Wide band frequency domain forward and inverse modelling code for GPR data

Geophysical imaging utilising the Ground Penetrating Radar (GPR) is a staple for observing the subsurface in high resolution. It produces images of the distribution of dielectric permittivity and electrical conductivity of the surveyed subsurface region. To produce these images, the data is supplied to an inversion algorithm. This algorithm iteratively improves the estimates of what the subsurface looks like. Each iteration is based on how close the response produced by the estimate is to the data.

To evaluate the response from estimate models, we require an efficient forward solver of the Maxwell's equations. It can be performed in time-domain, as very capably done via gprMax, or in frequency-domain. Working in frequency-domain allows for spectral analysis of subsurface response, choosing particular frequencies of interest, and unique parallelisation and inversion strategies. For wide-band antennas used in a GPR survey, we are developing an efficient frequency domain forward modelling code in 3D. Simulating in 3D allows us to account for the antenna's complete radiation pattern. Moreover, to account for the wide band data, the forward solver will be equipped with a custom meshing algorithm that creates separate meshes for different frequency ranges of the data spectrum. Care needs to be taken that adequate boundary conditions are implemented in the forward solver for accurate domain truncation.

We will present the capabilities and features of the forward solver with synthetic test cases. Supplying our forward engine to a suitable Full Waveform Inversion (FWI) strategy that maximises the adaptable meshing and frequency domain evaluations could result in high performance. The exact strategy for such an FWI will be defined and tested such that it leads to accurate and efficient reconstruction of the electrical subsurface properties.