CSEM — Controlled-Source Electromagnetics

Overview

A powerful transmitter dipole injects a low-frequency current while receivers along a line record the diffused electric and magnetic fields. Resistive bodies such as hydrocarbon or salt distort the field in a way that is measurable tens of kilometres away.

How the physics works

The transmitted field diffuses through the conductive earth and decays exponentially over one skin depth. A thin resistive layer guides energy laterally, so the received amplitude at large offsets rises above the background half-space response. Normalising a survey line against a background model isolates that anomaly.

Where it is used

• Offshore hydrocarbon de-risking before drilling
• Salt and basalt imaging where seismic is blind
• Deep groundwater and aquifer resistivity mapping
• Gas hydrate and permafrost characterisation