Dear expert,
How to do Distributed Acoustic Sensing for MASW purpose, if it can be done can the geophone planted on surface in vertical, for subsurface imaging ?
Really appreciate if it can be done?
Mel,
Distributed Acoustic Sensing (DAS) can indeed be used for the Multichannel Analysis of Surface Waves (MASW) purpose, and it offers a unique and versatile approach to seismic data acquisition. Typically, DAS utilizes a fiber optic cable as a distributed sensor to capture ground motion along its entire length. However, using traditional geophones planted on the surface in a vertical arrangement may not be the most effective way to complement DAS for MASW. Here's how you can effectively use DAS for MASW:
1. Fiber Optic Cable Installation:
- Install a fiber optic cable along the surface or in a borehole at the site of interest. The choice between surface and borehole installation depends on your specific objectives, geological conditions, and depth of investigation.
2. DAS Interrogation System:
- Connect the fiber optic cable to a DAS interrogation system. DAS works by sending laser pulses down the fiber and measuring the backscattered light to detect acoustic signals.
3. Seismic Source:
- Generate seismic waves using appropriate sources, such as a sledgehammer impact, a seismic vibrator, or a controlled explosive source. These sources create ground motion that interacts with subsurface geological layers.
4. DAS Data Acquisition:
- The DAS system continuously records ground motion along the entire length of the fiber optic cable in real-time as the seismic waves propagate.
5. Data Processing:
- Process the acquired DAS data to extract dispersion curves. Dispersion curves provide valuable information about the shear wave velocity as a function of depth, which is the primary goal of a MASW survey.
6. Inversion:
- Invert the dispersion curves to obtain a shear wave velocity profile of the subsurface. This profile aids in characterizing geological layers and assessing geotechnical properties.
Advantages of Using DAS for MASW:
- Continuous sensor coverage along the entire length of the fiber optic cable allows for high-resolution data collection.
- DAS can be deployed in various environments, including boreholes, which can be beneficial for deeper subsurface imaging.
- It offers real-time data acquisition and the ability to capture subtle seismic signals effectively.
Considerations:
- The choice between surface installation and borehole deployment of the fiber optic cable depends on site-specific conditions and survey objectives. Borehole deployment is often preferred for deeper investigations.
- Ensure proper calibration and validation of the DAS system to obtain accurate and reliable results.
- DAS for MASW requires careful planning, execution, and data processing to achieve meaningful subsurface imaging.
While DAS is a powerful tool for MASW surveys, the use of traditional geophones in a vertical arrangement on the surface may not be necessary in this context. DAS alone can provide comprehensive and high-resolution data for MASW purposes, making the additional use of geophones less common in such applications. However, the choice between DAS installation on the surface or in boreholes should be guided by the specific requirements of your project.
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