FM256 Dual Gradiometer System

The dual gradiometer system uses two instruments carried together either to double the speed at which a survey can be made or to increase the sampling density of a survey. Basing the system on two individual gradiometers gives optimum flexibility since they can also be used separately at different sites when required.

A three sided carrying frame, CF6, supports the two gradiometers. One FM256 acts as a master sample trigger which controls a second slave gradiometer - this can be either another FM256 or an FM18/36. Once data sets have been collected in the two gradiometers they are downloaded, and assembled into two individual composites as normal. The two data sets are then easily merged together to form the final composite - Geoplot provides for this in one simple operation.

The system can be used in either parallel or zig-zag survey mode. When used in zig-zag mode the operator, not the frame, turns around at the end of a traverse, thereby avoiding the introduction of direction dependent heading errors (see movie clip below). Since there is no need for restrictive harnesses, turnaround is very rapid. The start/stop sample trigger button on the FM256 is replicated twice on the frame to cater for operator orientation during zig-zag surveying. Logging is simply performed by pressing the start/stop button. The four legs allow the system to be rested on the ground anywhere during survey - no need to return to a centralised support tripod. The frame is designed to support just one gradiometer at a time when resting on the ground, allowing each gradiometer to be aligned individually. The frame can also be used with the legs removed for operation over very overgrown areas.

Variations in angular orientation can introduce heading errors into the data collected. Using a handheld frame, in contrast to other instrument support systems, provides the user with greater awareness and control of angular variations which significantly improves the quality of the data collected. The sturdy frame is well balanced and easy to use even in long undergrowth, and can be lifted easily over other obstacles. The frame is very lightweight, packs flat and is easy to assemble and transport.


Double speed surveys are carried out by making traverses with the system every 2m, rather than the normal 1m traverse interval for a single gradiometer. This mode is very useful for rapid evaluations. Traverses may be either parallel or zig-zag and sample interval can be up to 16 samples per metre. A guide marker on the centre of the frame  helps you maintain alignment with guide tapes (if used). Area coverage for evaluation surveys is very rapid : a 20m x 20m grid can be surveyed in about 3 minutes using zig-zag traverses, sample interval of 0.25m, sample trigger rate of 0.7s/m and 1m traverse interval.


Increased sample density surveys can be achieved by modifying the traverse pattern. Surveys with traverse intervals of 0.5m (double density) and 0.25m (quad density) can be achieved in half the normal time for a single gradiometer. The increased sampling density  mode is useful for detailed evaluations or research applications where high resolution maps of sub-surface structures are required. The quad density mode is especially useful in this respect - see example survey 2 below. Traverses may be either parallel or zig-zag and the sample interval can be up to 16 samples per metre. Walking the modified traverse pattern is straightforward, and guide markers on the frame help you maintain alignment with guide tapes (if used). The resulting interleaved data pattern is simply merged together using software - again Geoplot provides for this in one simple operation.

See Example Survey Results for further movie clips of the FM256


The survey shown below was collected in double density mode. Grid dimensions were 20m, data was collected in zig-zag traverses at 4 samples per metre and the resultant traverse interval of the merged data is 0.5m. Trigger rate was set at 0.8s/m. Roughly a third of the site was covered with grass and nettles to the height of the frame yet this was successfully navigated with the dual system with no obvious signs of impairment in data quality. Several sub 1nT traces of ridge and furrow running E-W are visible, along with many ferrous responses - no destagger corrections were required to align the ferrous responses between adjacent traverses.

Double density survey made with a dual gradiometer (FM256 and FM36). Plot range -3nT to +3nT. Area 40m x 60m.


The results below compare the difference between raw data collected with a normal 1m traverse interval and data collected in quad density mode. Data were collected with parallel traverses in both cases at 4 samples per metre and with a trigger rate of 1s/m. The resultant traverse interval of the quad density data is 0.25m. The improvement in detail shown in the quad density survey is very striking. Even greater resolution is possible with the FM256 if sampling is increased from 4 to 16 samples per metre.

                  Quad density survey (0.25m x 0.25m)
                            Plot range -5nT to +5nT

Standard survey (0.25m x 1m)
      Plot range -5nT to +5nT







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