Annoying Geology 2

Look at the magnetometer image on the left. There is some pretty obvious archaeology there, a couple of tracks and a small enclosure, but what are those large blobs? They must be archaeology too right? I certainly thought so, I even flagged them as archaeology in the report. Similarly with the image on the right, showing a track next to these features. Actually, this is more annoying geology. This time, going under the name of Gley. Gley is often formed geologically adjacent to streams or other water courses. The example to the left is next to a stream, and the right next to the floodplain of a river. You don't want to dig that stuff up, very sticky.

Which way do I walk?

When doing a magnetometer survey, questions arise of which way should you be walking across the grid, and which corner do you start in? Here are some things to consider when making that choice. They may well conflict with each other, so it is up to you to decide how much weight to give each of them.

• If there is a slope in your field, you will want to walk along it rather than up and down the hill. This not only saves your legs, but will help to avoid stagger in the results.
• A funny shaped field can affect your choice, and the correct choice will mean less messing about with dummy readings. For example, if you have a rectangular field with many partial grids along one edge, then you will want to walk parallel to that long edge and start in the corner away from it. That way, you will only need to press 'end grid'. Another example, this time with a field which has a straight edge, and opposite to it a very curving edge. Again you will want to start in a corner away from the curving edge, but instead of walking parallel to it, which would result in problems when you hit the edge, walk towards it.  Magnetometers have a feature where you can finish the line and automatically enter dummy readings to the same point on the next line. Starting opposite the curved edge avoids you entering dummy readings at the start of a line. Counting marks on the string to start a line sucks.
• If you are surveying a field, its present condition may have an effect on which way you walk. It will be easier on the legs to walk along lines of ploughing or stubble than across them. 
• The archaeology itself can have an impact on which way you walk. If, for example, you are expecting a Roman road crossing your survey area, you will want to walk perpendicular to it for two reasons. Firstly, you will have a greater resolution of readings across the feature, and are more likely to pick it up. Secondly, use of the destripe filter may hide features that are parallel to the way you walk.
• Converselywise, if you have a known utility pipe across your survey area, walking parallel to it will help stop the extreme readings from adversely affecting the destripe filter across the rest of the grid.
• The hardware itself may play a part. For example, with the Bartington GRAD601-2, the data collection procedure expects the left sensor to always be on the edge of the grid with the first line that you survey, so picking a direction to walk will restrict you to two out of four corners to start in.
• Hardware again. Most machines in use today are fluxgate gradiometers, and produce striping in the results. With the Bartington at least, the balancing procedure results in less striping heading north-south than east-west. Filters can deal with striping though, so you only really need to take this into account if you have a flat square field and no idea about the archaeology underneath. 

Earth Resistance Replacement?

I've been pondering the nature of earth resistance in archaeology, mostly because it is so darn slow. There has to be a quicker way of getting this sort of data. I'm sure you all know the standard way of doing things, with a twin-probe array. You also know that the main thing it reacts to is a change in the moisture in the ground. There are other ways that you can measure ground moisture, however, without sticking metal probes in it. I must admit that I know little about these methods, and hence I am looking for comments on their possible effectiveness.

Electrical Conductivity

Conductivity is already established as an archaeological tool, albeit a little used one (at least in the UK). It works by inducing a magnetic field, similar in a way to a metal detector, though not quite. As you can see (with the Geonics instruments), it has a similar collection method to magnetometers, which is good. Unfortunately, the effectiveness compared to traditional resistivity has been questioned, with the results apparently being a lot less clear.

Thermal Imaging

As everyone knows, water is very good at holding heat, but can this be exploited for archaeological purposes? Thermal imaging cameras are commercially available, for example, from FLIR. They can be cheap to astoundingly expensive, depending on the sensitivity and resolution of the instrument. The question is, does any temperature difference under the surface have a measurable effect on the surface, where the camera is looking? If so, it would mean a very quick means of collection, perhaps with the aid of a small tethered balloon to dangle the camera from, or a UAV, assuming it can carry the weight. Most likely though, it wont be as effective in getting the depth needed.

Microwave Radiometry

Jumping past the other end of the visible spectrum, there is another means of measuring ground moisture, with a microwave radiometer. I was first alerted to the possibilities by this article. Their general use is for meteorology, and deployed in satellites, but you can buy smaller versions. Is it possible to point one of these at the ground and measure the moisture? What would be the depth penetration? How would it be affected by plants? Again it may suffer from the same limitations as thermal imaging.

If anyone knows more about these the science involved, then please post a comment. It's all hardware to me, and I'm a software guy.

Overlaying Geophysics Results on Google Earth

I've been asked before (Hi Lisa) how I overlay geophysics results on Google Earth. My method requires the use of a total station, which though not cheap, is considerably less expensive than an RTK GNSS setup. You can pick up a decent 2 person setup (i.e. non-robotic) on ebay for about £3000.

So as an example, let's take this field :

Step 1) Set up your total station in a position where you can see all of the features you wish to record (see below). This may be difficult on undulating terrain, so think before you set up.

Step 2) Optionally, you can use the total station to lay out any grids rather than using tapes, which is more accurate over a large area, especially if it is hilly. You need to use an arbitrary grid for this, so set your x,y coordinates to be, for example, 500,500, and set the horizontal angle to a suitable value whilst pointing the total station in the direction of the grids. On my total station at least (Topcon), 0 corresponds to grids north, 90 is grid east, 180 is grid south and 270 is grid west. Once this is done, you can send out your staff bearer with a walky-talky and give them directions to go so many metres/centimetres (grid) north, south, east or west until they hit the right spot to mark a corner of a survey grid.

Step 3) If you have not set yourself up in an arbitrary grid, do so now. You will need to record three things :

Step 3a) Record the corners of the survey area, which is only 4 if you have a square or rectangular survey.

Step 3b) Record two resection points. These are points which you can accurately describe, on features that are unlikely to move any time soon, for example, telegraph poles or gateposts. An example description would be 'Centre of south face of western gatepost of gate north edge of field'. This is a point which someone should be able to find to within a few centimetres. When you come back to the field and want to re-establish the arbitrary grid, you enter the coordinates you have recorded for the resection points, and re-survey them. The total station can then work out where it is in relation to the original grid, setting its own position accordingly.

Step 3c) Record several features that you can see on Google Earth. The edge of the field is good. Several point recorded along a fence will give you a nice line to match up with the aerial photograph. You will need to record more points if the fence turns or curves. Recording where the orientation changes is a good idea. Trees at the edge of a field can obscure the fence on the aerials, you will generally need at least two sides of the field to have a visible fence for this process to work well.

Step 4) Do your geophysics survey

Step 5) Download the data from the total station and export as a dxf file. Load this into a CAD program. If you don't have AutoCAD and don't want to spend a lot of money, something like QCad is a cheap option. Join the dots of the survey area, and any features like the edge of the field. If your survey area is less than 100x100 metres, you will need to add a two dimensional scale of at least that size, anywhere you wish on the image. You should end up with something like this :

Step 6) Save the CAD image in a lossless raster file format that supports transparency, I tend to use PNG. White lines tend to show up on Google Earth better than black lines, so make the background black. Load this image into a decent paint program, and make the black background transparent. The method of doing so will differ between paint programs, so you will need to consult the documentation.

Step 7) Add the resulting image to Google Earth as an image overlay. Use the measure tool on the two dimensional scale, or your survey area, to get the overlay to the correct scale. Rotate and shift the image so that the features you have surveyed, such as the field edge, match up with the aerial photo. Remember that the lines drawn on are a guide. It's the points that really matter. If there is a curve in the field boundary, and you have not taken enough points, then the lines between the points will not exactly match up to the aerial. You should end up with something like this. Note how the white lines match up to the features surveyed :

Step 8) There will now be a nice box into which you can drop your geophysics image. Don't forget that if there are any dummy readings or missing grids in your survey, then you will need to make that area transparent in the same way as the grid image. If you are using a standard grey scale for your geophysics plot, it is best not to make your dummy readings black or white, otherwise that may interfere with the transparency. Make dummy readings a completely different colour, such as red. And finally you are done. As you can see, the field has been completely trashed during the building of the adjacent reservoir, which is a shame.

A Note on Google Earth Imaging

You may be thinking, why not just take four readings of your survey area with an RTK GNSS system and save yourself all the hassle. To an extent, you would be right. Apart from the cost, there is a problem with the Google Earth imagery itself, in that it is usually not exactly placed. It may be only a couple of metres out, but that may be enough to confuse things, meaning that your survey will not be correct in relation to the image.

Converselywise, if you think you can get decent geographic coordinates from your total station measurements using Google Earth, you can expect a similar error going the other way.

If you have an RTK setup, don't forget that they generally record in a coordinate system that is tied to a continent, for example ETRS89, whereas Google Earth works with the world average, WGS84. The difference may only be half a metre, but it increases every year.