Sains Malaysiana 41(12)(2012): 1621–1627

 

Beamforming Technique for Investigation of Lateral Variability at Geotechnical Sites

(Teknik Alur Bentuk untuk Mengkaji Kebolehubahan Sisi di Tapak Geoteknik)

 

 

Norfarah Nadia Ismail*, Sung-Ho Joh & Raja Hassanul Musa Raja Ahmad

Department of Civil and Environmental Engineering, Chung-Ang University

221 HeulSeok-Dong, DongJak-Gu, Seoul, 156-756 Republic of Korea

 

Diserahkan: 7 Mei 2012 / Diterima:  6 Ogos 2012

 

ABSTRACT

A beamformer in seismology is a signal receptor with a series of geophones, in which a beam of elastic waves is formed like a light beam by adjusting signal delays at individual geophones. Recently, beamforming has extended its applications to surface-wave measurement. In surface-wave measurement, beamforming provides unique advantages over other surface-wave methods, such as full automation in data analysis as well as directional signal reception to minimize scattered noise and multiple reflections in signals. However, certain defects depreciate the value of beamforming in terms of its practicality and feasibility. These include the requirement of having many receivers and the loss of small wavelength data due to spatial aliasing. It leads to insensitivity in identification of lateral variability, which creates the problem of having to smooth out geologic features and complexities like folding, faults and fractures. In this paper, advances in the refinement of beamforming were described on two counts: improvement of sensitivity in identification of lateral variability and recovery of aliased wave numbers, which enables evaluation of shallow material. On the passage to refinement, synthetic waveforms for typical layering systems were generated to figure out characteristics of beamformer velocities in comparison with SASW velocities and theoretical normal-mode velocities.

 

Keywords: Beamforming theory; phase velocities; refined beamformer; spatial aliasing

 

ABSTRAK

Alur bentuk dalam konteks seismologi ialah pengesan isyarat yang menggunakan sebilangan geofon, dengan alur gelombang kenyal seperti jalur cahaya yang dibentuk dengan mengubah lengah isyarat pada setiap geofon. Kebelakangan ini, aplikasi alur bentuk telah dikembangkan dalam kaedah pengukuran gelombang permukaan. Dalam kaedah pengukuran gelombang permukaan, alur bentuk mempunyai kelebihan yang unik berbanding kaedah gelombang permukaan yang lain, seperti automasi penuh dalam menganalisis data serta penerimaan isyarat terarah untuk meminimumkan taburan hingar dan pantulan berganda dalam isyarat. Walau bagaimanapun, terdapat beberapa kekangan yang boleh menurunkan nilai alur bentuk daripada segi praktikal dan pelaksanaan. Ia termasuklah keperluan untuk menggunakan banyak penerima dan kehilangan data pada panjang gelombang yang pendek akibat herotan ruang. Ini membuatkan tahap pengenalpastian terhadap kebolehubahan sisi menjadi kurang sensitif dan boleh menimbulkan masalah kerana terpaksa melicinkan fitur geologi serta struktur rencamnya seperti lipatan, sesaran dan retakan. Dalam kertas ini, kemajuan dalam tapisan alur bentuk diterangkan berdasarkan dua penekanan: penambahbaikan kepekaan dalam pengenalpastian kebolehubahan sisi dan pemulihan nombor gelombang yang terherot, bagi membolehkan penilaian dilakukan ke atas bahan cetek. Dalam proses penapisan ini, sejumlah gelombang sintetik untuk sistem berlapais yang tipikal telah dihasilkan bagi mencari dan menentukan ciri-ciri halaju beamformer, berdasarkan perbandingan dengan halaju SASW dan halaju teori mod-normal.

 

Kata kunci: Alur bentuk tertapis; halaju fasa; herotan ruang; teori alur bentuk

RUJUKAN

Joh, S-H. 1996. Advances in interpretation and analysis techniques for Spectral-Analysis-of-Surface-Waves (SASW) method, Ph.D. dissertation. The University of Texas, Austin (unpublished).

Johnson, D.H. & Dedgeon, D.E. 1993. Array Signal Processing: Concepts and Techniques. Indiana: PTR Prentice-Hall.

Stokoe, K.H., Joh, S-H. & Woods, R.D. 2004. Some contributions of in situ geophysical measurements to solving geotechnical engineering problems. Proceedings ISC-2 on Geotechnical and Geophysical Site Characterization, Viana Da Fonseca, A. & Mayne, P. W. (Eds). Millpress, Rotterdam.

Stokoe, K.H., II. Wright, S.G., Bay, J.A. & Röesset, J.M. 1994. Characterization of geotechnical sites by SASW method. Geophysical Characteristics of Sites. Volume prepared by ISSMFE Technical Committee #10, XIII ICSMFE, New Delhi, India.

Zywicki, D.J. & Rix, G.J. 2005. Mitigation of near-field effects for seismic surface wave velocity estimation with cylindrical beamformers. Journal of Geotechnical and Geoenvironmental Engineering 131(8): 970-977.

Zywicki, D.J. & Malladi, S. 2007. Forward-backward cylindrical beamformer and geometric spreading weighting for seismic surface wave parameter estimation. Proceedings of Geo-Denver 2007.

 

 

*Pengarang untuk surat-menyurat, email: norfarahnadia@gmail.com