From Archaeological Methods
Magnetometry is the workhorse of archaeological geophysical prospection. Magnetometers are devices that measure the properties of the magnetic field near the surface. Magnetometry is especially effective in archaeology because human processes and modification of the landscape alters the magnetic properties of the soil which can be detected long after the presence of human activity is no longer visible on the surface . With magnetometry, large areas can be surveyed quickly in more detail than is possible with conventional surveying methods. Processing magnetometry data is relatively straightforward, making it a favorite among archaeologists.
The purpose of this manual is to get the user up and running quickly with the Bartington 601-2 Gradiometer. A gradiometer is a special type of magnetometer with multiple sensors: one closer to the ground to collect magnetic data about the surface, and the other 0.5 m to 1 m above the first sensor to collect information about the Earth's magnetic field. Subtracting one reading from the other essentially filters out the noise from the Earth's magnetic field, allowing subtle features of archaeological interest to be detected.
This manual is broken down into four sections: setting up the Bartington 601-2 gradiometer, conducting a survey, downloading data, and processing data.
Magnetometers assist archaeologists by detecting subtle changes in the magnetic properties of the near-surface. In order to acquire accurate results it is important for operators to practice good magnetic hygiene—ferrous metal must not be anywhere on the operator; this includes the metal from zippers, buttons, shoes, jewelry, hats, watches, keys, glasses, and any surgically implanted metal plates or screws. When using a magnetometer it is best to leave any accessories that are not absolutely essential at home or in the car. After assembling the gradiometer (discussed below) it is important to always check the operator by putting the instrument in scan mode and running it close to their body to make sure that they are metal free. If in doubt, pajamas make ideal magnetometry attire.
In addition to personal magnetic hygiene, it is also necessary to practice good magnetic hygiene on-site. The use of metal flags or nails in the course of excavation or survey can seriously impact the magnetic data. Plastic pin flags should be used to mark surface finds, and fiberglass rods should be used instead of rebar for marking site datums. Metal nails and spikes should be avoided at all cost.
Assembling the Gradiometer
The gradiometer breaks down into three pieces for easy storing and transporting in its case. It consists of a) the crossbar with data logger and batter pack that holds b) two 1m long cylindrical sensors at each end (Figure 1).
There is a port for connecting the sensors to the data logger closer to one end of the cylinders; this end marks the upper portion of the sensors; slide the cylinders into the crossbar such that the bottom of the sensors are approximately 10cm from the ground and tighten the screws to secure the sensors in place. The connectors on the sensors should be a few centimeters above the crossbar so that the bottom of the sensors are approximately 10 to 20 cm above the ground surface during operation (Figure 2).
There is a red arrow on the top of each sensor's data logger connection port; these arrows should be perpendicular to the crossbar facing ahead as the user walks. A backpack also comes with the gradiometer and acts as a counterweight to make it easier to keep the instrument steady while walking traverses. Once the gradiometer is assembled, the operator should put on the backpack and clip it to the rubber loops on the gradiometer.
With the gradiometer assembled and the sensors connected to the data logger, the kickstand should be deployed and the instrument turned on and left sitting while the survey grids are established. The instrument requires 15 minutes to warm up before use.
Setting the Parameters
Once the gradiometer is assembled and attached to the operator, the settings must be adjusted for the survey. It is important to always check the settings prior to starting a new project to make sure everything is set correctly. The following paragraphs explain the various parameters that can be adjusted from the Set parameters entry on the data logger screen; to get to the Set parameters menu, press the down arrow until the > precedes Set parameters, and then press Enter. The Set parameters menu has the following options: Pace, Gridsize, Start, Pattern, Lines/m, Samples/m, Range, Audio, Volume, Threshold, Sensors, Reject, and Save. Some, such as audio and volume, are self-explanatory and will not be discussed.
Pace is used to adjust the walking speed of the operator. The pace should be set to a comfortable walking speed that also allows the instrument to remain relatively stable. A good pace to start off at is 1.5 m/s. Rarely does the operator complete a traverse on their first try; depending on the user's height and gait, 1.5 m/s provides a good starting point from which to adjust the speed.
Gridsize determines the size of the survey area in meters. The Bartington 601-2 gradiometer is capable of 10m x 10m, 20m x 20m, and 30m x 30m gridsizes; the newer models are also capable of 40m x 40m grids . Using the Archeosurveyor program, many grids can be stitched together to create a large composite image of the magnetometry data.
Start refers to the starting traverse walking direction of the grid, not the corner from which you are starting. For example, select north as the Start parameter if beginning the first traverse in the southwest corner and walking towards the northwest corner. Because of the nature of the Archeosurveyor software, beginning a survey in the southwest corner with the Start parameter set to north is highly recommended.
Pattern refers to the traverse method the operator will use: ZigZag or Parallel. Using the example above with a start direction of north, the first traverse in the grid with the ZigZag pattern will be to the north, the second traverse will be towards the south, and so on. If using the Parallel pattern, after collecting one traverse worth of data the operator would have to walk back to the southern extent of the grid without collecting data before beginning the next traverse (Figure 3). ZigZag is the recommended pattern to use as it reduces surveying time by minimizing the amount of unproductive walking.
Lines/m sets the number of traverses the operator will walk per meter – 1, 2, or 4. Depending on the types of features the operator is attempting to detect, 1 or 2 Lines/m are recommended. If if using 4 Lines/m, data are extremely sensitive to variations in the operators pace. Also, for each increase in Lines/m the number of traverses doubles, increasing the time required to complete a survey.
Samples/m refers to the number of readings the gradiometer takes per meter along each traverse: 1, 2, 4, or 8. The Lines/m and Samples/m essentially make up the resolution of the data. For example, if Lines/m is set to 2 and Samples/m is set to 4, the resolution for each cell of the raster image is 0.5m x 0.25m. Archeosurveyor can interpolate to provide a higher resolution during the processing phase. Similar to Lines/m, increasing the number samples per meter can introduce artifacts in the data due to operator pace. The recommended setting for Samples/m is 4.
Range refers to the sensitivity of the instrument – 100 nanoteslas (nT) or 1000nT. For detecting anomalies of archaeological interest, it is essential that the Range be set to 100nT. The 100nT range provides a resolution of 0.01nT, which is very important for detecting prehistoric anomalies that generally range from -5nT to +5nT 
Threshold is used in scan mode to determine at what increment in nT the alarm bell of the gradiometer should increase or decrease in pitch to alert the operator to the changing strength of the magnetic field. Since this is limited to scan mode it can be left at 1nT.
Sensors tells the instrument how many sensors are are to be used in the survey. The Bartington 601-2 has two sensors, so this setting should be left at 2 the majority of the time. It is possible to only attach one of the sensors if the situation requires, such as in heavily vegetated areas where it is impractical to navigate with the gradiometer completely assembled.
Reject can be set to either 50Hz or 60Hz depending on the frequency of the electrical grid in the survey area. This feature helps to reduce the influence of the magnetic field generated from powerlines in the vicinity of the survey. In the United States, this should be set to 60Hz. Keep in mind that though this reduces the influence of the powerline's magnetic field, surveying in close proximity to large metal towers is not recommended.
Once all of the parameters have been adjusted, move the > to the Save line and press Enter.
Finding a Low Gradient Area
Once the parameters are set, the operator needs to calibrate the gradiometer to the field conditions. Ideally calibration occurs in an area with a low gradient all around the operator; in reality, however, this can be hard to accomplish. As long as the readings from the sensors do not change too much as the operator rotates about a point, the adjusting the gradiometer should complete successfully. If a poor location was chosen, the instrument will inform the operator at the end of the adjustment procedure.
To find a good location for adjusting the gradiometer, put the instrument into scan mode by moving the > to Scan and pressing enter. Depending on the strength of the magnetic field at the operator's current position, the instrument will begin beeping (slowly for low magnetic signatures, and fast for strong magnetic signatures). The operator should search for a location where the beeping stays relatively constant as the instrument and operator turn in a circle about a fixed point. When such a location is found, mark it with a non-ferrous pin flag and leave scan mode by pressing the escape button. Calibrating the Gradiometer
While still standing at the low gradient location, use the arrow keys on the data logger to navigate to >Adjust Gradiometer and press enter. The screen on the data logger will give the operator a series of instructions, beginning with face north and press Enter/Pb. The operator should follow the directions, pressing the green button to the left of the right hand grip (Figure 6). After facing in the appropriate direction and pressing the green button, the instrument will make some adjustments; until the instrument beeps and presents a new instruction, the operator should remain facing north while keeping the instrument steady. This process will be repeated with the operating facing south, east, and west. After the west instruction is completed, the instrument says to invert the sensors and press Enter/Pb. The operator should rotate the crossbar around so that the bottom sensors are pointing up, press Enter/Pb, and wait for the instrument to beep. Continue following the directions.
When the adjustment is complete the instrument will either inform the operator of success or failure; if setup did not succeed, search for a new location with scan mode and repeat the setup process.
Conducting the Survey
Establishing a Grid
Magnetometry data must be collected in a controlled manner so that anomalies can be precisely located; collecting data on a grid is the predominant method used. Grids can either be established manually or with a total station. Each has its benefits, depending on the amount of area to be surveyed.
Manual grid establishment is quick, easy, and ideal for smaller surveys or for conducting exploratory surveys. Establishing grids manually is the same as establishing a square excavation unit: it is based on geometry. 10m square grids can be established using 10m leg length measurements and 14.14m hypotenuse length measurements; 20m grids can be established using 20m leg lengths and 28.28m hypotenuse length measurements (Figure 4).
Total Stations can be used to optically establish large areas to be surveyed by “staking in” the boundaries of the survey area. With this method an established grid is necessary; the coordinates of the survey area boundaries must be known. A total station can then be used to find and mark those points. The interval for staking in points depends on the maximum tape measure available. The longer the tape measure, the fewer the number of points that must be staked in. a 100m tape works well in this situation, allowing hectare-sized grids to be established rapidly. The four corners, 100m apart, are shot in and the 100m tape stretched between them. Using the tape, plastic pin flags can be set at the appropriate intervals for the planned survey. Explaining how to use a total station to setup such a grid is beyond the scope of this manual.
Once the grids are established, tape measures should be laid at both ends of the grid perpendicular to the traverse direction to facilitate placement of the traverse line guide that the operator will walk. As many tapes have metal ends on them, it is standard practice to stretch the tape out 5 m past the beginning of the grid; thus the grid begins at 5m instead of at 0.
With the Gradiometer assembled and adjusted, and the survey grids established, it is time to start walking traverses. This involves stretching a rope or tape marked every 1 m across the grid. Beginning at one end of the grid, the operator walks along the rope or tape being sure that they are over the 1m marks at each beep of the gradiometer's metronome. The frequency of the metronome is automatically adjusted based on the pace set in the parameters screen of the gradiometer. After each traverse is walked successfully, the line is moved to the next traverse and the operator walks back.
To start the survey, navigate on the main menu so that the screen reads >Start survey and press Enter. The next available grid number will be displayed; press Enter again. The next screen tells the operator the traverse number and sample number along the traverse. To start the survey at traverse one, begin a few paces back from the edge of the grid and start walking at a comfortable pace. As you enter the grid, press the green Enter/Pb button. Each time the instrument beeps the operator should be over a meter mark on the traverse tape. At the end of the traverse the instrument will beep twice to let the operator know they should be at the edge of the grid. If satisfied with the pacing, move the traverse to the next increment (discussed below) and begin again.
If the gradiometer signals that it should be at the end of the traverse but the operator was off pace, the traverse will need to be redone. Press Esc, then navigate to >Back 1 trav entry and press Enter. The traverse number should decrease. After moving back one traverse be sure to press Esc before pressing Enter, otherwise you will delete two traverses! Press the Green Enter/Pb button to begin the traverse again.
Traverse Increment Tables
The Bartington 601-2 gradiometer has two sensors, allowing two lines of data to be collected at the same time. Though this does save time, it makes the traverse intervals slightly more complicated; Always be sure to have a traverse increment reference sheet available, and make sure that both 'line movers' are on the same measurement. Traverse increments and tape measure readings (assuming the grid starts at 5 m as discussed above) for several grid sizes are listed in the appendix.
On the back of the data logger is an RS232 (serial) connector. Plug one end of the serial cable into this port, and the other into the serial port on the computer. If the computer does not have a serial port, a serial-to-usb adapter can be used. Launch Archeosurveyor and in the Navigation bar, click New Site (Figure 5). In the pop up window, enter a name for the site and press OK. Enter any other relevant information in the Site Data window and press Save. With the site created, press Download in the Navigation bar. In the Download window, be sure Bartington Grad 601 is set as the Protocol, and then press next (Figure 6). The download wizard walks through downloading data. After everything is downloaded, close the download window and return to the Navigation bar.
With the data downloaded, grids must be assembled in the proper order. A composite image will be generated from the grid assembly so as to preserve the original data. In the Navigation Bar, press Assemble Grids. A window similar to that in Figure 7 will appear.
Assemble the grids by dragging them from the available grids area and placing each onto its proper square in the Assembly Area, The compass arrow in Figure 7 does not point to north; instead, it shows the direction the traverses were walked. If one starts the survey in the southwest corner and walks the first traverse heading north, the grid will be rotated clockwise 90 degrees so this must be taken into account during grid assembly. In this example, the first grid would go in the top right corner, and there would be a line of grids extending to the bottom right corner; the row to the left would then be filled in and so on. Good field notes and sketches are essential for accurately placing grids. When all of the desired grids are assembled, press Save As to save the grids as a single composite image. It is this composite image that will undergo processing (Figure 8).
Available composites for a site are displayed in the box at the bottom of the Navigation bar. Double-click on a composite listed there to open it.
Processing Data – the essentials . This manual will close with an explanation of how to add data to existing grids so as not to have to reapply processing techniques. All of the processing tools in Archeosurveyor are found on the left side of the screen (Figure 9).
Destriping removes the stripe-like patterns visible in Figure 11 due to variations in the height of the sensors as the operator walks the traverse. The strength of a magnetic field decreases rapidly with distance, so the striping effect can be especially pronounced in sites with anomalies at the sub-nanotesla level .
Despiking removes the dipole signatures of metal objects with very high readings that obscure the low readings. Ferrous metal objects create a dipole signature that can be equated with a bar magnet; in the magnetic data, there is a large black spot (the positive end) right next to a large white spot (the negative end) that looks like a large blob. The size of the combined white and black areas is much larger than the actual object itself, which is somewhere in between the positive and negative areas. However, it is not directly in the middle of the positive and negative because of the orientation of the magnetic field; the exact position depends on many factors including one's location on the earth.
Clipping data is a process that can be used to enhance only the areas that are of interest. For example, because of various historical artifacts in a survey, the data range in value from -300 to +300 nT. This large range of values makes it difficult to see the much lower anomalies that are often of interest at prehistoric sites. With the Clip tool, all values above and below a specified range or standard deviation will be capped at the specified value, allowing the fainter signatures to be seen.
Finally, data can be added to existing grid assemblies, carrying over any processing that they have undergone. There are several steps needed to accomplish this. First, after downloading data from the instrument, the previously surveyed grids need to be deleted. (>Clear Data from the main menu on the gradiometer). After collecting the data for the new grids, connect the gradiometer to the computer and download as before. When downloading, one of the download window screen will ask what number to start numbering the grids with; set this number to one higher than the highest grid number that has already been downloaded to the site in Archeosurveyor. After the grids have downloaded, Press the Open Grid Assembly button on the Navigation Bar, expand the Assembly Area and add the grids into their appropriate slots. After saving the new composite image, it will need to be opened by double clicking it from the Navigation Bar. It should appear with all previous processes still intact.
The following tables represent 1, 2 and 4 traverse increment measurements for 20m grids for use with the dual sensor Bartington 601-2 Gradiometer. The traverse number, center position, and tape measure readings are given in each table. Center position (m) refers to the raw tape measure reading that the operator should walk along during the traverse. Increment refers to the traverse spacing between measurements. This value alternates when conducting a survey with more than one traverse per meter because of the dual sensor configuration of the Bartington 601-2 Gradiometer; in the case of a survey with only one traverse per meter, the increment is always 2m. Tape Measure Reading (m) refers to the actual reading on the tape measure that the traverse marker will be positioned at. This is to account for metal in the reel and on the end of the tape measure. Standard practice is to stretch the tape measure out 5 m beyond the grid; so instead of a 20m grid ranging from 0m to 20m on a side, it will range from 5m to 25m.
20m Grid with 1 Traverse per Meter
|Traverse||center position (m)||Tape Measure (m)|
20m Grid with 2 Traverses per Meter
|Traverse||Center Position (m)||Increment (m)||Tape Measure Reading (m)|
20m Grid with 4 traverses per Meter
|Traverse||center position (m)||Increment (m)||Tape Measure (m)|
- ↑ 1.0 1.1 Kvamme, Kenneth L. 2006b Magnetometry: Nature's Gift to Archaeology. In Remote Sensing in Archaeology, edited by Jay K. Johnson, pp. 205-233. University of Alabama Press, Tuscaloosa.
- ↑ 2.0 2.1 Kvamme, Kenneth L. 2006a Data Processing and Presentation. In Remote Sensing in Archaeology, edited by Jay K. Johnson, pp. 235-250. University of Alabama Press, Tuscaloosa.
- ↑ DW Consulting 2009 ArcheoSurveyor User Manual. The Netherlands. Electronic Document, http://www.dwconsulting.nl/Download/ASv250x-Manual.pdf, Accessed November 12, 2009.