*3.1. Instrumentation*

The proposed hybrid method is based on the data obtained during the automatic longterm survey performed using the two instruments MISM and MILTS. These new/original instruments were designed, realized and calibrated by the authors [14–16] for the purpose of measuring magnetic fields, being suitable in monitoring magnetic field exposure for the general population.

Although there are small differences between MISM and MILTS, each of the two instruments consists of an isotropic field sensor (three linear coils), an electronic signal processing circuit (amplifier and integrator), a data acquisition module and a laptop that hosts several virtual instruments realized in LabVIEW.

Both measurement systems record and process the waveforms for the three components of B. The processing and LabVIEW module can perform a frequency analysis and compute the rms and peak-to-peak values, as well as a statistical post-processing of the recorded data. These two instruments can perform a long-term survey of the magnetic field, recording samples of B every second for a frequency range of the B components up to 100 kHz, which is appropriate for observing the biological effects of the magnetic field.

The calibration of these instruments was conducted using a reference magnetic field (standard-field method), with an uncertainty of less than 5%. This uncertainty of the measurement instrument was also verified by comparison with commercial instruments.

The experimental recording performed with MILTS lasted for 5 h and 31 min, between 9:37 am and 3:08 p.m. At the same time interval, the transversal profiles near the pillar (in z = 0) and at the mid-distance between two pillars (z = d/2), and also the longitudinal profile B (0, y, z), z ∈ [0, d], were traced with MISM at a heighty=1m above the ground.

The OPL considered in experiments (Figure 1b) is a double three-phase line 2 × 220 kV with RST-RST phase disposition, situated in Iasi County, Romania. The geometrical configuration of the line as well as the current rms values are known and are used in the numerical simulations.

## *3.2. Measurement Results*

During the experiments, two profiles for B were obtained: one in transversal direction for z = 0, PT, and a longitudinal profile PL for z ∈ [0, d]. A supplementary verification and validation of these results is performed using measurements taken for a transversal profile at the mid-distance between two consecutive pillars, PTM.

Figure 3 depicts the transversal profile near the pillar using 61 consecutive measurements in points 1 m apart, at different moments of time (since only one instrument MISM was used). The maximum value for |B| is obtained at approximately 5 m from the central symmetry plane of the OPL, due to the fact that the currents in the second line (conductors (1 ), (2 ) and (3 ) in Figure 1) were zero (line out of service).

**Figure 3.** Transversal profile for |B| obtained in successive measurements near one pillar (z = 0).

The longitudinal profile between two pillars, PL, was traced using spot measurements in points 3 m apart for the distance d = 282 m, Figure 4. In the considered case, the experimental procedure for PL lasted 40 min.

**Figure 4.** Longitudinal profile for |B| obtained in successive measurements (PL).

The plots in Figures 3 and 4 show rather large variations of |B|, even for neighboring points. This is due to the variation of |Ik| in time, allowing for the instrument MISM to be moved to the next position.

However, most of these variations can be smoothed (leveled) using the proposed method for estimating the magnetic field profile at a given time, this being equivalent to the case when all the measurements are carried out simultaneously. Some irregularities in |B| still persist even after applying this method due to errors produced by the fact that the Earth's surface is not flat (small differences in the distance between the line and the instrument occur).

Although the profiles PT and PL give sufficient data in order to plot a map of |B| in a rectangular region beneath the line, a second transversal profile PTM was recorded at a distance z = d/2. This profile, represented in Figure 5, is used to verify the proposed method for field estimation. The procedure is based on the comparison of the data in PTM with the transversal profile estimated using the data in PT, taken in z = 0, the data in PL and the time variation of B obtained with MILTS during the long-term survey.

Thus far, only the data from spot measurements with MISM were presented. In the following, the data from the long-term survey performed with MILTS will be presented and discussed. Figure 6 presents the recordings made for B rms with MILTS for the entire observation interval of 5 h and 31 min. In this recording, a significant temporal variation of the magnetic field can be observed, justified by the time variation of the rms current in the line. These variations can be correlated with the variations of B identified in the measured data for the longitudinal profile (plotted in Figure 4).

**Figure 5.** Transversal profile measured experimentally at the mid-distance between two pillars.

**Figure 6.** Values recorded for B (rms) during the long-term survey at a fixed point.

Using the long-term survey instrument, MILTS, a time and frequency analysis of the recorded signal can be performed. Figure 7 presents both the time variation and the spectral analysis for the field components Bx, By and Bz.

**Figure 7.** Waveforms and spectral analysis for the B components.

A statistical post-processing of the recorded results over the observation time interval can be also performed. Table 1 presents the results of post-processing, i.e., date and time of observation, number of measurements, minimum, maximum and average values of B, standard deviation, number of recorded data larger than the average value and percentage of exceeding values.


**Table 1.** Post-processing statistical results for the data recorded with MILTS.

The currents in the line, further used in simulations, are known only for six moments in time (10 a.m., 11 a.m.,..., 3 p.m.), but the number of values recorded for B during the long time survey is 17,530, with a time step of about 1 s (17,530 measurements in 19,860 s). Moreover, if the range for the current rms values is from 118 to 196 A, the range of values for B recorded by MILTS is from 724 to 1375 nT.
