THG using the remote passive MASW method in an industrial setting

Remote Passive MASW Method in Industrial Setting

In a recent project at an industrial site, Multichannel of Analysis of Surface Waves (MASW) methods were used to determine the depth to the top of rock.  MASW data are typically collected by utilizing an active source to generate a seismic signal, such as a sledge hammer or pneumatic source.  It can be challenging to get high-quality seismic data in industrial settings because the signal is easily dominated by ground vibrations from facility activities.  To overcome this, passive MASW techniques utilized the ambient noise to produce high-quality models despite a challenging field setting.  Ambient noise sources can include vibrating machinery, generators and traffic from a busy road.

Active MASW data acquisition uses a linear geophone array oriented in line with the source.  Although passive MASW data can be acquired using the same linear array, at industrial facilities noise is usually generated from multiple sources travelling from multiple directions.  Therefore, it is optimal to use a geometric array (e.g. circle, square, equilateral triangle) designed to record ground motion from multiple directions.

At this project, the active signal was overwhelmed with site noise, so THG used an equilateral triangle array to acquire passive signals.  Eighteen (18) geophones spaced five (5) feet apart were deployed to collect ten (10) events that were combined during processing.  Dispersion curves from the active and passive records were combined and interpreted together.  The dispersion curve generated from the passive record had a more reasonable trend at frequencies below 10 Hz; whereas the dispersion curve generated from the active data provided realistic trends at higher frequencies.

The lower frequency data generated from the passive record was instrumental in mapping the depth to bedrock at depth. The inverted dispersion curves resulted in shear-wave velocity profiles consistent with nearby geotechnical borings.

Combined active and passive MASW data