APPLICATIONS OF MULTICHANNEL ANALYSIS OF SURFACE WAVES (MASW) FOR DETECTION OF TUNNELS

Multi-channel analysis of surface waves (MASW) method is a non-invasive geophysical technique that uses the dispersive characteristic of Rayleigh waves to estimate low strain shear modulus and damping coefficient of near-surface soil. The acquisition of the MASW data normally uses twenty-four low-frequency (4.5 Hz) vertical geophones, typically placed at 5 ft intervals centered on each test location. Acoustic energy is generated at an offset (distance to nearest geophone) of about 30 ft using an active seismic source such as a 20 lb sledge hammer and metal plate. The generated Rayleigh wave (desired type of surface wave) data are recorded using a 24-channel seismograph. MASW data sets are then transformed into 2-D shear-wave velocity profiles using a dispersion curve technique.

MASW uses lower frequencies that are more conducive to placement of uncoupled ground sensors. MASW linear arrays can also be towed behind a moving vehicle where weight drops are employed to generate surface waves for surveys. Source-towed geophone spreads with on-board acquisition, GPS location recordation, basic processing transmission by wireless connection to a command vehicle (m to km away), all under the direction of a single non-geotechnical operator is potentially achievable (Miller, 2006.)

Attenuation Analysis of Rayleigh Waves (AARW) is a new technique that shows promise for void detection and depth to bedrock. It is based on the observed damping effect of the void on the surface responses. These observations have shown good agreement with the observed numerical results, were verified with other field and laboratory data and shows promise as a tool for void detection (Nasseri-Moghaddam, 2006.)

Challenges to the development of MASW software applications include overcoming prejudicial algorithms that are premised on horizontal layering and the standardization of geophone geometry, an inability to distinguish between air and water filled voids, and the effects of back-scattering reflections that preclude confident differentiation between utilities and tunnels in urban environments. Theoretically, non-standard geophone array spacing as well as passive data acquisition is plausible. However, these developments are predicated upon some understanding of range limits and soil condition effects.