Multi-probe array PA20 is a versatile modular frame system for use with the RM85, RM15-D or RM4 Resistance meters. Different size beams clip onto the handle frame giving a wide range of probe arrays and detection depths. Stainless steel probes can be easily added, removed or interchanged to make up desired configurations. The PA20 can be configured into a number of different probe arrays including : Twin of spacing 0.25m, 0.5m, 0.75m, 1m, 1.25m, 1.5m, 1.75m or 2m, 0.5m Wenner, 0.5m Double Dipole, and mini- Schlumberger. You can easily vary the spacing of a Twin array configured with a PA20 probe array in order to optimise detection depth for a particular application.

Use of an RM85 with multiplexer card or addition of the MPX15 multiplexer to an RM15-D opens up a much wider range of arrays and applications: parallel Twin arrays (2, 3 or 4) for much faster ground coverage, multiple Twin arrays of different spacings for depth investigations, simultaneous measurement of Wenner alpha, beta and gamma arrays etc. The multiplexers are also fully programmable allowing the user to configure and store their own measurement sequences and configurations. The auto-log mode of the RM85 and RM15(-D) makes multiplexing totally automatic, surveys even faster and more efficient, whilst reducing operator errors and fatigue (see RM85 and MPX15 data sheet for more details).

The Twin array is a popular configuration designed for rapid detailed area surveys, at depths roughly equal to the probe spacing - see figure on opposite page. It requires only two probes on a frame to be moved from station to station, whilst the remote pair of probes remain fixed. Placement, orientation and insertion depth of the mobile probes is non-critical. If combined with the auto-log mode of the RM85 or RM15(-D), this results in a rapid data collection system.

Modular frame system for RM85, RM15-D and RM4  resistance meters
Quick release system for rapid interchange of probe array beams - no fasteners to lose
0.5m and 1m beams as standard. Additional 1.5m and 2.0m beams available.
Lightweight glass fibre beams save weight compared to the PA5 system : save 0.7 Kg for a 1m array, save 1.3 Kg for a 1.5 m array
Junction box fixed securely to main frame
Glass fibre beam construction minimises measurement errors
Optional remote probe Extension leads for improved performance.
Offers many probe configurations with adapters and beams :
     Twin (spacings from 0.25m to 2.0m, 0.25m steps)
     Wenner, Double-Dipole (spacing 0.5m)
     Gradient, Pole-Pole, Mini Schlumberger
Addition of an RM85 multiplexer card or MPX15 multiplexer extends the PA20 capability :
     Parallel Twin for faster ground coverage or more detailed area
     Multiple Twin arrays for depth investigations etc
     Simultaneous  configurations eg Twin and Wenner alpha, beta,


The standard PA20 array (017-020) comprises a clip-on handle frame with quick release latches (see figure), one 0.5m beam, one 1.0m beam, 2 stainless steel probes and associated fasteners, one cable drum containing 50m of heavy duty arctic grade twin cable, two stainless steel remote probes, cables to connect the remote probes to the cable drum. A range of different length jumper leads are provided for connecting the probes to the frame terminals, depending on the lengths of additional beam sizes ordered. The standard PA20 array can be configured as a 0.25m, 0.5m, 0.75m, or 1m Twin.

Additional beams may be purchased to configure other arrays, such as Wenner, Double Dipole, or multiplexed arrays. Their relative sizes are shown opposite, along with the standard beams for comparison. Additional beam sizes are 1.5m (019-019) and 2.0m (019-020). Remote probe Extension leads are available for easier operation and improved accuracy with larger beam sizes. Where more than the standard 2 probes are required additional stainless steel probes (019-023) must also be ordered (a maximum of six probes can be mounted at any one time). The RM15(-D) includes a mounting kit (019-003) to attach it to the PA20; the RM85 requires a mounting bracket (003-013). An Adapter or MPX15 multiplexer is also required to interface the PA20 to an RM15-D or RM4 resistance meter. The RM85 can interface directly to the PA20 but adapters may be required for some of the more specialist arrays.

Standard PA20 System --0.5m and 1.0m beam

Additional beam--1.5m
Additional beam--2.0m
Possible probe locations

ADAPTERS (RM15-D only, some RM85 adapters are different)

The lead from the PA20 frame terminates in a 6-way waterproof plug which must be connected to the 2-way connector on the RM15(-D) or RM4 resistance meter. This may be achieved using either individual adapters for different arrays or the MPX15 multiplexer (RM15-D only) which will interface virtually all arrays.

AD1 and AD3 The most popular adapter is the AD1 (029-020) which is suitable for configuring a single Twin probe array of any spacing (0.25m to 2m). The next most popular adapter is the AD3 (029-022) which enables a 0.5m Wenner, 0.5m Double-Dipole or mini-Schlumberger array to be configured (the remote probes and cable are not used in these cases, but two additional stainless steel probes, 019-023, are required).

AD2 The Gradient array adapter, AD2 (029-022), connects the two potential pins of the resistance meter to the frame, and connects the two current pins of the resistance meter to leads terminated with 4mm socket. Two additional 50m small cables (019-015 for a pair) are required to connect these to two remote current probes which can be the pair provided with the standard system. Alternatively, two of the leads and probes from the PA3 probe array (see separate data sheet) may be used instead.

AD4 The AD4 adapter (029-023) will convert all Twin arrays into the Pole-Pole equivalents. This adapter converts the "Remote Probes" two-pole connector on the RM15(-D), RM4 or MPX15 into two separate leads (current and potential), each terminated in a flying 4mm socket. As for the AD2, two additional 50m small cables (019- 015 for a pair) are required to connect these to the remote probes. The AD4 may be used with both the MPX15 multiplexer and AD1 adapter.

Extension Leads
Remote probe Extension leads, each 25m long, are advised when using longer beams in order to minimise variation of readings over a grid due to geometry effects. They will enable larger areas to be surveyed from the same remote probe position and can simplify grid matching if a wide remote probe spacing (5-10m) is used.


The MPX15 multiplexer (not compatible with RM4) effectively integrates adapters AD1, AD2 and AD3 into one box, though an AD4 will still be required for the Pole-Pole array, and additional 50m cables will be required for both Pole-Pole and Gradient arrays. The AD1 may still be a useful lightweight addition to your system if a single twin probe array is to be used frequently. Of course, the MPX15 offers much greater scope than is possible with just single measurement sequences and adapters. The MPX15 may be used to configure single, parallel or multiple arrays, offering much faster or more detailed area coverage, or depth investigation (see MPX15 data sheet for more details).


Great care has been taken to maximise the conditions in which the PA20 can be used. However, regular cleaning by the user is required to avoid tracking problems in wet and muddy conditions. Since all probes need to make simultaneous contact, some terrains may cause problems, especially if there are 6 probes on a wide frame. For example very dry, hard and stony ground may be difficult to survey with a 2m frame. However, on more moist ground, surface contact is usually sufficient to obtain good readings, even in auto-log mode. If the ground is moist then even where there is a pronounced undulation, the probes need only be inserted 1 cm to obtain good readings.


The standard 0.5m Twin array finds greatest application in archaeological surveys as many features are near surface, and speed, ease of use, simple response and excellent resolution (approximately 0.5m) is often of greater importance than detection depth limitations (0.5m - 0.75m for typical archaeological targets). Where more substantial structures are involved, such as capped mine shafts, detection depth can be greater. Detection depth can be improved by increasing the Twin array size, giving a commensurate increase in detection depth. However, this will be at the expense of resolution, and if there are near surface features present, narrower than the array dimensions, these may give rise to multiple responses (or artefacts). Nevertheless, providing one remains aware of such possibilities during interpretation, useful information at greater depths can be achieved. The standard PA20 can be configured as a Twin array up to 1m wide, whilst the addition of beams can increase the array size up to 2m, giving up to a four-fold increase in detection depth.

Some possible PA20 and MPX15 configurations

Single 0.5m Twin

Single 1.0m Twin

Multiplexed 0.5m, 1m Twin
Parallel and Multiple Mode

Multiplexed 0.5m Wenner and Double-Dipole

Multiplexed 0.5m, 1m, 1.5m
Twin Aligned centres


Dimensions configured as 0.5m Twin (including probes)
Dimensions configured as 1m Twin (including probes)
Frame weight (configured as 0.5m or 1m Twin)
Cable Drum and remote probes weight

Height 980mm, max width 665mm
Height 980mm, max width 1165mm
3.3 Kg (0.5m Twin), 3.9 Kg (1m Twin)
4.1 Kg

Upgrading a PA5 probe array system to PA20 probe array system

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