Applied Mechanics and Materials Vol 771 (2015) pp 179-182 © (2015) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.771.179 Submitted: 2014-07-24 Revised: 2015-03-05 Accepted: 2015-03-07 Combination of Active and Passive Seismic Analyses for Embankment Characterization Yekti Widyaningrum1, a, Sungkono1,b Alwi Husein1, Bagus Jaya Santosa1,c, Ayi S. Bahri2 1 Department of Physics, Faculty of Mathematics and Natural Sciences, 2 Department of Geophysical Engineering, Faculty of Civil and Urban Planning Engineering, Institut Teknologi Sepuluh Nopember, Jl. Arif Rahman Hakim, Surabaya 60111, Indonesia a yekti12@mhs.physics.its.ac.id, bhening_1@physics.its.ac.id, cbjs@physics.its.ac.id Keywords: Embankment, Rayleigh wave, shear wave velocity Abstract. Rayleigh wave dispersion is intensively used to determine near surface of shear wave velocity (Vs). The method has been known as non-invasive techniques which is costly effective and efficient to characterize subsurface. Acquisition of the Rayleigh wave can be approached in two ways, i.e. passive and active. Passive seismic is accurate to estimate dispersion curve in low frequency, although it is not accurate for high frequency. While active seismic is vice versa of passive seismic. The high frequency of Rayleigh wave dispersion reflects to near surface and vice versa. Therefore, we used the combination of both passive and active seismic methods to overcome the limitations of each method. The shear wave velocity (Vs) which is resulted by inversion of the combining data gives accurate model if compared to log and standard penetration test (N-SPT) data. Further, the approach has been used to characterize LUSI (Lumpur Sidoarjo) embankments. The result shows that embankment material (0-12 m) has higher Vs than that lower embankment material. Introduction Subsurface structure is an important data in geological site characterization. The importance to characterize subsurface structures in local area becomes an interesting topic in the recent years among the geology and geophysics researchers, such as ground characterization at railway station [1], bedrock depth in Chorzow Stary-Poland [2], UBC/IBC site classifications in Nevada and California [3]. Subsurface structure is usually analyzed by Rayleigh wave dispersion curve. The method has been known as non-invasive techniques which is costly effective and efficient to characterize subsurface [3-6]. In layered media, Rayleigh waves have a characteristic where it will be dispersed [7,8]. As a result, Rayleigh wave amplitude decreases exponentially with increasing depth [8]. Rayleigh wave is an evanescent of primary wave (P) and secondary wave in the vertical direction (Sv) which propagates along the free surface [7,8]. Therefore, Rayleigh wave is one of surface waves which velocity is affected by the shear wave velocity (Vs). Shear wave velocity can be used to determine each layer characteristic where this wave propagates. Acquisition of the Rayleigh wave can be approached in two ways, i.e. passive and active. Passive seismic is accurate to estimate dispersion curve in low frequency, although it is not accurate for high frequency. While active seismic is vice versa of passive seismic [4]. The high frequency of Rayleigh wave dispersion reflects to near surface and vice versa [3,4]. Therefore, in this paper we focused in the acquisition data which used the combination of both passive and active seismic method to overcome the limitations of each method. This method was applied at Line P90-91 of mud volcano embankment in Sidoarjo (LUSI). Active-Passive Seismic Methods Active measurement technique is the acquisition of seismic Rayleigh waves on the surface of the soil which can be done using active vibration sources, including seismic hammer, weight drops, All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 36.84.103.115-06/05/15,12:50:07) 180 Instrumentation and Measurement Systems electromechanical shakers, vibroseis, and bulldozers. These techniques include SASW (Spectral Analysis of Surface Waves) and MASW (Multichannel Analysis of Surface Waves) [4]. Park et al. mention the advantages of MASW (Multichannel Analysis of Surface Waves) array [5]. MASW is able to acquire measurement data of Rayleigh waves with a shorter time because it uses more geophones as receivers. This configuration is similar to the measurement of body wave, so that the overall seismic energy can be recorded. While the measurement technique for passive seismic data acquisition techniques Rayleigh waves is using natural resources, or better known by measurement of noise [5][4]. This technique consists of array of microtremor and refraction microtremor (REMI). In this paper, we used REMI (Refraction Microtremor) using a straight line arrangement, because this method is the most suitable passive seismic method to be applied at LUSI embankment. Data Acquisition Data acquisition for the active-passive seismic used MASW array as shown in Fig. 1. In this study, for the active seismic MASW technique we used a hammer as a seismic vibration source. However, the use of seismic sources in the form of a hammer seismic recording technique produces MASW surface waves with smaller depth and shear wave velocity profile in 1 dimension. There are two ways to correct these deficiencies. First, measurement of the MASW technique was carried out repeatedly by varying the distance between the sources to produce 2-D shear wave velocity profile. Second, the use of passive seismic methods was done. In addition, 12 pieces of geophones are arranged linearly with the distance between two nearest geophones of 5 meters. While the distance between the source of vibration and the first geophone varied from 5, 10, 15 ... 50 meters. S R a) 1 5 5 m S 3 2 4 5 6 7 8 9 10 11 12 3 4 5 6 7 8 9 10 11 m R b) 1 10 m 2 5 12 m Fig. 1. Variation of distance between the source and first receiver in active seismic acquisition data. (a) First measurement array; (b) Second measurement array Data Processing In this paper, we used Geogiga Surface Wave Plus 6.0 to build dispersion curve from data acquisition and also to do inversion process. Firstly, this program transforms the data from timedistance domain into frequency-wave number using f-k transformation. This transformation process produces Rayleigh waves’ energy spectra in velocity-frequency domain. Then, the inversion process in this program used genetic algorithm (GA). Active phase velocity pick Fig. 2.(a) has done by choosing the maximum energy spectra, red brick color. While passive phase velocity pick Fig. 2.(b) has done by choosing the lower energy spectra at given frequency, electric blue color [3,4]. In the active phase velocity pick given in Fig. 2.(a), there are many maxima of energy spectra at the same frequency especially at lower frequency less than 4 Hz. This can be confusing to determine which maximum that we are going to choose. While in Fig 2.(b), we cannot choose any phase velocity at high frequency which is more than 4 Hz. This is caused by the natural source of vibration in passive seismic record come from deep depth and every direction. Therefore, in higher frequency it is better to use the active seismic method than the passive seismic method and vice Applied Mechanics and Materials Vol. 771 181 versa. Clearly, it is the best way to combine the active and passive methods in order to overcome the limitation of each method. a) b) Fig. 2. Rayleigh wave dispersion energy spectra in velocity-frequency domain. (a) Active phase velocity pick; (b) Passive phase velocity pick Fig. 3. (a) 1-D shear wave velocity model; (b) 2-D shear wave velocity model Figure 3.(a) shows one of 1-D shear wave velocity structures at P90-91 LUSI embankment. The 2-D shear wave velocity structure was built by 10 of 1-D shear wave velocity structure Fig. 3.(a). Based on Fig. 3.(b), 12 meters depth of subsurface structure was LUSI embankment. Embankment structure has higher shear wave velocity than that of the subsurface structure below it (at a depth of 12-22 m). These results are proportional to the N-SPT data given in Table 1. N-SPT data state that a layer at a depth of 0-12 m has a value between 7-58 m. This means the embankment structures denser than the underlying layer which N-SPT value is less than 7. The embankment (0-12 m of depth) depicted in Fig. 3.(b) has shear wave velocity structure varies from 210-250 m/s. The shear wave velocity is influenced by the type of medium and it cannot propagate in the medium of gas and liquid. Therefore, if a medium have lower shear wave velocity, then the medium is not just composed of solids. It is also possible composed of gases or liquids. This means that the embankment has been saturated by fluids. The reduced value of shear wave velocity in this layer indicates that this layer saturated fluid. Saturation is possible because the 182 Instrumentation and Measurement Systems water comes from seep water storage pond at the north embankment P90-91. Therefore, the shear wave velocity decreases when passing through the interface between the embankment layer and the layer under the embankment. Table 1. N-SPT data at south LUSI embankment [9] Soil Description Depth (m) Embankment material has brown color, consist of mixed loose 0 – 1.95 material, silt size, sand, silt stones, sand stones, andesite chunks, 1.95 – 4.90 very moist, non-plastic, firm, loose and strong Silt clay with some of organic materials, dark grey, moist, medium- 4.90 – 5.50 high plasticity, medium dense, at 5.20 – 5.50 m depth contain of 5.50 – 8.00 smooth sand, silt brown color Clay, some organic materials, a bit silt, dark grey, stiff, very moist, 8.00 – 9.70 medium plasticity Silt sand, brown color, loose, smooth-medium size of sand, 9.70 – 14.00 sometimes there are many egg cell fragments, grey, loose, bad gradation Grey clay, sometimes there are some of 1 cm egg cell fragment, 14.00 – 32.70 firm, sometimes there are some organic material, very moist, high plasticity N-SPT 58 32 32 13 9 7 6 Summary Combination of active and passive analyses has been shown to be effective to characterize the embankment. The active method could overcome the limitation of passive method and vice versa. The characterization results at line P90-91 LUSI embankment showed that overall the embankment structure is denser than the layer under the embankment. References [1] J. Fernández, L. Hermanns, A. Fraile, E. Alarcón, I. del Rey, Spectral-analysis-surface-wavesmethod in ground characterization, Procedia Engineering 10 (2011) 3202–3207. [2] M. J. Mendecki, W. M. Zuberek, P. Hrehorowicz, S. Jarek, An inversion of Rayleigh waves dispersion curves as a tool to recognize the bedrock depth in Chorzów Stary, Poland, Contemporary Trends in Geoscience 1(1) (2012) 39-44. [3] A. J. Martin, J. B. Shawver, J. G. Diehl, Combined use of active and passive surface-wave techniques for cost-effective UBC/IBC site classification, Association of Environmental and Engineering Geologists, Conf. Proc., 2005 Annual Meeting. Zonge International (2005). [4] Information on http://geovision.com/seismic.php. [5] C. B. Park, R. D. 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