Interpreting geophysics results is like sorting wheat from chaff, and you have to be able to recognise a lot of different chaff, otherwise you end up digging it up. A while ago, I had the dubious fortune of surveying an area that contained pretty much all the types of chaff you are likely to come across in a magnetometry survey. The site is in a valley that runs roughly north-south down the centre of the image.
A is the actual archaeology we are looking for. It's a small Roman period iron bloomery that was excavated by Henry Cleere in the 1960's*.
B is a water pipe. An off-shoot of this leads to a water trough. These are easily recognised by their stripey character and high readings.
C is an electricity cable, leading to the house at the northern end.
Covering the survey are are land drains, such as at D. They are generally ceramic pipe, and help drain the land of excess water. Modern land drains may be plastic and less easy to spot.
Just to the right of E is a small feature, half black and half white. This is known as a dipole, and is generally indicative of modern metal junk. You tend to get these more on arable land as bits break off tractors as they work.
F is a plastic pipe dug into the centre of the valley to take the small stream underground. Whilst the magnetometer does not pick up the plastic, it does pick up the cut into the underlying geology in which the pipe sits.
Just in the corner at G, we surveyed close to the metal fence. Fences tend to show up as negative readings.
The two blobs at H are actually down to the landowners landrover pulling up as I neared the end of the survey line. Be sure to make a note of such things as you survey, otherwise you might mistake them for features
*Cleere, H. The Romano-British industrial site of Bardown, Wadhurst. Sussex Archaeological Society occasional papers no. 1