Physics

Purpose

Demonstrate the fundamental physical principles governing the airborne TDEM experiment.

Most of the physics related to airborne TDEM is available in Maxwell I: Fundamentals and Maxwell IV: TDEM. Because an airborne TDEM system uses small horizontal loops as the transmitter and receiver, geophysicists have developed some representative physical models. Readers are supposed to be familiar with the following concepts.

• Sphere excited by a transient magnetic dipole

• Transient magnetic dipole in a whole space

• Transient magnetic dipole above a half space

The basic physics of airborne TDEM is illustrated by the animation below. Click through the radio buttons to see how an airborne TDEM system senses a buried conductive object.

Airborne TDEM survey.

A loop is towed by the helicopter.

A current is applied to the loop, generating a magnetic field.

The transmitter current is shut off, changing the primary magnetic fields. The change in flux through the target induces electric currents.

A secondary magnetic field results from the induced currents.

Two typical earth models are of particular interest in airborne EM: the 1D layered earth model and the sphere-in-halfspace model. Here we illustrate the basic physics involved in airborne TDEM with a small horizontal loop source (1 m radius) over a three-layer earth model. The receiver makes point measurement of the magnetic field. Both loops are at the same altitude 20 metres above the surface and are separated by 10 metres. Click through the radio buttons to explore the problem.

Airborne TDEM survey over a layered 1D Earth.

The 1D conductivity earth model. Top layer: 0.1 S/m and 20 m thick; Second layer: 1 S/m and 20 m thick; Basement: 0.1 S/m. The source and receiver are marked by the black and red dots respectively.

The transient vertical magnetic dipole source (black dot) induces electrical current (showing amplitude at 0.01 ms after source current turn-off) below the surface. Note the geometric fall-off of the current intensity from the source location to large depths/distances, and the enhanced current in the conductive layer.

The transient vertical magnetic dipole source (black dot) induces electrical current (showing amplitude at 1 ms after source current turn-off) below the surface. Note the stronger concentration of current in the conductive layer and the disappearance of current near the surface, as the induction diffusses into greater depth at a later time.

The magnetic field everywhere in the whole-space generated by the induced current in the earth at 0.01 ms after source current turn-off.

The receiver (red dot) measures the magnetic field (showing the vertical component) at multiple time channels. Because the primary field is turned off, the received magnetic field is always secondary. The responses from a uniform haf-space of 0.1 S/m is plotted for reference. The higher conductivity in the second layer is manifested as a bump on the decay curve.