Evaluation of the Sensitivity of Compressional Waves to Assess the Presence of a Macroscopic Flaw in a Pre-Fractured Agaria Marble Specimen
Authors: Verma Anita , Shirole Deepanshu , Sinha Sankhaneel 
Published: August 2024

ABSTRACT:
Rock mass inherently contain various types of discontinuities that can be accessed using seismic signals. To investigate the effect of flaw on seismic signals, laboratory-based experiments were conducted in this study utilizing high-frequency seismic P-wave (ultrasonic) signals, with the aim of gaining further insights into how the presence of these flaws affect the ultrasonic waveforms. For this purpose, prismatic Agaria marble specimens (dimensions of 151 mm × 76 mm × 26 mm) containing a pre-existing flaw were employed. A novel multi-channel test setup was developed to enable the transmission of seismic signals across the specimen. Seismic waveforms were acquired in real-time using a LabVIEW-controlled system with a sampling frequency of 20 MHz. Piezoelectric transducers with a central frequency of 1 MHz were operated at 0.5 MHz, 1 MHz, and 2.25 MHz in this study to probe the influence of varying the operational transducer frequency. The findings quantitatively demonstrate that modulating the transducer frequency does not significantly impact the propagation behavior of P-waves through the flawed Agaria marble specimen.

1. INTRODUCTION
In engineering and geological investigations, elastic wave (ultrasonic) measurements are widely employed as a non-invasive technique to analyze material characteristic (stiffness, flaws, weathering, etc.) and its modulations over a period of time (time-lapse monitoring) (Benavente et al., 2020). The key advantage of ultrasonic/seismic measurements lies in its non-destructive nature, making it ideal for both field (geophysical) and laboratory investigations, with minimal intrusion and interventions, which allows engineers to assess the dynamic properties and structure of the target medium without causing any damage (ASTM D2845-00, 2000; Kahraman, 2002a, 2002b). The test itself involves measurement of the velocities/amplitudes of specific high frequency (0.1 MHz to 15 MHz) sound/elastic waves, either compressional (P) waves or shear (S) waves, via two piezoelectric sensors (in through transmission mode) attached to the rock sample. Specifically, one of the sensor acts as a source/transmitter, emitting the seismic signal, while the other acts as a receiver, capturing the signal as it propagates through the material (Zarie et al., 2019). The P-wave velocity, also known as ultrasonic pulse velocity (UPV) or seismic velocity, is the most commonly measured parameter due to its relative ease of determination. In some cases, shear wave velocity (S-wave velocity) is also measured alongside P-wave velocity to calculate dynamic elastic moduli, such as Young's modulus and Poisson's ratio (Benavente et al., 2020). Research suggests that changes in these seismic velocities can potentially predict impending failures within rock formations (Ghazvinian, 2015; Sayers et al., 1990; Shirole et al., 2019, 2020, 2021). Additionally, conventional methods for evaluating rock strength, such as strain measurement and passive monitoring, fall short in providing an objective and precise assessment of imminent fracturing and damage in materials. In contrast, ultrasonic monitoring, which is an active monitoring approach, can potentially offer valuable insights into the internal structure and integrity (e.g., location, size, growth, propagation) of rock-like materials (Shirole et al., 2017). However, the sensitivity of the ultrasonic measurements, whether based on changes in signal amplitude or velocity, in detecting the material modulations is influenced by the frequency of the transmitted seismic signal, as demonstrated by displacement discontinuity theory (Chen et al., 1993; Hedayat et al., 2014; Pyrak-Nolte et al., 1990a, 1990b), and in the experimental study of Shirole et al., (2020).