The surface wave test evaluates the experimental dispersion curve from field measurements and shear wave velocity profile is determined from iterative comparison with theoretical dispersion curve in inversion process. Therefore, to obtain reliable experimental dispersion curve is essential for surface wave test. In the conventional SASW method, the experimental dispersion curve is determined using phase difference between receivers. Although SASW method is effective, it is difficult to determine correct results when there are noise, body waves and lateral variation. Recently, two-dimensional (or three-dimensional) imaging of shear wave velocity profile becomes un-omissible demand because of the lateral non-homogeneity of soil stiffness. Until now, subsurface is assumed to have many horizontal soil layers to bedrock when executing surface wave method. Because subsurface includes several curved/non-horizontal layers, unfortunately, errors can be generated during analysis task of surface wave measurements. Though there are many tomography techniques based on electric-magnetic wave / elastic wave, they cannot give detailed modulus information and bring much cost. Also, the conventional SASW method has weakness to realize the imaging of subsurface stiffness variation for its horizontal constancy assumption.
To overcome these weaknesses, new seismic site characterization method using the harmonic wavelet analysis of waves (HWAW) was proposed. HWAW method is based on time-frequency analysis using harmonic wavelet transform. HWAW method mainly uses the signal portion of the maximum local signal/noise ratio to evaluate the phase velocity and it can minimize the effect of noise. The HWAW method can eliminate near field effect and obtain reliable shear wave velocity profile for any soil condition without expertise. Also, the HWAW testing method can explore deep layer properties below about 30m depth with only one short receiver spacing setup. HWAW method not only can obtain the dispersion curve of various points quickly but also can sample the detailed shear wave velocity profile of whole depth.
The proposed two-dimensional Vs imaging method consists of four steps: field testing, evaluation of dispersion curve, determination of Vs profile using single array inversion process, and interpolation of individual Vs profiles. Field testing of HWAW method is relatively simple and fast because one experimental setup which consists of one pair of receivers is needed to determine the dispersion curve of the whole depth, and field testing is repeated within interested line. HWAW method uses the near field information and can sample much deeper part of the site than the conventional phase unwrapping method. This method uses single array inversion which considers receiver location without increasing calculation time and any complexities because the whole dispersion curve is determined from one experimental setup. Interpolation of individual Vs profiles enables 2-D imaging of shear wave velocity.
The accuracy of details on the shear wave velocity profile and reliability of long wavelength components should be verified to confirm the applicability of proposed method. Also, the assumption that subsurface consists of several horizontal layers with different shear modulus should be notified as unnecessary for precise exploration such as two-dimensional shear wave velocity imaging. To achieve these objectives, initially several numerical simulations using ABAQUS/Standard were performed. In the numerical simulations, horizontal/inclined soil layer models of shallow/deep depths were used for dispersion curves and comparisons. Finally, the 2-D shear wave velocity imaging methods were tried from those dispersion curves to verify the applicability of proposed method. Also, 2-D $V_s$ imaging job was applied to other simulation model with complex variation shape.
In the end, the applicability of 2-D shear wave velocity imaging to in-situ construction site was verified. Initially, the reliability of deeper layer properties was proved to support 2-D shear wave velocity imaging method. To do that, several field testing methods were applied at several sites. To estimate the reliability, phase velocity dispersion curves or shear wave velocity profiles determined by HWAW method were compared with results of other field tests such as SASW and Down-hole test. Then, 2-D shear wave velocity imaging for that tested construction sites were executed and compared with boring geologies of that site. Through several numerical simulations and field applications, the potential of proposed 2-D $V_s$ imaging method can be advanced to a practical level.
공학적 물성치로서 저변형율 $(<10^{-3}%)$ 에서의 전단탄성계수의 결정은 다양한 토목분야에서 매우 중요하며, 최근에는 지반의 횡방향 불균일성이 대두되면서 2차원적인 조사가 요구되고 있다. 본 논문에서는 저변형율에서의 전단탄성계수를 결정하기 위해 박형춘 등이 개발한 HWAW (Harmonic Wavelet Analysis of Wave) 방법의 실제 현장 적용성을 검증하는 연구를 하였으며, 이를 바탕으로 실제 지반의 2차원 영상화를 구현하고자 하였다.
우선 HWAW 방법의 현장 적용성을 검증하기 위하여 여러 수치 모의 시험 및 현장 실험을 수행하였다. 수치 모의 시험은 ABAQUS를 이용하여 여러 지반 조건을 모델링하여 수행하였으며, 현장시험은 HWAW방법을 검증하기 위하여 동일한 현장에서 수행된 SASW시험 및 Down-hole 실험의 결과와 비교하였다. 이를 통하여 HWAW방법이 배경잡음과 지반의 횡방향 불균일성에 의한 오류를 기존 방법에 비하여 최소화 할 수 있으며, 좁은 감지기 간격의 사용으로 인하여 국부적이며 정밀한 대상지반의 실제 전단파 속도 주상도를 매우 효과적으로 결정할 수 있음을 검증할 수 있었다.
마지막으로 여러 수치 모의 시험 해석 모델과 실제 지반에 대한 2차원 영상화 작업이 수행되었다. 2차원 영상화 작업은 HWAW 시험의 반복수행, 분산곡선 도출, 단일 어레이 역산을 통한 국부적인 전단파 속도 주상도 획득, 내삽의 4단계로 구성되어 있다. 수치해석 모델의 2차원 전단파 속도 영상화 결과는 원래의 모델 형상과 잘 일치하였다. 또한 실제 현장에서의 2차원적인 전단파 속도 영상도 얻을 수 있었으며, 2차원 영상 및 시추 주상도를 이용하여 풍화토와 풍화암의 경계도 산정할 수 있었다. 이로써 제안된 방법을 통한 지반 강성의 2차원 영상화 작업이 높은 적용성과 신뢰도를 가짐을 확인할 수 있었다.
