Electromagnetic methods

Question 1

What is the goal with electromagnetic surveying?

Answer 1

The same as with resistivity methods. To determinate resistivity as a function of depth

Question 2

What is the electromagnetic field made up of?

Answer 2

The electric and the magnetic field. They are perpendicular to each other. They both propograte like waves.

Question 3

What does Ampere’s law say?

Answer 3

A magnetic field is generated by an electric current or time varying electric field

Question 4

What does Faraday’s law of induction say?

Answer 4

The law of induction means that an electric voltage is generated (induced) in an electrical conductor, if a magnetic field in its vicinity varies with time.

Question 5

What does Lenz’s law say?

Answer 5

An electric current arising from electromagnetic induction has such a direction that it seeks to counteract the cause of its origin

Question 6

Describe the principle of electromagnetic measurements?

Answer 6

A transmitter, a coil with manu turns of wire, generates a time-varying electromagnetic field by putting electrical current into the coil. Called primary magnetic field (PMF). It propagates above and below the ground.

The changes of flux (magnetflöde) of the primary magnetic field induces electric current in the conductor. This induced current produces its own alternating magnetic field, the secondary magnetic Field.

The secondary magnetic field induces electric currents into a reciever. Properties of the secondary magnetic field provides information about the conductor. 3-component measurements fully describe the orientation of the secondary magnetic field.

Question 7

What happens if you increase the number of turns on the coil or size of current in the transmitter loop?

Answer 7

It increases the strength of the magnetic field.

It increases the strength of the induced currents in the conductor.

Question 8

Electromagnetic measurements can be done in two ways, which?

Answer 8

Frequency domain measurements - Current in transmitter is a continuous sinusoidal. Produces sinusoidally varying currents in conductor.

Time domain measurements - Current in transmitter is a steady current abruptly turned on and off in intervals. Produces current pulses circulating in the conductor for a short period and quickly decaying as they lose energy.

Question 9

What is meany by off-time and on-time measurements?

Answer 9

When doing time domain measurements you can do off-time which means that measurements are taken after the pulse is turned off, when there is no primary magnetic field. This is the most common.

Measurements could also be taken on-time, in the presence of the magnetic field.

Question 10

What is meant by delay time?

Answer 10

The strength of the secondary magnetic field as a function of time after primary field turn-off ( or turn-on) when doing time domain measurements.
There are channels (time gates) for measurements of a given delay time. Channel 1 has the least delay time and then the delay time gets longer.

Question 11

What in terms of channels indicate a deep conductor?

Answer 11

Little signal in the early channels because it takes time for the field to propagate up to the surface.

Question 12

What indicates a good conductor?

Answer 12

The transient signal is long and values of all channels are large

Question 13

What indicates a poor conductor?

Answer 13

Loses the energy fast because of their higher resistivity. The values decrease quickly.

Question 14

How is data acquisition done with a moving loop system?

Answer 14

During ground surveys the transmitter and receiver are moved along the survey line with their relative position fixed. Loop size of transmitter between 50m -500m.

Airborne surveys can also be done. A low costs method that covers a large area. Use of large coils or currents needed to compensate reduction of signal due to flight level.

Question 15

How is data acquistion done with a fixed loop system?

Answer 15

This system is used for ground and downhole surveys.
Gives deep penetration.
The transmitter is fixed during the survey and receiver moved along the survey line which gives simpler operational logistics. Lower resolution than moving loop though.

Question 16

How does the depth of penetration decrease? And what is meant by skin depth?

Answer 16

Waves become weaker or attenuate as they propagate. The amplitude decreases exponentially.

The skin depth is defined as the depth at which the amplitude of the electromagnetic field is decreased by a factor 1/e compared with its surface amplitude

Question 17

What happens if the target is vertical?

Answer 17

No anomaly is detected by horizontal receiving coil straight above the body due to the secondary field being horizontal.
The greatest field values will be observed when the conductor lies midway between the two coils.
Similarly there will be no anomaly when the transmitter coil is vertical above the body, no significant current will be induced in the conductor.

Question 18

What is ground penetrating radar (GPR)?

Answer 18

It has by far the highest frequency of any e-m method and displays the wave aspect most clearly. It depends on reflection of pulses of waves, and so resembles seismic reflection. Although it is limited in penetration to a few metres and is almost useless in high-conductivity rocks such as clays.

Question 19

What is magnentotelluric (MT) used for?

Answer 19

Measures natural magnetic and electric field variation at the Earth’s surface. Investigation depth ranges from 300 m below ground by recording higher frequencies down to 10,000 m or deeper with long-period soundings.

Question 20

How does the dielectric constant effect GPR velocity? How does water effect?

Answer 20

The higher the dielectric constant of a material, the slower speed. Water has a constant of 80 which is super high so the amount of water in rocks is of great importance

Question 21

How does the frequency effect GPR penetration depth?

Answer 21

A higher frequency gives a lower penetration depth. Although GPR only works in a small range of frequencies

Question 22

What is the Very Low Frequency method?

Answer 22

The VLF method uses powerful remote radio transmitters. In radio communications terminology, VLF means very low frequency. Relative to frequencies generally used in geophysical exploration, these are actually very high frequencies.
These radio transmitters are very powerful. Under normal conditions, the fields produced are relatively uniform in the far field at a large distance (hundreds of kilometers) from the transmitters.

The VLF method uses relatively simple instruments

Question 23

Which frequencies are used in GPR?

Answer 23

Frequencies between 25-1000 MHz

Question 24

Which frequencies are used for VLF?

Answer 24

Frequencies around 20 000 Hz