Antonios Giannopoulos

A nonsplit complex frequency-shifted PML based on recursive integration for FDTD modeling of elastic waves

In finite-difference time-domain FDTD modeling of elastic waves, absorbing boundary conditions are used to mitigate un- fields — it is difficult to adopt a complex frequency--shifted stretching function. We present an alternative implemen-tation of a PML that is based on recursive integration and does not re- quire splitting of the velocity and stress fields. Modeling re-sults show that the performance of our implementation using a stan- dard stretching function is identical to that of the convention-al split-field PML. Then we show that the new PML can be modi- fied easily to include the complex frequency-shifted stretching function. Results of models with an elongated domain show that this modification can substantially improve the performance of

gprMax: open source software to simulate electromagnetic wave propagation for ground penetrating radar

gprMax is open source software that simulates electromagnetic wave propagation, using the Finite-Difference Time-Domain (FDTD) method, for the numerical modelling of Ground Penetrating Radar (GPR). gprMax was originally developed in 1996 when numerical modelling using the FDTD method and, in general, the numerical modelling of GPR were in their infancy. Current computing resources offer the opportunity to build detailed and complex FDTD models of GPR to an extent that was not previously possible. To enable these types of simulations to be more easily realised, and also to facilitate the addition of more advanced features, gprMax has been redeveloped and significantly modernised. The original C-based code has been completely rewritten using a combination of Python and Cython programming languages. Standard and robust file formats have been chosen for geometry and field output files. New advanced modelling features have been added including: an unsplit implementation of higher order Perfectly Matched Layers (PMLs) using a recursive integration approach; diagonally anisotropic materials; dispersive media using multi-pole Debye, Drude or Lorenz expressions; soil modelling using a semi-empirical formulation for dielectric properties and fractals for geometric characteristics; rough surface generation; and the ability to embed complex transducers and targets.

A Realistic FDTD Numerical Modeling Framework of Ground Penetrating Radar for Landmine Detection

A three-dimensional (3-D) finite-difference time-domain (FDTD) algorithm is used in order to simulate ground penetrating radar (GPR) for landmine detection. Two bowtie GPR transducers are chosen for the simulations and two widely employed antipersonnel (AP) landmines, namely PMA-1 and PMN are used. The validity of the modeled antennas and landmines is tested through a comparison between numerical and laboratory measurements. The modeled AP landmines are buried in a realistically simulated soil. The geometrical characteristics of soils inhomogeneity are modeled using fractal correlated noise, which gives rise to Gaussian semivariograms often encountered in the field. Fractals are also employed in order to simulate the roughness of the soils surface. A frequency-dependent complex electrical permittivity model is used for the dielectric properties of the soil, which relates both the velocity and the attenuation of the electromagnetic waves with the soils bulk density, sand particles density, clay fraction, sand fraction, and volumetric water fraction. Debye functions are employed to simulate this complex electrical permittivity. Background features like vegetation and water puddles are also included in the models and it is shown that they can affect the performance of GPR at frequencies used for landmine detection (0.5-3 GHz). It is envisaged that this modeling framework would be useful as a testbed for developing novel GPR signal processing and interpretations procedures and some preliminary results from using it in such a way are presented.

An Improved New Implementation of Complex Frequency Shifted PML for the FDTD Method

A new implementation of the perfectly matched layer absorbing boundary for finite-difference time-domain grids is presented. The approach which is based on the complex co-ordinate stretching perfectly matched layer (PML) formulation uses the complex frequency shifted stretching function and is based on the simple concept of the recursive evaluation of an integral avoiding the calculation of time derivatives. This recursive integration PML is simple to implement, efficient and exhibits a modest gain in performance over the convolutional PML without requiring any extra computational resources or an increase in the algorithmic complexity of the PML implementation.

A large-scale magnetic survey in Makrygialos (Pieria) Greece

A large-scale magnetic survey was conducted in the archaeological area of Makrygialos. The site was threatened due to the construction activities carried out in the area, as part of the national highway re-route project. Geophysical prospection contributed to the archaeological evaluation of the site, which was based mainly on the salvage excavations that took place prior to and after the geophysical survey. Magnetic prospecting was applied on a routine base, in order to cover a large area in a short period of time. Also, magnetic susceptibility was used to acquire detailed information of the stratigraphy of the ditches revealed by the excavations. The Le Borgne contrast was calculated and was used as an index of the magnitude of the magnetic anomalies. Geophysical data were processed by a number of filtering techniques, including the removal of regional trends and Hanning smoothing. Fourier transformation was applied and bandpass filtering procedure was based on the examination of the power spectrum of the data. In addition, two-dimensional inversion filtering was performed at specific parts of the data set, in an effort to rectify the significant geophysical anomalies of the site and obtain more information about their width and magnetization. The results of the geophysical survey were able to highlight a system of three curvilinear ditches, which excavation data suggested were probably dug during the Neolithic period. Various linear and geometrical anomalies, related to subsurface structures, are included among the other geophysical features encountered at the site. The geophysical prospecting techniques were able to map more than 60,000 m2 of the site, a large portion of which has now been destroyed by the construction activities for the national road. In this way, geophysical maps can be used as a valuable source of information for the future study of the site. The present case study illustrates the impact of geophysical exploration in the management of archaeological sites threatened by large-scale construction projects.

Creating finite-difference time-domain models of commercial ground-penetrating radar antennas using Taguchi’s optimization method

Very few researchers have developed numerical models of Ground-Penetrating Radar (GPR) that include realistic descriptions of both the antennas and the subsurface. This is essential to be able to accurately predict responses from near-surface, near-field targets. This paper presents detailed three-dimensional (3D) Finite-Difference Time-Domain (FDTD) models of two commercial GPR antennas—a Geophysical Survey Systems, Inc. (GSSI) 1.5 GHz antenna and a MALA Geoscience 1.2 GHz antenna—developed using simple analyses of the geometries and the main components of the antennas. Values for unknown parameters in the antenna models (due to commercial sensitivity) were estimated by using Taguchi’s optimisation method, resulting in a good match between the real and modelled crosstalk responses in free-space. Validation using a series of oil-in-water emulsions to simulate the electrical properties of real materials demonstrated that it was essential to accurately model the permittivity and dispersive conductivity. When accurate descriptions of the emulsions were combined with the antenna models the simulated responses showed very good agreement with real data. This provides confidence for use of the antenna models in more advanced studies.Very few researchers have developed numerical models of ground-penetrating radar (GPR) that include realistic descriptions of both the antennas and the subsurface. This is essential to be able to accurately predict responses from near-surface, near-field targets. We have developed a detailed 3D finite-difference time-domain models of two commercial GPR antennas — a Geophysical Survey Systems, Inc. (GSSI) 1.5-GHz antenna and a MALA Geoscience 1.2-GHz antenna — using simple analyses of the geometries and the main components of the antennas. Values for unknown parameters in the antenna models (due to commercial sensitivity) were estimated by using Taguchi’s optimization method, resulting in a good match between the real and modeled crosstalk responses in free space. Validation using a series of oil-in-water emulsions to simulate the electrical properties of real materials demonstrated that it was essential to accurately model the permittivity and dispersive conductivity. When accurate descriptions of the emu...

An advanced GPR modelling framework: The next generation of gprMax

gprMax is a freely-available set of electromagnetic wave simulation tools based on the Finite-Difference Time-Domain (FDTD) numerical method. gprMax was originally written in the mid-1990s and has primarily been used to simulate Ground Penetrating Radar (GPR). Current computing resources offer the opportunity to build detailed and complex FDTD models of GPR to an extent that was not previously possible. To enable these types of simulations to be more easily realised, and also to facilitate the addition of more advanced features, significant modernisations have been made to gprMax. The original C-based code has been completely rewritten using a combination of Python and Cython programming languages. Standard and robust file formats have been chosen for geometry and field output files. New advanced modelling features have been added including: an unsplit implementation of higher order perfectly matched layers (PMLs) using a recursive integration approach; uniaxially anisotropic materials; dispersive media using multiple Debye, Drude or Lorenz expressions; improved soil modelling using a semi-empirical formulation for dielectric properties and fractals for geometric characteristics; rough surface generation; and the ability to embed complex transducers and targets.

Creating FDTD models of commercial GPR antennas using Taguchi’s optimisation method

Very few researchers have developed numerical models of Ground-Penetrating Radar (GPR) that include realistic descriptions of both the antennas and the subsurface. This is essential to be able to accurately predict responses from near-surface, near-field targets. This paper presents detailed three-dimensional (3D) Finite-Difference Time-Domain (FDTD) models of two commercial GPR antennas—a Geophysical Survey Systems, Inc. (GSSI) 1.5 GHz antenna and a MALA Geoscience 1.2 GHz antenna—developed using simple analyses of the geometries and the main components of the antennas. Values for unknown parameters in the antenna models (due to commercial sensitivity) were estimated by using Taguchi’s optimisation method, resulting in a good match between the real and modelled crosstalk responses in free-space. Validation using a series of oil-in-water emulsions to simulate the electrical properties of real materials demonstrated that it was essential to accurately model the permittivity and dispersive conductivity. When accurate descriptions of the emulsions were combined with the antenna models the simulated responses showed very good agreement with real data. This provides confidence for use of the antenna models in more advanced studies.Very few researchers have developed numerical models of ground-penetrating radar (GPR) that include realistic descriptions of both the antennas and the subsurface. This is essential to be able to accurately predict responses from near-surface, near-field targets. We have developed a detailed 3D finite-difference time-domain models of two commercial GPR antennas — a Geophysical Survey Systems, Inc. (GSSI) 1.5-GHz antenna and a MALA Geoscience 1.2-GHz antenna — using simple analyses of the geometries and the main components of the antennas. Values for unknown parameters in the antenna models (due to commercial sensitivity) were estimated by using Taguchi’s optimization method, resulting in a good match between the real and modeled crosstalk responses in free space. Validation using a series of oil-in-water emulsions to simulate the electrical properties of real materials demonstrated that it was essential to accurately model the permittivity and dispersive conductivity. When accurate descriptions of the emu...

Evaluating four-dimensional time-lapse electrical resistivity tomography for monitoring DNAPL source zone remediation.

Practical, non-invasive tools do not currently exist for mapping the remediation of dense non-aqueous phase liquids (DNAPLs). Electrical resistivity tomography (ERT) exhibits significant potential but has not yet become a practitioners tool due to challenges in interpreting the survey results at real sites. This study explores the effectiveness of recently developed four-dimensional (4D, i.e., 3D space plus time) time-lapse surface ERT to monitor DNAPL source zone remediation. A laboratory experiment demonstrated the approach for mapping a changing NAPL distribution over time. A recently developed DNAPL-ERT numerical model was then employed to independently simulate the experiment, providing confidence that the DNAPL-ERT model is a reliable tool for simulating real systems. The numerical model was then used to evaluate the potential for this approach at the field scale. Four DNAPL source zones, exhibiting a range of complexity, were initially simulated, followed by modeled time-lapse ERT monitoring of complete DNAPL remediation by enhanced dissolution. 4D ERT inversion provided estimates of the regions of the source zone experiencing mass reduction with time. Results show that 4D time-lapse ERT has significant potential to map both the outline and the center of mass of the evolving treated portion of the source zone to within a few meters in each direction. In addition, the technique can provide a reasonable, albeit conservative, estimate of the DNAPL volume remediated with time: 25% underestimation in the upper 2m and up to 50% underestimation at late time between 2 and 4m depth. The technique is less reliable for identifying cleanup of DNAPL stringers outside the main DNAPL body. Overall, this study demonstrates that 4D time-lapse ERT has potential for mapping where and how quickly DNAPL mass changes in real time during site remediation.

Unsplit Implementation of Higher Order PMLs

An unsplit implementation of higher order perfectly matched layers (PMLs) using a recursive integration approach is presented. The formulation, which is based on the complex coordinate stretching of space, is developed for a general complex frequency-shifted stretching function but is applicable to PMLs employing the standard stretching function or a mixture of either types. The approach results in the development of two general formulae that could be used to easily generate PML correction equations for any PML order. Numerical results from finite-difference time-domain models are presented to illustrate the validity of the approach.