Geoscan FM36/256 magnetometer: methods for field use

 With a single fluxgate gradiometer and 30 m grids.

Check batteries have charged by switching on the machines (battery should read between 2.9 and 3.1 V if they have charged OK – this data is given on the display screen immediately when switched on).

Collect the following equipment: one magnetometer, one ‘twizzle stick’, 2 x 30 m base lines (yellow or blue) and a N-S line (green) with metre divisions marked with sticky tape, 2 non-magnetic pins for each N-S line, map of the site for marking grids on, sharp pencil, sharpener, rubber.

Select personnel: you need 3 people as a minimum

Roles can be rotated to give everyone a go at the magnetometer if they want to use it. But they MUST prove they can make the pace precisely. Always have the same person for a complete grid (don’t change halfway across). The recorder should record who is on the magnetometer for each grid (i.e. initial the grid with the walker’s initials in case of problems that need sorting).

On reaching the site set up the grid:

Set up the two 30 m base lines, as the E-W lines, on the measured 30 x 30 m grid (posts will be put into the 4 corners of the grid to mark them).

Place the first 30 m N-S line at 1 m in from the start point of the grid. Start point is always bottom left (i.e. the SW corner). The N-S line is then moved at 2 m intervals.

Switch machine on and check the people who are to use the magnetometer for magnetic clothing (walk up to the machine a few times and note if there is a change in values). The machine should not fluctuate as they approach. Only magnetic free people can use the machine, those on the lines can wear what they like, but stay 5 m back from the line when the magnetometer is working.

Balance the machine. A form of calibration that ensures accurate readings and must be repeated every 2 or 3 grids. To do this, switch machine on, and find a ‘quiet’ spot i.e. an area with no dramatically variable readings. Mark this point with a cane and spray, and always use this throughout the survey. Focus on this one spot to hold machine over.

Go to ‘Menu’ and the right arrow to change the reading value to 1 nT. Then ‘End M’ which is ‘end menu’.

Holding the machine always over this same spot look at the reading when facing the north direction of grid (determine N precisely with a compass and mark on the ground with spray or see what horizon feature to line up with).

Then check that the ‘upside down’ reading – in a north direction is the same as the north reading in the normal machine holding position. The mag ‘box’ must always point north.

If the upside down value is not the same as the ‘down’ value both pointing N, then use the twizzle stick in the side hole. When the same now turn to the S. If the reading is not the same (to within 1nT) then adjust using the N-S dial. Then do the same in the E-W direction, using the E-W dial to adjust, until all 4 compass points give the same reading. Then a final check to see that nothing has moved in the N-S and E-W directions.

If all balanced, then press ‘Menu’ then left arrow to change back to 0.1nT for the survey. Press ‘End M’.

Then, holding machine exactly in the direction of your first run (i.e. north) press ‘zero’ holding it down for a 4 sec period (4 beeps). Hold the mag. steady while the machine is doing this. It takes a little while to do it. It is now zero’d and ready to use.

Start the grid measurements:

Press ‘enable log’. You will be told by the machine which grid you are on. For instance, at the start of the day this will be the first grid, first line, first position thus the dial reads: G1 L1 P1.

Check the machine is the correct way round! Magnetometers should always face the same way within the grid. The start point is always the bottom left corner of the grid (SW corner).

Record start direction and direction of line increment on the map.

Press ‘start’ switch and keep a good steady machine, vertical probe, and a steady pace, no lolliping walks! The normal pace is about 1 metre per c. 0.8 secs, if your pace is kept correctly with the metre marks, then the machine comes to an end automatically as you cross the tape at the other end. Switch to off position.

NB. From the start position the mag. must be positioned correctly for first beep over the E-W tape and then kept a uniform 0.5 metres from the N-S line as you walk (hence hold it in your left hand) and green N-S line on your right side.

At the end of the line you turn round (but do not turn the machine!). Keep the meter facing in the same N direction, but you turn and position yourself, 0.5 m from line, mag in left hand, and press ‘start’ to do the return run other side of line.

For a partial line at the end of the line:

If the lines are cut short by obstacles such as hedges then you need to stop the machine manually (press ‘stop’ on a metre marker). The P number should be a multiple of 4 plus 1. If not delete to get to this nearest number of the intended marker. Then you need to tell it to ‘finish line’ ‘image line’ – NB only press VERY lightly as the result is a ‘double press’ and you have finished the next line too! Don’t worry if this does happen for the ‘finish line’, just press ‘delete line’ holding it down for a 4 sec period to remove the extra one.

However, if done incorrectly for the ‘image line’ this could scramble your whole grid which will need starting again from the beginning. Be warned. 

After a ‘finish line’command the machine automatically now knows that your next return line will be short (clever) so you need do nothing for the return line. (You will notice that the points are already in the right position with a higher number than the usual ‘P1’ for the start of that line.). Step forward to the marker in front of the marker you stopped at, press start, and switch off as you pass the line. The meter should read ‘P1’ for the next row number.

For a partial line at the beginning of the line.

You need to tell the mag. that you are x points short at the start of the line. See how many metres short you are, and knowing that there is one initial count on the line followed by 4 counts per metre you press ‘dummy points’ to the appropriate number. For instance, if you are just one metre short then 4 + 1 = 5 points. Three metres short is 12 + 1 = 13 points, and so on.

Partial grid – with missing whole lines:

When you come to the end of the last line in the incomplete grid simply press ‘finish line’ for the appropriate number of missing lines, until the grid is complete.