PSI - Issue 64

A. Di Benedetto et al. / Procedia Structural Integrity 64 (2024) 2254–2262 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

2257

4

(Spalling), while negative values denote upward deformations (Swelling). The search sphere's radius, a function of distress type and average size, is the sole parameter to be set. 5. Radiometric correction of intensity values and analysis of output values to detect potential water leakages. Figure 2 illustrates the developed workflow. All processes have been implemented in the MATLAB environment.

Fig. 2. Workflow.

3.1. Radiometric correction of intensity values. Prior to analysis, intensity values require radiometric correction (Kashani et al., 2015). They are influenced by the chemical and physical characteristics of the materials on which the laser beam impacts (target) and the scanning geometry, particularly the distance to the target and the incidence angle of the beam. Accordingly, correction of intensity values from MLS data is usually required to obtain consistent values independent of these variables. There are four levels of intensity processing. Each level increases in accuracy and quality of information: starting from level 0, representing basic intensity values provided by the manufacturer or vendor in their native storage format, up to level 3, which represents rigorous radiometric correction and calibration. For our purposes, we will focus on level 1 correction. In this process, corrections are made to intensity values to reduce or ideally eliminate variation caused by scan geometry (range and angle of incidence). Experimental results indicate that the range effect is mainly driven by instrumental factors, while the angle of incidence effect is primarily related to target surface properties. Road surfaces are homogeneous reflecting surfaces and behave more like Lambertian scatterers than other materials. The angle of incidence has less influence on road pavements. In round section tunnels, the laser beam is almost orthogonal to the surface of the intrados, hence intensity values are less influenced by it, whereas range has a greater influence. Only the dependence on range will then be evaluated, as it weighs more than the angle of incidence. Several studies have indicated that the intensity of TLS becomes proportional to 1/r 2 from 10-15 m onwards, meaning that the near range and non-near range are two different functions (Tee-Ann and Hui-Lin, 2015). For distances up to 10-15 m, the intensity trend is not inversely proportional to the distance. This is often observed for the part of the point cloud belonging to the tunnel. To separate the amplitude-range function into two functions, a cut-off point for near-range (f 1 ) and far-range (f 2 ) has been calculated as follows:

= + + + 2 f r a a r a r a r ( )

 r Max

(f ) q

3

1

0

1

2

3

(1)

1 1 1

( ) f r b b b

b

 r Max

(f ) q

= + + +

2

0

1

2

3

r

r

r

2

3

The cut-off point refers to the turning point "Max(f q )" of the second-order polynomial function (f q ). The Least Squares interpolation has been applied to estimate the polynomial coefficients. To convert the original recorded intensity I o into normalized intensity I c , the equations are as follows: