GPR attenuation analyses using spectral ratios of primary and multiple arrivals: examples from Welsh peat bogs

Wednesday, 17 December 2014
Adam Booth, Imperial College London, London, United Kingdom, Donna Carless, Swansea University, Geography, Swansea, United Kingdom and Bernd Kulessa, Glaciology Group, Swansea University, Swansea, United Kingdom
Ground penetrating radar (GPR) is widely applied to qualitative and quantitative interpretation of near-surface targets. Surface deployments of GPR most widely characterise physical properties in terms of some measure of GPR wavelet velocity. Wavelet amplitude is less-often considered, potentially due to difficulties in measuring this quantity: amplitudes are distorted by the anisotropic radiation pattern of antennas, and the ringy GPR wavelet can make successive events difficult to isolate. However, amplitude loss attributes could provide a useful means of estimating the physical properties of a target.

GPR energy loss is described by the bandwidth-limited quality factor Q* which, for low-loss media, is proportional to the ratio of dielectric permittivity, ε, and electrical conductivity, σ. Comparing the frequency content of two arrivals yields an estimate of interval Q*, but only if they are sufficiently distinct. There may be sufficient separation between a primary reflection and its long-path multiple (i.e. a ‘repeat path' of the primary reflection) therefore a dataset that is rich in multiples may be suitable for robust Q* analysis. The Q* between a primary and multiple arrival describes all frequency-dependent loss mechanisms in the interval between the free-surface and the multiple-generating horizon: assuming that all reflectivity is frequency-independent, Q* can be used to estimate ε and/or σ.

We measure Q* according to the spectral ratio method, for synthetic and real GPR datasets. Our simulations are performed using the finite-difference algorithm GprMax, and represent our example data of GPR acquisitions over peat bogs. These data are a series of 100 MHz GPR acquisitions over sites in the Brecon Beacons National Park of South Wales. The base of the bogs (the basal peat/mineral soil contact) is often a strong multiple-generating horizon. As an example, data from Waun Ddu bog show these events lagging by ~75 ns: GPR velocity is measured here at 0.034 m/ns (relative ε of 77.9) and spectral ratios suggest Q* of 19.9 [-6.6 +19.4]. This Q* implies that the bulk σ of the bog is 21.7 [-10.7 +10.8] mS/m. Our measurements require in situ verification (e.g. comparison with co-located electrical resistivity profiles) but our method provides a promising addition to the suite of GPR analysis tools.