3D FWI Research Articles

3D Full-Waveform Tomography of Standard Penetration Test-Seismic Wavefields in Karst Florida Limestone

The shallow subsurface across large parts of Florida consists of weathered karst limestone, a spatially variable porous stratigraphy with significant influence in the design and load capacity of deep foundations. For reliable load transfer of the super structure into the bearing soil/rock, detection of voids and weak zones is crucial. This study investigated the capabilities of a recently developed standard penetration test (SPT)-seismic testing method for characterizing a large volume of soil/rock properties with a single SPT. The method utilizes a three-dimensional full-waveform inversion (3D FWI) of wavefields induced by SPT blows at depths (in-depth source) to characterize the subsurface around the SPT borehole. A site in Florida that presents shallow, surface karst limestone, was the location of a field experiment that consisted of two 36- × 18-m areas, each with an SPT at the area center. Seismic wavefields induced by SPT blows at depth were recorded by 72 vertical geophones on the ground surface and analyzed by the 3D FWI. The results revealed that subsurface soil/rock properties were characterized in submeter pixels over a large 3D domain of 24 × 36 × 18 m (depth × length × width). Multiple voids at various depths from 5 to 17 m were successfully detected at the site and confirmed by SPT data. The results suggested that the SPT-seismic method is an efficient tool for site investigation, as a bridge pier or pile group could be designed with only one SPT.

Imaging of voids beyond boring depth with 3D SPT-seismic full-waveform tomography

Identification of voids in bedrock is important for the design of deep foundations. Driven piles are often socketed near the top of bedrock due to driving refusal, thus it is crucial to image voids below the pile tips for the reliability of foundations. This study investigates the capabilities of a new SPT-seismic testing method in imaging voids beyond the boring depth. The method utilizes a 3D full-waveform inversion (3D FWI) of wavefields induced by the standard penetration test (SPT) blows at depths (in-depth source) to characterize subsurface material properties. After being verified on synthetic data, the method is applied to field data recorded by 72 vertical geophones placed in a 36 × 18 m array on the ground surface during a SPT. Using the seismic data from a single SPT to 11-m depth, subsurface soil/rock properties were characterized in a large 3D domain of 24 × 36 × 18 m (depth × length × width), or up to 18 m away from the SPT location. A deep void at 13 to 17-m depth is successfully imaged, showing the capability of the method to detect a void beyond the depth of an SPT boring. The results suggest the SPT-seismic method is a powerful tool for site investigation particularly for imaging of voids in weathered and karst rock that are difficult to identify by conventional geophysical methods or invasive tests.

3D Time-domain electromagnetic full waveform inversion in Debye dispersive medium accelerated by multi-GPU paralleling

Electromagnetic full waveform inversion in Debye dispersive medium (EFWI-D) is a promising technique to reconstruct the inner structure and electrical properties of the medium such as soil, rock and biological tissues. Same as conventional full waveform inversion, EFWI-D requires high computational cost, especially in the 3D case. To reduce the long computation time, we design and implement the EFWI-D algorithm in time domain using multiple GPU cards. The inversion method is based on the L-BFGS optimization algorithm, which can increase the convergence of the misfit function, while the auxiliary differential equation (ADE) method is employed for modeling the Debye dispersive medium by using exponential time differencing (ETD) finite-difference time-domain (FDTD) approach. Moreover, a multi-stream strategy is performed in the workflow to improve the computation performance. Numerical results illustrate the improvement of the computational performance and the preliminarily feasibility of the proposed inversion algorithm. Program summaryProgram title: 3D Electromagnetic Full Waveform Inversion in Debye Dispersive Medium Based on Multi-GPU ParallelingCPC Library link to program files:https://doi.org/10.17632/mjd9pp5dcm.1Licensing provisions: LGPLProgramming language: C/C++, CUDANature of problem: Electromagnetic FWI derives high-resolution distribution of electrical parameters by using additional information provided by the amplitude and phase of the received signals. It goes beyond ray-tracing tomography techniques, which use only the travel time kinematics of the signals. Mathematically, electromagnetic FWI is a kind of conditional extremum problem which is to minimize the difference between observed and modeled waveform of received signals under the condition of Maxwell's equation.Solution method: The basis of inversion is the solution of the forward problem. In this program, the forward simulation is based on Auxiliary Differential Equation (ADE) method. Since the inverse problem is nonlinear, it is solved by using iterative solution which is an optimization method. As it is well known that 3D FWI requires tremendous computation, multi-GPU paralleling is used in this program to accelerate the computing process.Additional comments including Restrictions and Unusual features: The program is mainly designed to solve the inverse problem in Debye dispersive medium, but it also can be used for solving the forward and inverse problem in nondispersive medium by setting the input parameter: poles_of_debye_dispersive_medium(non_dispersive=0,dispersive=1-5)=0.

Coupling of SPT and 3D full waveform inversion for deep site characterization

Detailed information of soil/rock variability and properties for the entire volume of supporting materials is crucial for the successful design and construction of deep foundations. Traditional invasive testing methods such as the standard penetration test (SPT) and the cone penetrometer test (CPT) only sample a small volume of soil/rock properties near the device's tip. Surface-based seismic methods can provide overall subsurface conditions but are limited to shallow depths due to the dominancy of surface waves. To address this issue, we present a novel SPT-seismic testing method for deep site characterization. Seismic wavefields generated by SPT blows at various depths are recorded by a 2D grid of geophones on the ground surface, and analyzed by a 3D full-waveform inversion (3D FWI) to extract subsurface material properties. Unlike surface-based wavefields dominated by surface waves propagating near the ground surface, the SPT-seismic wavefields are rich in body waves propagating from great depths that allow extracting detailed material properties at depths. The method is tested on in-depth source synthetic data and SPT-source field data. The results of the synthetic experiment indicate that the method successfully images soil layering with a buried anomaly. Field experiment results provide new insights into its utility as a viable geophysical tool for deep site characterization. 3D subsurface S-wave and P-wave velocities within 9 m around the SPT boring are well characterized, including two deep voids at 14–18 m depth. Comparison with the surface-based 3D FWI method proves the superiority of the presented method in imaging deep structures. S-wave velocity values of the final inverted result are also compared with SPT N-values and a good overall agreement over the whole depth is observed.