Helicopter EM
A transmitter and receiver were carried beneath a helicopter to map responses across the property.
THOR IN 3D · GEOPHYSICAL SIGNALS
EM Anomalies.Tracing conductive
rock at depth.
Four generations of electromagnetic investigation provide complementary views of Thor—from near-surface conductors to kilometre-scale resistivity architecture.
- Survey families
- Four
- Latest airborne survey
- 2022 MT
- Common depth range
- 1–2 km
- Measured contrast
- Conductivity
01 · SYSTEM VIEW
Signals that depart from the geological background.
An EM anomaly is a localized electromagnetic response produced by material whose electrical behavior differs from its surroundings. At Thor, apparent-conductivity axes are evaluated beside geology, drilling and interpreted three-dimensional bodies.
Integrated conductivity model
Frequency adds a depth dimension.
Low- and high-frequency apparent-conductivity axes are plotted with the interpreted geological framework. Their distribution helps prioritize where conductive rock may continue through the system.
02 · SURVEY RECORD
Four methods.
Four scales of evidence.
Thor’s EM history combines active and passive methods. Each survey responds to conductivity differently, so the datasets are most useful when compared rather than treated as interchangeable.
Fixed transmitters
Very-low-frequency surveying used distant submarine communications transmitters as the field source.
EM-37
Large transmitter loops installed on the ground produced time-domain responses for more detailed follow-up.
Airborne MT
Passive magnetotellurics measured natural low-frequency fields to model substantially deeper resistivity structure.
Airborne coverage
Built for rugged terrain.
Expert Geophysics conducted airborne magnetic and magnetotelluric work at Thor, allowing broad coverage where steep terrain complicates ground deployment.
Ground follow-up · EM-37
Responses through time.
The Z-component profile plots channels Z5 through Z21. The source page identifies anomalies A and B as responses associated with massive sulphide mineralization.
Passive MT does not require surface transmitter loops. Its lower frequencies can support deeper investigation—commonly around one to two kilometres in the Thor program context—while targeted ground methods can refine individual airborne responses.
03 · SIGNAL PATH
From electromagnetic field to subsurface model.
Active EM systems transmit a changing field and measure the response. Passive MT instead measures naturally varying electric and magnetic fields. Both approaches use the Earth’s response to infer contrasts in electrical properties.
The field
An active transmitter—or natural atmospheric and magnetospheric activity for MT—provides a time-varying electromagnetic field.
Induced current
Conductive material responds to the changing field. In active systems, eddy currents are induced within the conductor.
Secondary response
The induced current produces its own electromagnetic field, with timing, geometry and amplitude related to the source body.
Measurement
Receivers record the response. Processing and inversion separate signal from background and estimate subsurface properties.
Interpreted outputsConductivityDepthScaleGeometry
04 · EXPLORATION MEANING
A map of conductivity—not a map of ore.
EM anomalies identify electrically unusual rock. They can help locate sulphides and alteration, but conductive responses can also be produced by graphite, clays, groundwater and other geological materials.
3D spatial relationship
Anomalies around the deeper system.
Black points mark apparent-resistivity anomalies; green volumes represent interpreted intrusive rocks and yellow volumes show the North and South Tusks below 70 ohm-metres.
READING THE RESPONSE
Context turns signal into a target.
The exploration question is not simply whether a conductor exists, but how its depth, form and orientation relate to mapped structure, alteration, mineralization and the broader intrusive model.
- 01Follow continuity
Trace responses across lines, frequencies and adjacent survey types.
- 02Compare geology
Test whether the response follows host rocks, faults, alteration or intrusive contacts.
- 03Prioritize drilling
Use geometry and supporting evidence to rank the most coherent targets.