Summary of "Metode Elektromagnetik (geofisika) bagian 1"

Lecture overview

This is an introductory lecture on the electromagnetic (EM) method in applied geophysics (part 1). The presenter connects the EM method to the previously discussed geoelectric method and places it within the framework of Maxwell’s laws — especially Faraday’s law of electromagnetic induction.

Core idea: EM surveys intentionally generate changing electromagnetic fields in the near-surface (via a transmitter). Those primary fields induce currents and secondary magnetic fields in subsurface materials (conductors). Receivers measure the resulting electric and/or magnetic responses; these measured responses are used to infer subsurface physical properties.

Target physical properties

The EM method can be sensitive to:

Electrical conductivity (σ)
Magnetic permeability (μ)
Dielectric permittivity (ε)

These properties help distinguish rock types, fluids, and mineralization, although some conductivity ranges can be ambiguous without additional information.

Principal EM survey modes

Frequency-domain EM (FDEM) — uses continuous or swept frequencies; measures amplitude and phase.
Time-domain EM (TDEM) — uses transient pulses and records the transient decay response.

Ground-penetrating radar (GPR) is also an EM-based method but primarily senses dielectric permittivity. GPR behaves like a wave-reflection method (similar to seismic) where contrasts in permittivity create reflectors.

Historical / practical context

EM methods have been widely used in mineral exploration and engineering since about the 1960s. Field instruments record electric and/or magnetic fields depending on the method and survey objectives.

Demonstration / physical intuition

A simple demo with a coil and a moving magnet illustrates key physical laws:

Faraday’s law: changing magnetic flux through a coil induces an electromotive force (EMF).
Lenz’s law: the induced magnetic field opposes the change in flux.
The induced current can produce a measurable potential (e.g., lighting a lamp). Polarity of the induced potential depends on the direction of flux change.

Attenuation and penetration (skin-depth concepts)

EM amplitude decays approximately exponentially with depth.
Penetration depth depends mainly on conductivity (σ) and frequency (f):
- Lower frequency and lower conductivity → deeper penetration.
- Higher frequency and/or higher conductivity → shallower penetration.
Conductive layers limit vertical penetration — a practical consequence similar to resistivity/geoelectric surveys.

Practical ambiguity

Some conductivity ranges overlap between rock types (for example, certain sedimentary versus igneous or carbonate units), so EM interpretations must incorporate geological context, borehole data, or other geophysical methods to reduce ambiguity.

Typical survey workflow

Planning
- Select method (FDEM vs TDEM) and survey parameters (frequency range or pulse shape) based on target depth, expected conductivity, and resolution needs.
- Use lower frequencies or TDEM pulses for deeper targets; higher frequencies for higher resolution but shallower penetration.
Field setup
- Install and power the transmitter (TX) that generates the primary time-varying EM field.
- Deploy receiver(s) (RX) to measure secondary fields (magnetic and/or electric).
- Record receiver positions and metadata in a data logger/collector.
Data acquisition
- Transmit EM fields (continuous-wave or pulses). For FDEM, sweep frequencies as needed; for TDEM, transmit pulses and record the transient decay.
- Measure received signal amplitude, phase (FDEM), transient decay curve (TDEM), and/or electric field.
- Save raw data with location and timing information.
Data processing
- Convert measured signals to useful quantities (e.g., apparent conductivity/resistivity, apparent susceptibility).
- Correct for instrument response, survey geometry, and noise.
Interpretation
- Use forward and inverse modeling to estimate subsurface distributions of conductivity, permeability, and permittivity.
- Integrate complementary data (geology, boreholes, other geophysical surveys) to constrain models.

Practical rules-of-thumb

Use low frequencies or time-domain methods to increase depth of investigation when the subsurface is not highly conductive.
Expect exponential attenuation of signal with depth; design survey geometry and sensitivity accordingly.
Choose to measure magnetic field, electric field, or both depending on the target signature and available instrumentation.

Additional notes and cautions

The lecture emphasized the physical basis in Maxwell’s equations (particularly Faraday’s law) and Lenz’s law for the direction/polarity of induced fields and currents.
The transcript included numeric conductivity examples (e.g., contrasts among igneous rocks, sedimentary rocks, seawater, graphite), but some numeric statements in auto-generated subtitles were garbled. The reliable takeaway is the existence of typical conductivity ranges for different materials and that intermediate ranges can cause interpretational ambiguity.
A short demonstration video was used to visualize induction in a coil and how induced currents create secondary fields and observable potentials.
The lecturer referenced a prior lecture on the geoelectric method and an external GPR demonstration video (UI) as related material.

Speakers / sources mentioned

Course lecturer / presenter (primary speaker; greeting in Indonesian; host of the “Vika” lecture series).
Theoretical references: Maxwell’s laws and Faraday’s law.
Demonstration clip: coil and moving magnet illustrating electromagnetic induction.
Referenced materials: previous geoelectric lecture and a GPR demonstration video (University of Indonesia).