper, we present work to detect and visualize near-surface damage in concrete using contactless wavefield imaging hardware that we developed. A wavefield data processing approach extracts non-propagating oscillatory fields set up by concrete damage while suppressing forward propagating and back-scattered surface waves. This study presents the first attempt to apply MEMS microphone-based wavefield imaging technology to concrete for visualizing near-surface cracking damage. The presented approach offers several advantages for nondestructive inspection of concrete over existing approaches: (1) a fully contactless (air-coupled) test setup that requires no special surface treatment of concrete enables rapid collection of ultrasonic wavefield data; (2) the extracted non-propagating wave energy using the proposed wavefield data processing approach enables detection and visualization of near-surface cracking damage; and (3) a low-cost wavefield imaging hardware configuration is used.