Backyard Magnetometry Survey, Using a Cell-Phone Magnetometer

(July 6th, 2013)

 

While reviewing the sensors in my (Droid Razr) cell phone, the presence of a magnetometer brought to mind the magnetometer surveys performed on the BBC TV series Time Team of various archaeological sites.  While it was unlikely that the magnetometer in a cell phone would be sufficiently sensitive, it seemed a simple enough project that I decided to give it a try.

 

I used a tape measure and bits of masking tape to mark 3-foot intervals along the back of my house, and stretched a piece of twine to act as a starting point for each survey transect.  I set my cell phone on a piece of cardboard and used self-adhesive strips of foam stuck to the cardboard to prevent it from sliding (essentially a shallow foam-rimmed box to lay the cell phone in).  I made a bend in the cardboard so that it would slide smoothly over the ground, and tied a piece of string to it so that I could pull it.

 

I installed a data-logging app called SensorLogger on the cell phone.  Setting the app on its fastest setting, I selected the option to record the AK8975 3-axis Magnetic field sensor, set the phone on the cardboard, and clicked Start.  Using a handheld compass to maintain a constant direction from start, I proceeded to walk a transect away from the house while dragging the cardboard behind me (at a fairly constant speed), and upon reaching the end of the transect (the far side of the yard), I clicked Stop to end the data logging.  I picked-up the cardboard slider and returned to the marker line and repeated this procedure 14 more times, altogether recording a total of 15 transects from my house to the back of the yard.  The datalogger stored each recorded transect as a separate file, and the order in which they were recorded was clear from the time the file was created.

 

I later opened the comma-delimited log files in Excel and combined them all into a single file with the existing column headings Second, X, Y, and Z, plus an added column M which took the square root of the sum of squares of each row’s X, Y, and Z to get the overall magnitude of the magnetic field in that row, plus a column E which was an integer indicating the transect offset from the initial transect, numbered 1 through 15.  I saved this file as a tab-delimited text file.

 

To visualize the results, I used the (free) R statistical software, plotting the local mean magnetic intensity with the lattice package using the following code:

 

library(lattice)

 

setwd("D:\\ClarkAndersen\\Magnetometer\\TrialPM")

dat=read.delim("Magnetometer.txt")

 

y=(10:( floor(max(dat$Second)*10)))/10

n=length(y)

x=rep(1:15,n)

z=rep(NA,n*15)

for(i in 1:15)

{

  for(j in 1:n)

  {

    z[(x==i)&(y==y[j])]=mean(dat$M[(dat$E==i)&(abs(dat$Second-y[j])<.05)])

  }

}

dat2=data.frame(x,y,z)

levelplot(z ~ x*y,data=dat2)

 

 

This yielded the following plot of the magnetic intensity in my backyard, bearing in mind that my start and stop times were a bit imprecise, and my walking speed was not quite constant:

Many features on this plot correspond to known features in my backyard, particularly large trees, a sandstone path, an old telephone switchbox, and the entirely buried foundation of an old fence which once ran from left to right about 2/3rds of the way up from the bottom.  I’ve marked these below:

The low intensity (purple) region at the top seems to be associated with the galvanized steel fence at that side of the yard.

 

Overall, it is clear that the cell phone’s magnetometer is indeed sufficiently sensitive to detect buried structures.  This may be helpful in amateur archaeology projects.