The dispersion curve is further processed through inversion analysis to obtain the substrate shear wave velocity profile in 1-D, 2-D, or 3-D formats. Further, a good resolution dispersion image helps to clearly identify the dispersion curve, which is the main requirement for inversion analysis. An accurate representation of the dispersion curve is imperative to the accuracy of the shear wave velocity profile obtained from the data analysis.
Motivation, Objective and Scope of the Present Study 6
BACKGROUND AND LITERATURE REVIEW
Factors Governing the Resolution of a Dispersion Image 25
EXPERIMENTATION PROGRAM AND ANALYSIS METHODOLOGY
INFLUENCE OF DATA PREPROCESSING PARAMETERS ON THE RESOLUTION OF
INFLUENCE OF DATA ACQUISITION PARAMETERS ON THE RESOLUTION OF
Validation of the Experimental Dispersion Curve 134
QUANTIFICTAION OF RESOLUTION OF DISPERSION IMAGE AND AUTOMATED
CASE STUDY: SUBSURFACE IDENTIFICATION USING ACTIVE MASW SURVEY ALONG 1.2 KM
Shear Wave Velocity Profile along the Bridge Alignment 204
CONCLUSIONS AND RECOMMENDATIONS
APPENDIX-I: WORKING STEPS OF SURFSEIS
5.14 (a) Total power of wave field records collected at site 1 for different offset distances (b) Frequency bandwidth of significant energy content of collected wave field records (sampling frequency – 7500 Hz, sample length – 5120, spacing between receivers – 1 m, Number of channels - 24). 8.14 (a) Typical representation of manual selection of dispersion points in SURFSEIS b) Screenshot of data obtained by manual selection. 8.20 2-D shear wave velocity profile obtained from along the active MASW along the route of the proposed bridge over Jia-Bharali (P5-A2).
LIST OF TABLES
MASW Multichannel Surface Wave Analysis SASW Surface Wave Spectral Analysis DAQ Data Acquisition System.
- Motivation, Objective and Scope of the Present Study
- Outline of the Dissertation
The shear wave velocity (Vs) is one of the elastic constants and is closely related to the shear modulus. Examine the effect of data preprocessing parameters (filtering and silencing) on the resolution of dispersion images. Examine the effect of data acquisition parameters (sampling rate, sample length, offset distance, receiver spacing, array length, source energy, stacking, and impact plate material) on the resolution of dispersion images.
The objectives and purpose of the research work are listed, along with the organization of the dissertation.
The effect of the impulsive source and the composition of the impact plate used on the obtained Vs profile is also demonstrated. The effect of initial model, numbers and density of manually selected distribution points and its corresponding frequency band were investigated and reported.
The information will be used for the design and analysis of the proposed well foundation for the bridge piers and abutment.
BACKGROUND AND LITERATURE REVIEW
- History of Surface Wave Methods
- Procedure of Active MASW Survey
- Dispersion Analysis
- Resolution of the Dispersion Image
- Factors Governing the Resolution of a Dispersion Image
- Critical Appraisal of the Literature Survey
The choice of geophone affects the resolution of the scattering image pertaining to the diffuse frequencies. It has been noted that the quality of the dispersion image obtained by the Refraction Microtremor (ReMi) method, or currently known as the passive MASW approach, depends on the collected wavefield data (Louie 2001). However, existing literature reveals that dispersion image resolution is mostly qualitative and based on visual inspections.
EXPERIMENTATION PROGRAM AND ANALYSIS METHODOLOGY
- Equipments of Active MASW Survey
- Location of the Test Sites
- Test Setup and Methodology
- Analysis Software: SURFSEIS
Receivers have their own natural frequencies below which the resolution of the received data becomes poor. A 40 kg thrust power generator (Fig 3.2) was used as a drop weight for some of the studies in the present study. The output voltage at the geophone reaches its maximum level at the natural frequency of the geophone.
The bandwidth of the system indicates the frequency range up to which a reliable signal can be received. When choosing a data acquisition system, care must be taken for the physical stability of the system. The impact energy reduction from the rubber plate, as used in the present tests, is not significant.
Rather, the composition of the plates controls and changes the frequency content of the transmitted waves. The use of a cast steel plate (Fig 3.5) increases the higher frequency content of the. 3.8: (b) Typical layout of linear array geophones for active MASW survey (c) Connection mechanism of the geophones with the connecting cable.
A summary of the entire procedure of a MASW survey (active or passive) is shown in the flow diagram in Fig.
INFLUENCE OF DATA PREPROCESSING PARAMETERS ON THE
- Resolution of a Dispersion Image
- Results and Discussions
The result is manifested in the form of the resolution of the dispersion image obtained from the dispersion analysis of the raw or processed signals. The detailed effects of the influence of various data acquisition parameters on the resolution of the dispersion image will be highlighted in Chapter 5. This chapter focuses on the influence of the effects of filtering and muting on the resolution of the dispersion image.
The few available documented reports on the resolution of scattering images focus on the quality of the resolution based on visual inspection alone. Filtering is performed based on the amplitude spectrum response of an accumulated wavefield recording. The dispersion images of the corresponding silent recordings (Fig. 4.9) of the unfiltered wavefields are shown in Figs.
Compared to the dispersion images obtained from unfiltered wave fields (Fig. 4.10), it can be clearly observed that the same images obtained from the filtered and damped wave fields exhibit superior resolution, as most of the. This chapter highlights the influence of preprocessing parameters on the resolution of the dispersion images obtained from an active MASW survey. Band-pass filter with the correct choice of the filter frequency range results in the best resolution of the dispersion image.
Applying muting to unfiltered wavefields causes aliasing effects in the low-frequency range of dispersion images.
INFLUENCE OF DATA ACQUISITION PARAMETERS ON THE
- Results and Discussions
- Validation of the Experimental Dispersion Curve
The current study uses different sample lengths available in the MAE seismograph, i.e. one can see that for a given location the normalized amplitudes are quite the same for different sample lengths. The conventional idea that the resolution of the dispersion image increases with the increase in sampling frequency is not always necessarily true.
In fact, the choice of sampling frequency depends on the time of sampling and the total length of the sample. Three vertical stacks of the dispersion image were used to increase the resolution of the images obtained for Site-1. The comparison of the Vs profiles obtained from the borehole survey and the active MASW survey is shown in Fig.
The properties of the shallower layers are derived from the velocity associated with the shorter wavelengths. It can be observed that the dispersion images are almost similar, although the 24-channel recording exhibits more distinctness due to the higher energy accumulation in the recording. The spectrum of the source is related to the frequency content of the generated signal.
It is worth noting that the low-frequency content of the signal is necessary for greater penetration depth. The primary goal of stacking is to improve the resolution of the dispersion image so that the dispersion curve can be efficiently extracted to the best possible accuracy. Increasing the number of receivers alone will not increase the resolution of the dispersion image.
- Results and Discussion
In this direction, the influence of the number of dispersion points chosen and the frequency band from which it is chosen is investigated. However, since the numbers of layers are constant, the optimization scheme leads to almost equivalent shear wave velocity profiles. Therefore, a parametric study was conducted to check the influence of the number of layers on the shear wave velocity profile.
The extent of the dispersion curve in the frequency domain significantly affects the outcome of the inversion analysis. Therefore, it is imperative to select the dispersion points along the dispersion curve so that complete information about the substrates can be obtained simultaneously. As shown in the previous section, the dispersion points must be selected from the entire base dispersion curve to obtain the most suitable profile of the substrates.
It is recommended that the number of dispersion points should not be too small, since the heterogeneity of the substrates cannot be recognized uniformly. Selecting a larger number of points does not necessarily increase the accuracy of the shear wave velocity profile. As mentioned earlier, the accuracy depends on the accurate identification of the locations of maximum energy in the dispersion image.
This chapter focuses on the parameters that affect the accuracy of the shear wave velocity profiles obtained from inversion analysis.
QUANTIFICATION OF RESOLUTION OF DISPERSION IMAGE AND
AUTOMATED EXTRACTION OF DISPERSION CURVE
The dispersion curve having the slowest phase velocities corresponds to the fundamental mode (M0), while the next faster one corresponds to the first higher mode (M1), and so on (Fig. 7.1). The theoretical dispersion curve obtained by any of the standard methods (Schwab and Knopoff 1972; Ke et al. 2011) is optimized towards the experimental dispersion curve, through the sequential and iterative model parameter updating scheme. However, the efficiency and accuracy of the inversion analysis depends on the accuracy of extracting the fundamental or higher order dispersion curves from the corresponding dispersion image, which, in turn, depends on the resolution of the dispersion images.
The effects of various data preprocessing and data acquisition parameters on the resolution of the dispersion image have already been discussed in Chapter 4 and Chapter 5, respectively. However, the resolution of the images was merely described qualitatively from visual inspection and classification, while the quantification of the resolution of the dispersion images was omitted. Commercially available software extracts the dispersion curve for the inversion process based on user discretion.
As clearly mentioned earlier, such manual extraction of fundamental or higher order dispersion points is subject to human error. Such deviations originate from the lack of unambiguous identification of the exact location of the locally highest peak energy phenomena in the dispersion image. Such incorrect inclusions of dispersion points in defining the dispersion curve that will later be used in the inversion would result in an inaccurate or inaccurate shear wave profile.
This chapter describes the efforts made to address the above two problems, namely (a) quantification of the quality of the generated dispersion image, and (b) automatic extraction of exact fundamental or higher order mode dispersion curves according to the exact peak energy trends of the distribution image.
Quantification of the Resolution of Dispersion Image