If you run exploration for a mining company, the decision you don’t want to get wrong is this: which anomaly on the processed magnetometer grid warrants a drill. The drill cost in the Indian iron belt runs between ₹8,000–18,000 per metre depending on geology and depth. A 150-metre exploratory hole is ₹12–27 lakh, plus mobilisation, plus downtime. Drilling the wrong anomaly is not a survey problem; it is a budget problem.
This post is a working guide — not a textbook — to how we read airborne magnetometer grids at Flybi before we recommend targets to a client.
The four things we do before the map becomes useful
A raw magnetometer flight is not a decision-ready product. Four processing steps must be done well, or every anomaly downstream is suspect.
1. Diurnal correction. Earth’s magnetic field fluctuates throughout the day — sometimes by tens of nanoteslas across a morning. Without a base station logging the diurnal variation at the site while the drone flies, you cannot separate field variation from geological signal. A survey flown without a base station, or with a base station located too far from the flight area, yields a grid with drift masking the real anomalies.
2. IGRF reduction. The International Geomagnetic Reference Field is the calculated baseline field for your latitude, longitude, altitude, and date. Subtracting IGRF from the corrected readings gives you the residual — the anomaly grid. Skipping this step, or using a stale IGRF epoch, biases every interpretation.
3. Levelling. Flight-line-to-flight-line noise creates corduroy artefacts on the grid. Proper levelling — matching tie lines to survey lines — removes these. An unlevelled grid will often show “anomalies” that are actually flight-line noise, which can look terribly convincing to a non-specialist.
4. Reduction to pole (RTP). In low-latitude areas (most of India and Africa), magnetic anomalies appear offset from the causative body because the field vector isn’t vertical. RTP mathematically relocates the anomaly over its source. Without RTP, your drill target is shifted sideways — sometimes by tens of metres.
If the team delivering your survey skips or short-changes any of the above, the anomaly map is noise with a professional-looking legend. Ask for the processing log.
The three kinds of anomaly you’ll see
Once the grid is clean, roughly every target on an exploration block falls into one of three categories.
High-amplitude, tightly bounded, roughly circular. These are the anomalies most likely to be worth drilling — they suggest a discrete magnetic source at a relatively shallow depth. In the Indian iron belt, they often correlate with banded iron formations or lamprophyre dykes. A good shape rule of thumb: if the anomaly is sharper than the regional field and sits on its own, it’s worth a second look.
High-amplitude, elongated, linear. These usually indicate a geological structure — a fault, a dyke swarm, a sheared contact — rather than a discrete body. Valuable for structural mapping, but rarely drill-first targets. Treat them as pathfinders for the adjacent circular anomalies.
Low-amplitude, broad, regional. These are background variations in basement depth or regional lithology. Almost never a drill target; often the canvas on which real anomalies sit.
The highest-value targets we flag to clients are the tightly bounded circular anomalies that sit off-axis from the regional linear trends. Those are statistically the cleanest signal.
The metrics that matter before you drill
When we hand over an anomaly map, we include four numbers per high-priority target:
- Amplitude (nT) — rough proxy for the mass and susceptibility of the source; higher is more discrete.
- Half-width (m) — diameter of the anomaly; small half-width on high amplitude suggests a shallow, discrete body.
- Estimated depth to source — depth of the top of the body, calculated from half-width and amplitude.
- Confidence score (low / medium / high) — our interpretation of how certain we are the target is drill-worthy.
The combination of those four lets the exploration lead rank targets for drill budget allocation. A high-amplitude, narrow-half-width, shallow, high-confidence anomaly goes at the top of the priority list. A low-amplitude, wide, deep, low-confidence one goes at the bottom, or is excluded altogether.
What we’ve learned flying the Indian iron belt
Across Karnataka, Odisha, and Chhattisgarh, the signature pattern we consistently see is this: the valuable targets cluster along the margins of large regional gradients, not at their centre. The centre of a high-amplitude regional feature is typically the top of a thick package of magnetic rock — excellent for geological interpretation, but often not economically drillable because grade dilution at depth is high.
The margins — where a steep gradient meets a quieter field — are where we find the tight, discrete anomalies that are more likely to be ore-grade structures. This is counter-intuitive for teams new to airborne magnetic data, who instinctively want to drill the brightest spot on the map.
What to ask for in a handover
If you’re commissioning an airborne magnetometer survey, ask for these five things at handover:
- The processing log (confirming all four pre-processing steps).
- The residual grid at two scales — block scale and target scale.
- A target table with amplitude, half-width, depth, and confidence for every prioritised anomaly.
- A short interpretation narrative — not just a grid and target markers.
- The raw data archive — so you can bring in a second interpreter later if you wish.
If the vendor delivers only the grid and the target markers, you’ve paid for a fly-by. The interpretation is what makes the survey a decision-ready document.
When to call us
If you’re scoping an exploration block, an EIA baseline, or a subsurface integrity assessment where magnetic signature carries weight — the form on our magnetometer page routes to a geophysics lead on our team, not a sales inbox. Scoping calls are 20 minutes, no proposal delay.
