This post describes my field method for magnetic survey in large open areas. I can’t recall when I first conceived of this, but the method was first implemented formally at during my survey of Zincirli Höyük in Turkey, a 40+ hectare site of which I surveyed 20 hectares in about 7 weeks’ time in the summer of 2007 and an additional 5 hectares in 2009. My objective was to find a balance between the amount of time spent using the total station to lay out the survey grid and the amount of locational error that comes from grid extrapolation through measuring simply with meter tapes and compass. A third benefit of this method is that the amount of energy used to lay out transect ropes to guide data collection is minimized.
The result is a rapid survey with the dual magnetometer system by which a team of three would regularly be able to collect 20 or so grid squares per work day (even during the heat wave in the summer of 2007 in Turkey!). My personal record was 28 completed grid squares in a single work day. The following assumes some familiarity with the general terms used in geophysical survey.
For this survey you will need some basic survey equipment that could cost anywhere between $150 and $700 depending on the quality of the materials you choose to buy (of course, not including the magnetometer itself and the GPS or Total Station!)
- Dual gradiometer (eg Grad 601-2)
- A digital measuring device for establishing points with known coordinates and measurements (eg GPS or Total Station or dumpy level, if that is what you have)
- Three (3) 100m fiberglass measuring tapes (anywhere from $15 to $70 each depending on brand and quality)
- Thirty (30) lengths of rope of 22meters or more in length that are clearly marked at 1 meter intervals
- Spray paint or tape for marking the ‘rope’
- Ample wooden or plastic stakes
- Hammer or mallet for pounding stakes in
- Three (3) people ideally
- one ‘data collector’ who operates the gradiometer and
- two ‘rope pullers’ to manage the grids and ropes.
This method is based on my observation that long measuring tapes are quite unwieldly when stretched over 60 m. I have found it quite difficult to keep the line straight, taut and relatively unaffected by any wind when they are rolled out more than 60 meters. I also assume that you are collecting data in 20 x 20 meter squares with 0.5 meter transect spacing.
So, to begin:
- Lay out a sparse grid with the dumpy level, total station or the GPS. This sparse grid could look one of two ways, depending on your personal preferences and constraints. Remember, the point is to minimize your effort here.
- A 60 x 60 meter grid over the entire site
- A 60 x 20 meter grid over the entire site.
- If you have set up a grid that is 60 x 20 with the TS or GPS then you are ready to move to step 3. If you have a 60 x 60 meter grid then you will need to use your measuring tapes to lay in the 20 meter stakes in one direction only before you begin your survey. The direction in which you lay the 20 meter grid is the also the direction of the survey transects.
- Establish baselines with your three 100 meter survey tapes by stretching them 60 meters between the stakes you set up in step one. Remember that your survey tapes most likely have ferrous (read: magnetic) metal parts on the end and in the reel, so you will want to tie the beginning off at the one meter mark and move the end away from the survey grid. This means that the other end of your tape will cross the stake at the 61 meter mark. You will want to pull the tape out another two meters or so and lay it well outside of the immediate survey area so that you are not mapping the metal in the reel!
- Once your baselines are taut, begin to lay out all 30 of your survey ropes at two meter intervals beginning one meter from the stake. If this is your first time carrying out a dual gradiometer survey then you will want to refer to page 36 of the Bartington Grad 601-2 manual. Make sure that the ropes cross the baselines at a meter mark. I don’t use stakes to hold down the end of the ropes anymore, I instead depend on there being low ground cover and heavy and straight enough ropes to guide me as I walk.
- You should now have three 20 x 20 meter grid squares ready to survey! The data collector can get going as soon as most of the lines are down, maybe leaving the last 5 or so for the rope-pullers to finish as the gradiometry survey begins.
- When the first 20 meters are completed, the rope pullers can start to move the ropes on to the next survey grid. All they have to do is drag the ropes in a straight line up to the next baseline, then go back through and make sure the meter marks match the baselines. You can sometimes get it pretty close without double checking if the ground cover is forgiving. The rope pullers will almost certainly have the next three survey squares set up before the data collector has completed the first three. It is good practice to leave at least one transect ‘behind’ the data collector, in the case that he or she needs to redo a line, but also I find it can be a little disconcerting to not have that extra little bit of reference information as you are collecting data.
- Once the data collector has moved on to the second row of grid squares, the rope pullers can detach the 100 meter tape from the first baseline. The first baseline can then jump up to the next row of stakes in the survey direction and the rope pullers can begin to move transect ropes as soon as they are ready. I find that a stretch of three survey squares takes about 45 minutes at a modest data collection speed. It’s a good time to take a drink of water and a short rest after finishing three in a row.
If all of the stakes are already laid out you should be rolling! You can probably get 12 squares in before lunch!
A word on rope maintenance:
It will simply save you money and time if you take some small precautions to keep your measuring tapes and ropes in good shape.
- Fiberglass measuring tapes are prone to break after being repeatedly wrapped around wooden or other stakes. To extend the life of the measuring tape, I recommend that, if possible, you reinforce the measuring tape at the 1 and 61 meter mark with some heavy duty but flexible tape. You could re-mark the important meter marks with a sharpie.
- Use the ‘over under’ method to wrap up the survey ropes at the end of the day. AV guys are serious about this and there are a number of instructional videos to be found on the internet to show the the best way to wind up these ropes so that you don’t have to wrestle 30 twisted and tangled ropes the next morning.
 The type of survey rope you use will affect the results of your survey, especially if it is a large area and you use your ropes for a long time. If you are lucky enough to have the budget for fiberglass or pvc surveyors rope, then you will not have a noticeable problem with ropes stretching over the course of the survey. The best possible option is heavy fiberglass or pvc survey ropes such as the Keson Surveyors Rope which I also see listed as a Depth Gauge. These do not stretch but can be quite expensive. If you must use regular rope from the hardware store, I recommend rope that is at least 7mm thick. You also want to select rope that is not prone to excessive stretching.
. If you are using braided rope from the hardware store, then you will surely notice that your ropes will stretch -likely at different rates- over the course of your field season. It will be apparent when the meter marks on the survey ropes do not meet both baselines. This will produce some staggering in your data. If you are stuck with stretched ropes you can minimize the amount of error in your data by making sure that the survey ropes are always lined up on the same baseline (I usually use the starting baseline).