PLAN DE AUDITORIA

The Passive Wire (also referred to as the long or stretched wire antenna in the literature) is primarily used for the absolute measurement of electric potential (with the ground as the zero reference potential) at a given height, but can also be used to determine the total air conductivity using a method described later in this section.

The basic principle of the PW is that a well-insulated horizontal wire positioned at a known height above ground level will naturally assume the atmospheric electrical potential for that height. The time taken to reach this potential depends on the air conductivity. Since the resultant voltage on the wire is not artificially generated by an equaliser, the instrument is called the passive wire. Crozier (1963) provides a thorough account of the PW principle. The voltage on the wire is measured using a high impedance voltmeter (Harrison, 1997a). The voltmeter also applies a guard voltage to the support wires to

ensure ultra-low leakage (<5fA) from the sensing wire electrode. Further details of the operation and use of the PW at the RUAO are given by Barlow (1999).

A picture of the PW installed at the RUAO can be seen in Figure 4.1, with the main components of this instrument labelled. The actual wire used is kept under constant tension by weights and a pulley at one of the support posts. The wire is thin (0.5mm diameter) and can not be seen in Figure 4.1. A close-up view of the wire and insulator end can however be seen in Figure 4.2.

Figure 4.1 The Passive Wire (PW) installed on the RUAO 1m above the surface, showing main features. The horizontal wire was positioned 1m above the surface and was approximately 20m long. An identical support structure was at the far end of the instrument, except without the electrometer.

Once the potential is known for the height of the wire, the mean PG between the surface and wire can be determined by simply dividing the potential by the height of the wire. If the PG is considered uniform between the wire and surface, the potential of the PW positioned at 1m above the surface is therefore the same value as the PG at 1m. The uniformity of PG at this height is demonstrated by the results of Barlow (1999), where the potential was measured simultaneously on wires positioned at 0.5m, 1.0m, 1.5m, 2.0m and 2.85m above the surface (Figure 4.3). The potential increased uniformly with height, implying a constant PG between the surface and 2.85m. Similarity in the time variability of

the passive wire potentials correspond closely to each other and to that of a nearby field mill (Barlow, 1999). As no calibration or geometrical corrections are required, the value of PG at 1m measured using the PW is considered absolute.

Figure 4.2 The thin wire used for the PW instrument shown in Figure 4.1, seen here emerging from the end of the insulator.

Figure 4.3 PW potential measured simultaneously for wires at 0.5m, 1.0m, 2.0m and 2.85m above the surface, as well as output potential from a field mill system (From Barlow, 1999).

4.2.1 Use of the PW to measure air conductivity

In addition to the absolute measurement of PG, the PW can also be used to measure total air conductivity (Bennett and Harrison, 2006a). This is achieved by grounding the wire (by

briefly touching the electrode wire with a wire connected to Earth) and measuring the rate of increase in potential of the wire as it comes back into equilibrium with the surrounding air. After being earthed the time taken by the wire to reach atmospheric potential will depend on the total electrical resistivity (i.e. inverse of conductivity) of the air, ρ and electrical capacitance, C. It is the total conductivity (σT) that is measured since the wire is

charged to that of its surroundings by both positive ions from above and negative ions from below. Since ρ and C both depend on the same geometrical factors, which cancel, the total conductivity is given by (e.g. Chalmers, 1967)

0 0 T C

ε

ε

σ

ρ

τ

= = (4.1),

where ε0 is the permittivity of free space and

τ

is the electrical relaxation time of the air

between the surface and the wire.

τ

can be found using the time series of voltage on the wire after being earthed. The passive wire potential V with respect to ground is described in equation (4.2) in accordance with established theory for RC circuits

0 1

t V = ⋅ −V _ e−τ _

  (4.2),

where t is the time elapsed since grounding and V0 is the potential of the air at the height

of the wire. Since the wire is at one metre above the surface, this corresponds to PG in the first metre above the surface. This provides a method to determine total air conductivity as an exponential fit can be made to the voltage time series, the relaxation time

τ

(typically of order 1000s) found and thereby total conductivity using equation (4.1). An example of the close agreement between measured and theoretical PW voltage after grounding is shown in Figure 4.4. The period prior to grounding shows the agreement between passive wire and field mill derived PG.

0 10 20 30 40 50 60 70 80 -60 240 540 840 1140 1440 1740

Time since grounding (s)

P G ( V m -1 ) PW Theoretical FM

Figure 4.4 PG from calibrated field mill (FM) and voltage on the passive wire (PW) after grounding, with modelled charging curve of theoretical equation (4.2) (from Bennett and Harrison, 2006a).

As the calculated relaxation time is a time-averaged quantity, the derived total conductivity is an average value for the sampling period. This method assumes that changes in the PG during the charging time are small compared to the rate of increase in the voltage on the wire. This assumption is valid either nearer the start of charging when the rate of increase in V is greatest, or as is more usually the case in practise after a duration long enough for the short-period (order one minute) fluctuations in PG to be averaged out. The time taken to achieve a consistent estimation of the total conductivity was found to be approximately 15 minutes (Figure 4.5), when the charging time exceeded the electrical relaxation time of the atmosphere (Bennett and Harrison, 2006a).

0 2 4 6 8 10 12 14 16 18 20 0 300 600 900 1200 1500 1800

Time since grounding (s)

D e ri v e d c o n d u c ti v it y ( fS m -1 )

Figure 4.5 Total air conductivity derived from the best model fits using different lengths of the passive wire charging curve (from Bennett and Harrison, 2006a).

In document Lineamientos de seguridad de la información para proteger la base de datos del aseguramiento del departamento de cundinamarca (página 47-56)