Multi-Channel-Analysis-of-Surface-Wave.pptx

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1. Multi-Channel Analysis of Surface Wave (MASW) Group 1: Yonten Jamtsho Kelzang Lhaden Nima Tamang Palden Sonam Yangden Sangay Dema Ngawang Tenzin Sonam Dorji Tamang Tandin Wangchuk

2. Surface Wave Surface waves are generated in the presence of a free boundary. Surface Wave Dispersion Curve

3. Surface Wave Analysis General Procedure Surface wave analysis aims at estimating the seismic shear wave velocity (VS) profile by solving an inverse problem of model parameter identification based on an experimental dispersion curve. Fig 1: General procedure of surface wave analysis

4. Survey Design The investigation depth depends on the maximum measured wavelength and the resolution decreases with depth. The maximum investigation depth is related to the maximum measured wavelength, which depends on: The frequency content of the propagating seismic signal (source and site attenuation) The array layout aperture used for the recording The frequency bandwidth of the sensors The velocity structure of the site.

5. Surface wave analysis (Acquisition layout) Active prospecting Equally Spaced Eg ; MASW and SASW Passive prospecting No need of artificial seismic source

6. MASW Equipment 1. Seismic Source The energy provided by the seismic source must provide an adequate signal-to- noise ratio over the required frequency band, given the target investigation depth. Vertically operated shakers or vertical impact sources are typically used for surface wave testing. Weight-drop systems and vertically accelerated masses are able to generate high signal-to-noise ratios and allow longer wavelengths to be gathered and investigation depths to reach several tens of meters.

7. Explosive sources also provide high S/N data over a broad frequency band, with the caution that if they are placed in a borehole the amount of surface wave energy could be limited. The cheapest and most common source is a sledgehammer striking on a metal plate or directly on the ground surface. The weight of the sledgehammer should be at least 5 kg; the 8 kg sledgehammer is the most common choice.

8. 2 . Receiver vertical geophones are typically used for the acquisition of Rayleigh wave data. Natural Frequency: Choose geophones that match the expected frequency band of surface waves to minimize sensor response distortions. Below Natural Frequency Use: Geophones can operate below their natural frequency, but may cause non-linear sensor response and phase distortions. Phase shifts of 180° occur at the resonant frequency. Phase Distortion Mitigation: Using multiple receivers helps reduce errors, potentially extending the frequency band to half of the geophones' natural frequency. Regular calibration and frequency domain inspection are essential for accuracy.

9. Geophone Selection for Depth; Shallow Targets (e.g., 30 m): 4.5 Hz natural frequency geophones are suitable. Deeper Targets (10–15 m): Avoid using higher frequency geophones (e.g., 10–14 Hz) as they may be unreliable. Alternative Sensors; Accelerometers: Can provide a flat response at low frequencies. Typically less sensitive than geophones but viable for active data acquisition.

10. Receiver Deployment Guidelines ; Coupling with Ground: Use spikes for secure ground contact; remove thick grass beneath sensors. For hard surfaces, use a base plate for proper coupling. Placement: Ensure sensors are level and avoid placing over utilities. Weather Protection: Protect sensors from rain and other weather conditions to maintain data integrity.

11. 3 . Acquisition device Apparatus Options: Various devices can be used for digitizing analog geophone output and recording signals. Multichannel Seismographs: Common choice for seismic data acquisition. Specifically designed for seismic surveys such as reflection and refraction. Limitations of Seismographs: Check specifications for frequency band limitations, particularly at the low-frequency end. Considerations: Verify that the equipment meets the requirements for the intended geophysical survey. Be aware of potential mismatches between the survey needs and the equipment’s capabilities

12. 4. Trigger system Used for synchronizing seismic data acquisition, especially when stacking is employed. Types of Triggering Systems: - Contact Closure - Hammer Switch Importance for Stacking: Essential for stacking to improve signal-to-noise ratio. Critical for some single station procedures. Surface Wave Methods: If stacking is not used, triggering accuracy is less critical. Focus is on analyzing incremental travel time (phase differences) rather than exact arrival times.

13. Acquisition Layout The acquisition layout is based on a linear array of receivers with the shot position in-line with the receivers. The geometry is then defined by the array length L, the receiver spacing DX, and the source offset Fig 2: Geometry for active acquisition. L—array length, DX—receiver spacing

14. The usual rule of thumb is to have the array length at least equal to the maximum desired wavelength, which corresponds to more or less twice the desired investigation depth. With a more conservative approach, it is suggested an array length longer than two or three times the desired maximum investigation depth. Meaning, an array length of 60–90 m is preferred when trying to profile down to 30 m depth. Acquisition/Recording parameters Geophone Frequency: Low frequency geophones (4.5 Hz used) Higher frequency geophones (10 Hz-20 Hz) Geophone/Receiver spacing ( Δ x): Ranges from 1-4 m

15. Number of geophones (N): Ranges from 12 to 48 Array length (L): 23-96 m Sampling rate/interval ( Δ t): It is amount of time between two consecutive samples in a data acquisition process measured in millisecond ( ms ) 0.5-1 millisecond ( ms ) Sampling frequency (fs): It is the no of data points recorded per second by data acquisition system, measured as inverse of sampling interval ( fs= 1/ Δ t) 2000 Hz to 4000 Hz ( Eg ; If sampling interval is 0.5 ms , fs is 1/0.0005s=2000 Hz)

16. Recording time/Record length (T): It is the total duration of time over which data acquisition system records data after source is triggered. Pre-trigger time: Duration of data recording before source is triggered (Usually 0.1 -0.2 s) Post-trigger time: Duration of data recording after source is triggered ( usually 2 s) Offset between source and 1 st geophone/ Source offset (x 1 ): Approximately ½ max. depth of investigation Ranges from 5-20 m