Downhole survey discipline for Australian exploration: five habits that protect your resource model
13 May 2026
A drill hole without an accurate survey is a data point without coordinates. At 300 metres depth, a 2-degree deviation in azimuth can place the bottom of hole more than 10 metres from where the model assumes it sits. Multiply that across a 200-hole program and the interpreted geometry of an orebody shifts in ways that affect every resource block estimate and mine design built on that data.
The cost is not abstract. A resource model built on inaccurate hole geometries produces block estimates that misrepresent grade distribution, ore widths, and structural controls. The errors compound silently through the estimation process, and by the time they surface, typically at feasibility study or during early mining reconciliation, the cost of correction is measured in months of rework and, in serious cases, a downgrade of the resource confidence classification.
Downhole survey discipline is one of the most underrated controls in an exploration program. These five habits keep the geometry honest.
1. Survey inside the collar
The first survey point sets the reference for every reading that follows. A collar survey taken after the hole has already deviated introduces an uncorrectable baseline error. Measure azimuth and inclination at the collar, confirm against the planned orientation, and record the measurement before the hole goes deeper.
This sounds elementary, and it is. The reason it appears on this list is that it is skipped often enough to matter. Rig pressures, shift changes, and the assumption that "we will pick it up at the first downhole survey" are the usual causes. The first downhole survey at 30 or 50 metres cannot correct for an unknown collar orientation.
2. Set a maximum survey interval and do not exceed it
Industry standard for most exploration programs is 30 metres between downhole survey readings. Longer intervals between readings increase the uncertainty in the interpolated hole path between measurement points. A hole surveyed every 50 metres instead of every 30 carries significantly wider positional uncertainty at depth, and that uncertainty propagates directly into the resource model.
The argument for wider spacing is always cost or time: fewer surveys per hole, fewer delays at the rig. The counter-argument is that the cost of a survey reading is negligible compared to the cost of a hole whose geometry is too uncertain to use at estimation stage.
3. Use a gyroscopic tool in magnetically compromised ground
Single-shot magnetic tools are standard in most geological environments, but they fail in ground with significant magnetic mineralisation. Banded iron formations, magnetite-bearing ultramafics, and some mafic intrusive sequences produce enough magnetic interference to render compass-based readings unreliable. If the project geology includes magnetic lithologies, specify a gyroscopic survey tool from the start.
Retrofitting a program with gyro data after the fact is expensive and sometimes impossible. Holes drilled with unreliable magnetic surveys in magnetic ground may need to be re-surveyed with a gyro tool (if still accessible) or excluded from the resource model entirely. The cost of specifying gyro from the outset is a fraction of the cost of dealing with the problem after 50 holes have been drilled.
4. Compare survey data against planned hole traces on the same day
A survey that deviates significantly from the planned trace is either a real deviation that needs to be recorded and managed, or an instrument error that needs to be identified before the hole goes deeper. Either way, the comparison must happen the same day the data is collected.
A deviation discovered after the hole is completed and the rig has moved cannot be corrected. A deviation discovered on the same day can be investigated: was it a tool error, a geological deflection, or a driller-induced deviation? The answer determines whether to re-survey, adjust the planned trace for the remaining hole, or flag the result for the database manager. None of those responses is available if the comparison waits until the end of the program.
5. Record the survey tool type, serial number, and calibration status for every hole
Auditors reviewing the data for a JORC-compliant resource report will check whether the survey equipment was fit for purpose and properly calibrated. A hole surveyed with an uncalibrated tool is a hole whose geometry is unverifiable. The record takes 30 seconds to complete. The cost of failing the audit is weeks of rework.
This is the kind of documentation discipline that separates programs that pass Table 1 scrutiny from those that do not. The JORC Code 2012 explicitly requires disclosure of "the type, results and accuracy of any surveys or instantaneous measurements." A complete survey record is not optional for a compliant resource.
The bottom line
The drill hole tells you what is there. The survey tells you where it is. Lose the second and the first is useless.
At @Norwest Exploration and Mining Services, we specialise in rapid deployment of experienced geologists and field technicians who understand that data discipline is not separate from technical competence. Whether you need one person or a full field crew, we place the right people fast.
Looking to build a field team that gets the fundamentals right? Contact Norwest directly or visit our profile.