Ground-penetrating radar method used for the characterisation of ornamental stone quarries

Abstract

In the work non-destructive ground-penetrating radar (GPR) was used to probe the texture and presence of anisotropies in different types of carbonated rocks intended for ornamental use.First, GPR on the different facies of a marble quarry in Macael (Almería, Spain) was applied where alternating layers of marble and mica schists can be found. GPR allows for the differentiation of the marble units from the mica schist units, which makes GPR a good tool for indirectly evaluating the reserves in a deposit. In addition, it allows for the detection of the different anisotropies in the marble units (holes, fractures), and this information could be utilised to formulate a work-plan design. Antennae of different frequencies were employed (100, 250 and 800MHz) for these purposes, and the 250MHz antenna had the most effective probing capacity for obtaining an accurate depth resolution.Second, a GPR study was conducted in three types of rock known commercially as Macael Marble, Crema Marfil and Red Travertine before the block-cutting process. For this configuration, the 800MHz antenna was used to differentiate the textures of each type of rock as well as the location of a variety of anisotropies, and the results showed that GPR is an effective tool for evaluating the block quality, determining whether resins must be injected to consolidate the block and estimating the orientation of the cutting process.In the work, the use of a supervised two-dimensional (i.e., radargram) probabilistic latent component analysis (PLCA) approach is proposed to highlight only the information from target objects (i.e., marble anisotropy) provided by the radargram, which facilitates the data interpretation by the user. This approach can search activations of the georadar pulse across both dimensions. Once the analysis has been performed, a variable-gain compensation process is proposed to outperform the energy losses that result from the reflections of the signal when a different material is found.The results have shown that the GPR method can be utilised as a tool for the diagnosis of stone materials prior to their use in artistic work or monuments; therefore, GPR may be considered a technique for material selection. The presence of discontinuities (sometimes visible to the naked eye) explain many of the phenomena and typologies of the stone after its eventual alteration, and such discontinuities could be avoided by means of this pre-emptive work.