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Shallow Reflection Method for Water-Filled Void Detection and Characterization

Shallow investigation is crucial in enhancing the characteristics of subsurface void commonly encountered in civil engineering, and one such technique commonly used is seismic-reflection technique. An assessment of the effectiveness of such an approach is critical to determine whether the quality of the works meets the prescribed requirements. Conventional quality testing suffers limitations including: limited coverage (both area and depth) and problems with resolution quality. Traditionally quality assurance measurements use laboratory and in-situ invasive and destructive tests. However geophysical approaches, which are typically non-invasive and non-destructive, offer a method by which improvement of detection can be measured in a cost-effective way. Of this seismic reflection have proved useful to assess void characteristic, this paper evaluates the application of shallow seismic-reflection method in characterizing the water-filled void properties at 0.34 m depth, specifically for detection and characterization of void measurement using 2-dimensional tomography.


Introduction
Shallow investigation is crucial in enhancing the characteristics of subsurface void commonly encountered in Civil Engineering, and one such technique commonly used is seismic reflection technique. Conventional testing suffers limitations including: limited coverage (both area and depth) and problems with resolution quality. Traditionally, the measurements are use laboratory and in-situ invasive and destructive tests. However geophysical approaches, which are typically non-invasive and non-destructive, offer a method by which improvement of detection can be measured in a cost-effective way. Mayne (2012) has published the use of geotechnical techniques that are commonly used to identify underground features such as penetration testing and soil trenching [1]. These methods involve direct observation of underground materials. However, the conventional methods are frequently failed to observe the void, provide insufficient data and can and misrepresent the subsurface geology [2]. This may lead to undiscovered of the void position. Cost and time constraint factors are the main reasons to explain the difficulty in investigating the subsurface using conventional methods [3].
For the past few years, a range of geophysical techniques have previously been utilized successfully in the detection of voids in various geologies: microgravity [4], magnetic [ [17]. Compounding this, geophysics for void detection has an uncertain reputation in some areas of the engineering industry [5].
Void characterization by using seismic reflection technique has been proved useful, however, based on the previous work, the testing was conducted has focused on simulation-based observations making detailed evaluation of this approach difficult. Previously, the applicability of P-waves reflection techniques have been successfully detected subsurface voids [15][18] [19][20][21] [22]. The main advantages of these approaches are they are non-destructive, non-invasive, and require quick assessment. Such a method provides information with greater resolution in the lateral dimension with better traces resolution and thus, can define the geology of subsurface better with better thinner subsurface layers identification across a site. Therefore, to achieve better certainty of the site investigation, a dynamic approach is needed to be implemented

Working principle
This approach uses a piezoelectric sensor as receiver and trigger source, seismic hammer as a seismic source and data logger for data acquisition as illustrated in Figure 1. The water-filled ball at diameter of 0.16 m was buried at 0.3 m depth as shown in Figure 2. Therefore, this study has proposed straightforward method based on time domain sequence of data collection that can be generated by reflection method to simplify the problem of void size estimation and obviously lateral distances. Later, the twodimensional contour of amplitude was aligned using first arrival as a reference. Thus, the voids dimensions at lateral distances was directly determined by scheming the 2D tomography of amplitude. This approach will provide the estimation of depth and dimension of water-filled void as well as P-wave velocity regarding the direct wave method that have been implemented into this approach as a reference point.

Results
The amplitude of reflection wave for water-filled void pattern change with distance. Figure 3 illustrated the 2D pattern of amplitude tomography along the horizontal shot position, R1 till R10. In this study, reflection technique which has 10 steps of gathering shot was proposed.  m indicates negative amplitude where the energy is concentrated in the void before reflected to the surface. The direct wave also indicates in the dark area at 0.15 m depth because of small energy were reflected to the ground before reflection occurs. The 2D pattern of amplitude contour was aligned using first arrival as a reference.

Conclusions
This approach has come to conclusion whereas this study gives a significant value to overcome the uncertainty of void characterization. The estimation of depth and dimension of water-filled void as well as P-wave velocity has successfully determined using shallow reflection method.