Magnetics¶
Magnetic data measures spatial variation in Earth's magnetic field, which tracks the distribution of magnetic minerals below the surface. It is the one dataset in TerraEye that can see through cover at anything approaching target scale, and only where you have a fine enough survey. Where you do not, it sees regional architecture and nothing more.
So the first question is never what the anomalies look like. It is what your survey can actually resolve.
Cell size is not resolution¶
A grid's cell size is a display parameter. What an airborne magnetic survey can resolve is set by the flight-line spacing across strike and the sensor's height above ground, and the grid is conventionally posted at a quarter to a fifth of the line spacing whether or not that detail exists in the data.
Check line spacing and altitude, then compare those against the size of your candidate. A grid posted at 80 m over ground flown on 1.5 km lines does not resolve an 80 m feature, and nothing in the display will tell you that. The grid can look as smooth and detailed as you like — smoothness is not evidence of resolution.
Which grid do you have?¶
Global: EMAG2 v3. Arrives with every area. A compilation of satellite, airborne and marine surveys posted on a 2 arc-minute grid, about 3.7 km north-south and narrowing east-west toward the poles.
That posting is not its resolution. Where the compilation had airborne or marine coverage the grid carries something close to survey detail. Over the large areas that had neither, its content is the satellite-derived lithospheric field, which resolves nothing finer than a few hundred kilometers of wavelength. It is also a merge of surveys of very different age, altitude and quality, leveled and continued to a common surface.
Read it for regional architecture and whether your ground sits on a magnetic high or a magnetic low — a positive or negative magnetic anomaly, which has nothing to do with topography. It is context, not support for a specific target, and on ground where EMAG2 is the only magnetics you have, magnetics is not the dataset that rescues you from spectral blindness.
National surveys: the US, Canada and Australia. Used automatically where they exist. These are much better than the global grid, but they are compilations too, and heterogeneous ones. Modern coverage flown on 400 m lines is genuinely target-scale; legacy coverage inside the same grid, at the same posted cell size, can be 1.5 to 5 km line spacing. Find the survey index for your ground before you trust an anomaly.
Your own survey. This is the route to reliable target-scale magnetics anywhere else. Send us the magnetic anomaly grid from your airborne or drone survey as a GeoTIFF, and we will process it through the same transformations. Self-serve upload is not available yet.
Send the survey metadata with it, because the transformations depend on it:
- Required: the field's inclination and declination at the time of the survey, if known — or, failing that, the survey date, since RTP needs those values for the correct epoch and both drift over decades
- Required: confirmation that the main field has been removed — a grid with the core field still in it carries a regional trend that will dominate every derivative
- Required: confirmation that the data are leveled and diurnally corrected
- flight-line spacing and direction, and terrain clearance or flight altitude
What you can and cannot do with it in the app¶
You can visualize TMI and its transformations, switch the display to shaded relief, rotate the illumination, and read lineaments off the result by eye.
Automated magnetic lineament extraction is not available yet. Lineament interpretation is manual work in the current version.
What a magnetic edge can be¶
A magnetic edge tells you a magnetic property changes across a line. It does not tell you why, and the candidate causes are wider than geology.
Geological. A fault, an intrusive margin, a lithological contact, a dyke, or an alteration front where magnetite has been destroyed or created. Note that a magnetite-destructive halo need not sit exactly on the structure that fed it.
Cultural. A pipeline, railway, powerline, fence, buried casing, or mine infrastructure. These are usually the straightest and strongest lineaments in a grid, and they are the first thing to rule out.
Processing. Flight-line leveling stripes, tie-line residuals, gridding artifacts, and seams where two surveys of different vintage were merged. Every national and global compilation contains some of these.
Regolith. Maghemite in a lateritic profile patterns the near-surface strongly and independently of the bedrock beneath it.
Check the flight-line direction and the survey index against the trend of your lineament before you interpret it. Then write down which cause you settled on and what made you choose it.
The trap that does not announce itself¶
At low magnetic inclination RTP becomes numerically unstable and manufactures streaks that look exactly like lineaments. The literature puts the danger zone at roughly 15 to 20 degrees; TerraEye's processing uses a 10-degree threshold. Strong remanent magnetization breaks RTP too, at any latitude, but in a different way and with different symptoms. Both are covered on the magnetic transformations page, and in either case you should lean on Analytic Signal.