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United States Department of Agriculture

Agricultural Research Service

Research Project: ACOUSTIC AND GEOPHYSICAL TECHNOLOGY DEVELOPMENT FOR IMPROVING ASSESSMENT AND MONITORING OF EROSION AND SEDIMENT TRANSPORT IN WATERSHEDS...

Location: Watershed Physical Processes Research Unit

Title: Self-adaptive method for high-frequency dispersion curve determination

Author
item Lu, Zhiqu -

Submitted to: Symposium on Application of Geophysics to Engineering and Environmental Problems Proceedings
Publication Type: Proceedings
Publication Acceptance Date: February 12, 2014
Publication Date: March 16, 2014
Citation: Lu, Z. 2014. Self-adaptive method for high-frequency dispersion curve determination. Symposium on Application of Geophysics to Engineering and Environmental Problems Proceedings. on line www.eegs.org.

Interpretive Summary: In order to investigate the properties of very thin top layer soils, a high frequency multi-channel analysis of surface wave method (HF-MASW) has been developed. It is found that the existing MASW methods both in data acquisition methodology and signal processing algorithm cannot achieve such task. In the study, a self-adaptive MASW method is developed to determine high-frequency dispersion curve. The new method includes innovated measurement system and data processing algorithm. An initial dispersion curve obtained by a fixed-offset MASW is used to estimate wavelengths at all frequencies. At each frequency, near offset and spread length are then set to be the ratios of the wavelength. In other words, near offset and spread length are self-adaptive to the corresponding wavelength. Receiver spacing is either fixed or linearly increased to reduce spatial aliases. A case study is presented, which demonstrates the capability of the self-adaptive method to preferentially identify the dispersion curves of either the fundamental mode or higher modes of Rayleigh waves. The new method will be applied to study soil crusting/sealing phenomena in the future research.

Technical Abstract: When high-frequency (from 50 to 500 Hz) MASW is conducted to explore soil profile in the vadose zone, existing rules for selecting near offset and receiver spread length cannot satisfy the requirements of planar and dominant Rayleigh waves for all frequencies and will inevitably introduce near and far field effects as well as spatial aliases. To solve the problems, a self-adaptive MASW method is developed to determine high-frequency dispersion curve. An initial dispersion curve obtained by a fixed-offset MASW is used to estimate wavelengths at all frequencies. At each frequency, near offset and spread length are then set to be the ratios of the wavelength. In other words, near offset and spread length are self-adaptive to the corresponding wavelength. Receiver spacing is either fixed or linearly increased to reduce spatial aliases. A case study is presented, which demonstrates the capability of the self-adaptive method to preferentially identify the dispersion curves of either the fundamental mode or higher modes of Rayleigh waves. A nonlinear phenomenon has also been observed which deserves future investigation.

Last Modified: 12/19/2014
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