Quantification of Optical Clarity of Transparent Soil Using the Modulation Transfer Function

Abstract

Transparent synthetic soils have been developed as a soil surrogate to enable internal visualization of geotechnical processes in physical models. Transparency of the soil dictates the overarching success of the technique; however, despite this fundamental requirement, no quantitative framework has yet been established to appraise the visual quality of transparent soil. Previous approaches to assess and optimize transparency quality included an eye chart assessment method, although this approach is highly subjective and operatordependent. In this paper, an independent method for quantitatively assessing the optical quality of transparent soil is proposed based on the optical calibration method, Modulation Transfer Function (MTF). The work explores this hypothesis and assesses the potential for MTF to quantify the optical quality of transparent soils for a number of aspects including (i) optimum oil blend ratio, (ii) depth of viewing plane, and (iii) temperature. The results confirmed that MTF offers a robust and reliable method to provide an independent quantitative measure of the optical quality of transparent soil. The impact of reduced soil transparency and the ability to track speckle patterns-thus accuracy and precision of displacement measurement-was correlated with MTF to evaluate the permissible viewing depth of transparent soil.

Transparent Soil to Model Thermal Processes: An Energy Pile Example

“…It is also interesting to note that the trend line turning point corresponds to the temperature at which the material was calibrated to yield optimum transparency (i.e., 20 C). This is in good agreement with previous research reported by Stanier (2011) and Black and Take (2015), and provides further confidence in the pixel intensity-temperature relationship established and presented in Fig. 6.…”

“…Works using this approach include examination of failure mechanics of helical screw piles (Stanier et al 2013), stone column groups (Kelly 2013), and tunnel induced settlements (Ahmed and Iskander 2010) soil plugging behavior during press-in piling of tubular piles (Black 2012;Forlati and Black 2014). The success of transparent soil modeling has long been considered reliant on producing a soil surrogate that offers the highest optical clarity, with low transparency being considered detrimental to the modeling technique (Black and Take 2015). However, recent work by Siemens et al (2010) and Peters et al (2011) embraced the loss of soil transparency as a positive characteristic for the purpose of modeling unsaturated soil phenomena.…”

“…The background (referred to herein as "calibration target") implemented in the current research consisted of two distinct regions: (i) black uniform intensity and (ii) alternating black and white stripes at a spatial frequency of 1 line pair per mm (lp/mm). This split target was adopted due to the positive outcome in the work of Siemens et al (2010) and Peters et al (2011), who reported success using a black uniform background to detect changes in pixel intensity to capture loss of optical transmission, and striped region due to complementary work by Black and Take (2015) that established a robust quantitative framework for assessing the optical quality of transparent soils using optical method referred to as "modulation transfer function" (MTF). MTF operates by relating the pixel contrast that is transferred from an object to an image and is commonly used to calibrate optical systems.…”

“…The loss in signal modulation is governed by the transparency; hence, MTF can be used to quantify and evaluate the reduction in clarity that occurs as temperature changes. This process is fully described in Black and Take (2015); however, the main principle is conveyed in Fig. 2, which demonstrates the loss in modulation when temperature increases in transparent soil.…”

“…The pore fluid was a blend of white oil (Baylube WOM 15) and paraffinic solvent (N-paraffin C10-13) mixed to volumetric proportions of 77:23, giving a refractive index match to the silica aggregates of 1.467 at 20 C. This ratio was previously calibrated by Stanier et al (2012) using a visual eye chart assessment method; however, as described previously, this was superseded by a newly established quantitative framework proposed by Black and Take (2015) based on an optical calibration method known as MTF. N-paraffin C10-13.…”