USE OF FINITE DIFFERENCE MODELLING TO UNDERSTAND DETECTING ABILITY OF GPR FOR WATER CONDUCTING STRUCTURES IN CRYSTALLINE BEDROCK OVERLAIN BY COMPLEX QUATERNARY DEPOSITS, CENTRAL LAPLAND, FINLAND

The Sakatti Ni-Cu-PGE mining development site in Central Finnish Lapland was used as a test site for finite difference (FD) forward modelling to understand the ground penetrating radar (GPR) signals observed on selected 50 and 100 MHz GPR profiles acquired in the area. Precambrian crystalline bedrock of the Sakatti area is affected by complex deformational history, Neogene weathering and multiple glaciations. The main aim of the study was to investigate how water conducting structures such as faults and fractures are detected with GPR in order to improve the understanding of hydrogeological connection between the bedrock and overlying Quaternary sediments. Two types of FD modelling were tested: conceptual models that measured detectability of one feature, and complex models that were generated to represent the measured GPR profiles as realistically as possible.

The FD modelling results illustrate how the width of the structure affects the detectability of the water conducting structure. Based on the results with 100 MHz antenna, bedrock structures wider than 1 m are detectable when they are overlain by sediments with contrasting dielectric properties. In FD modelled profiles, water conducting structures with high dip (higher than 40 degrees) are hard to detect. The geometry of the bedrock topography affects such that V-shaped/concave topography results to hyperbolas similar to signals produced by the water conducting structures. The upper bedrock zone occasionally contains weathered units with higher dielectric permittivities and electric conductivities which dampen the detectability of these structures. The complex FD model profiles indicate that the relative dielectric permittivity values in overlying saturated Quaternary sediments must be about 10–12 to produce similar appearance as in the real GPR profiles. Previously mentioned aspects indicate that water conducting structures can be challenging to detect in real GPR profiles because they get easily mixed with topography-related effects. Joint interpretation with other surface geophysical data such as electrical resistivity tomography or seismic refraction might support the differentiation of water conducting structures.