Abstract
Accurately obtaining acoustic velocity parameters of soft rock formations (e.g., near-surface soils, seabed sediments) is essential for calculating their key mechanical parameters (Young's modulus and Poisson's ratio). However, existing in-situ acoustic velocity measurement techniques suffer from limitations: cross-hole testing has a depth range incompatible with shallow engineering applications and neglects guided wave characteristics and dispersion effects; the shallow shear wave refraction method relies on the assumption of near-surface horizontal layering, yielding poor performance under complex geological conditions. To address these issues, this paper proposes an in-situ measurement method based on guided wave propagation characteristics in fluid-filled pipes, aiming to directly acquire acoustic velocities of soft rock formations to provide critical input for subsequent mechanical parameter calculations. The methodology comprises three key aspects: First, the guided wave dispersion equation for the fluid-filled pipe-soft rock formation coupling model is theoretically solved, the propagation characteristics of modal waves are thoroughly analyzed, and it is discovered that their attenuation features are highly sensitive to variations in formation acoustic velocity, thereby establishing a theoretical model for inverting formation acoustic velocity using L(0,1) mode guided waves. Second, based on this model, an experimental prototype system suitable for in-situ measurements (attachable to seabed cone penetration test apparatus) is successfully developed, incorporating key components such as acoustic transmitting/receiving transducers and core functional circuit boards, with a complete testing platform constructed. Finally, through systematic performance experiments, acoustic signal characteristics of guided waves excited within the pipe are comparatively analyzed under varying external media (air, water, and simulated sediments). Theoretical and experimental results consistently demonstrate that the attenuation characteristics of guided waves (L(0,1) mode) in fluid-filled pipes are significantly controlled by the acoustic velocity of the external soft rock formation. This key feature provides an effective pathway for in-situ inversion of formation acoustic velocity. Experimental data fully validate the feasibility and effectiveness of the proposed method, offering a novel technical approach for in-situ measurement of mechanical parameters in soft rock formations.
Full Text
In-situ Measurement of Acoustic Velocity in Soft Rock Formations Using Fluid-Filled Pipe-Guided Wave Characteristics
Zhang Xiumei, Wei Qian, Che Chengxuan, Liu Bin
- State Key Laboratory of Acoustics, Institute of Acoustics, Chinese Academy of Sciences, Beijing 100190, China
- University of Chinese Academy of Sciences, Beijing 100049, China
- Beijing Engineering Research Center of Sea Deep Drilling and Exploration, Institute of Acoustics, Chinese Academy of Sciences, Beijing 100190, China
Abstract
Accurately obtaining acoustic velocity parameters in soft rock formations (such as near-surface soils and seabed sediments) is essential for calculating their key mechanical parameters (Young's modulus, Poisson's ratio). However, existing in-situ acoustic velocity measurement techniques suffer from limitations: cross-well testing depth ranges are mismatched with shallow engineering requirements and ignore guided wave characteristics and dispersion effects, while shallow shear wave refraction methods rely on assumptions of near-surface horizontal layering that perform poorly under complex geological conditions. To address these issues, this paper proposes an in-situ measurement method based on guided wave propagation characteristics in fluid-filled pipes, aiming to directly obtain acoustic velocities in soft rock formations to provide critical input for subsequent mechanical parameter calculations. The core of this method comprises three aspects: First, the guided wave dispersion equation for a coupled fluid-filled pipe and soft rock formation model was solved theoretically, and the propagation characteristics of modal waves were thoroughly analyzed. This analysis revealed that attenuation features are highly sensitive to changes in formation acoustic velocity, leading to the establishment of a theoretical model for inverting formation acoustic velocity using the L(0,1) modal guided wave. Second, based on this model, an experimental prototype system suitable for in-situ measurement (attachable to seabed cone penetration test apparatus) was successfully developed, incorporating key components such as acoustic transmitting/receiving transducers and core functional circuit boards, along with a complete testing platform. Finally, systematic performance experiments were conducted to comparatively analyze the acoustic signal characteristics of guided waves excited within the pipe when the external medium varied (air, water, and simulated sediments). Theoretical and experimental results consistently demonstrate that the attenuation characteristics of guided waves (L(0,1) mode) in fluid-filled pipes are significantly controlled by the acoustic velocity of the external soft rock formation. This key feature provides an effective approach for in-situ inversion of formation acoustic velocity. Experimental data fully validate the feasibility and effectiveness of the proposed method, offering a new technical approach for in-situ measurement of mechanical parameters in soft rock formations.
Keywords: soft rock formation; in-situ acoustic velocity; guided wave; Poisson's ratio