Wenner Array and Extension Bars

The TAR-3 Resistance Meter system is supplied with all the components needed to carryout a resistance survey according to the current edition of 'EAC Guidelines for the use of Geophysics in Archaeology'

0.5m Twin Probe Technique

This is the method recommended in the current EAC Guidelines. It uses four probes, two attached to the frame (0.5m apart) and two 'remote' probes with the same spacing (0.5m apart). The stationary remote probes should be located at a distance of at least 30 * 0.5m (15m) away from the frame mounted traveling probes. The four probes (two sets) are connected to the TAR-3 Meter using the red and blue leads and one (or two) of the 50m extension leads

1.0m Extension Bar

A 1.0m Extension bar can be attached to the 0.5m frame, (1.0m Twin Probe Technique). It uses four probes, two attached to the frame (1.0m apart) and two 'remote' probes with the same 1.0m spacing). The stationary remote probes should be located at a distance of at least 30 * 1.0m (30m) away from the frame mounted traveling probes. The two sets of probes are connected to the TAR-3 Meter using the red and blue leads and two 50m extension leads (100m in total).

The 1.0m probe spacing enables the TAR-3 Resistance Meter to 'See' deeper below the surface but the resolution is reduced and smaller features may not be detectable.

Wenner Array Bar (0.5m)

The Wenner Array bar is 1.5m long and has four holes. The four probes are mounted on the bar C1, P1, P2 and C2 and connected to the TAR-3 Meter using two red and two blue leads. Note: There are no 'Remote' probes. When using the Array Bar all of the probes are moved at the same time. The Array Bar is attached to the TAR-3 Frame using the P1 and P2 probes and bushes.

The Wenner Array Bar measures the resistance of the grid to the same depth as the standard 0.5m Twin Probe Technique, but due to the probe geometry the resistance displayed on the LCD screen differs by a factor of TWO from the 0.5m Twin Probe reading. This is due to the distance between the C1P1 and C2P2 pairs of probes being one (1 * 0.5m) and not thirty (30 * 0.5m) as required by the Twin Probe method.

1.5m Extension Bar (using the Wenner Array Bar)

The Array Bar can be used as a 1.5m extension bar. The bar is mounted to the frame using the bolts and bushes supplied. It uses four probes, two attached to the frame (1.5m apart) and two 'remote' probes with the same 1.5m spacing. The stationary remote probes should be located at a distance of at least 30 * 1.5m (45m) away from the frame mounted traveling probes. The two sets of probes are connected to the TAR-3 Meter using the red and blue leads and two 50m extension leads (100m long). Using the Wenner Array Bar as a 1.5m extension bar enables the user to measure the resistance of the ground at a greater depth. (Three time greater than the standard 0.5m Twin Probe Technique.) A 1.5m probe separation also results in a lower resolution. For best results users should modify their technique. At the end of each row leave the probes in the ground and walk around the TAR-3 unit so that the display is upside down and then the start the next row. At the end of the row walk around the unit again so that the display is in the correct orientation and continue you onto the next row. (do not rotate the frame at the end of each row)

Comments

The EAC Guidelines recommends the 0.5m Twin Probe method for professional use. Users may decide to use one (or more) of the other methods if, in their judgment, the technique is warranted. Users may want to use 1.0m (or 1.5m) probe separation if they suspect that the ground surface has been subject to fluvial deposits, hill wash or other soil deposits (including landscaping or industrial use). The increased probe separation results in a lower spatial resolution.

The Wenner Array was the default technique (in the fifties, sixties) but has been superseded by the Twin Probe technique. The Twin Probe method is physically easer, the user only has to move two probes and ensure the two they are in good contact with the ground for each reading. Ease of use, less physically demanding and increased resolution have made the Twin Probe technique the preferred method for most surveys.

The Wenner Array suffers from another other disadvantage over the Twin Probe technique – the results show 'twin peaks'. As the probes C1 and P1 pass over an anomaly a variation in resistance is recorded. When the C2 and P2 probes pass over the same anomaly a second variation is recorded but at a different position which can give rise to a halo in the results. In many cases the advantage of not having a trailing lead are offset by the extra weight, extra probe contacts and halo effect.

If the ground surface is suitable the Wenner Array may be the preferred technique when searching for Roman Roads or other linear features. A single user can mark out one base line and by setting a 30m by 5m grid cover a long linear traverse across the landscape. (Hopefully, perpendicular to the feature).