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Reversals are random events. They can happen as often as every 10

thousand years or so, and as infrequently as every 50 million years or

more. Questions about reversals are very popular with the general public,

and further information can be found in the references given in the Further

Reading page of the given website.

Nothing causes the magnetic field to reverse is polarity. That answer might

surprise you, but the fact that the field occasionally reverses is simply a

property of the continuous, on-going behavior of the Earth's dynamo.

There is no ‘cause’ per se. In answering the previous question we

discussed the phenomenology of polarity reversals, what they are and how

they might affect a (hypothetical) compass, but with respect to the physics

of the process itself, some lessons can be learned from the laboratory. It is

possible, for example, to design a machine, an electrical-magnetic-

mechanical dynamo consisting of spinning metal disks and coils of wire

which, when supplied with mechanical energy, sustains its own magnetic

field. Depending on the details of the apparatus, the magnetic field can be

steady, with no time dependence at all, or it can reverse periodically, like

the Sun’s magnetic field does every eleven years, or it can reverse

randomly, bouncing back and forth in an orbit around two preferred states

(opposite polarities) like the Earth’s magnetic field does. It is also possible

to write down the mathematical equations that describe the behavior of this

laboratory system – the equations describe what is popularly known as

‘chaos’, and, even though the laboratory system is relatively simple, its

equations have some similarity to those describing the dynamics of the

Earth’s core. In summary, then, nature allows for different kinds of

dynamos, some of which just simply have the property that they undergo

occasional random reversals. The Earth' core happens to be one of those

dynamo types.

5. One of the most important jobs that a scientist has is to determine, from

among all the possible causes and effects in nature, which are the most

important and strictly and necessarily causally related, and which are

simply insignificant and essentially unrelated. Although extremely

unlikely, we will admit that it might be possible for a reversal of the

Earth’s magnetic field to be triggered by a meteorite or cometary impact,

or even for it to be caused by something more ‘gentle’, such as the

melting of the polar ice caps, as you suggest. But remember, from our

discussion following the previous question, self-contained dynamical

systems, some of which can be built in the laboratory, can exhibit

randomly reversing behavior. They can do this without any outside

influence. The Earth's dynamo is a natural example of such a self-

contained, randomly-reversing dynamical system. Therefore, invoking an

external mechanism for causing the Earth’s polarity reversals is, quite

simply, a ‘solution’ to a non-problem. Reversals would happen anyway.

6. Almost certainly not. Direct historical measurements of the intensity of

the geomagnetic field have been possible ever since Gauss invented the

magnetometer in the 1830’s. Since then the average intensity of the field

at the Earth’s surface has decreased by about ten percent. And we know,

from paleomagnetic records, that the intensity of the field does indeed

decrease, by as much as ninety percent, at the Earth’s surface during a

reversal. But those same paleomagnetic records also show that the field

intensity has often exhibited significant variation, with both decreases

and increases in intensity, without there always being a coincident

reversal. So, an intensity low does not necessarily mean that a reversal is

about to occur. Moreover, the recent decrease in intensity is not really

that dramatic of a departure from normality, and for all we know the field

may actually get stronger at some point in the not-so-distant future. It's

worth remarking that predicting the occurrence of a reversal based upon a

knowledge of the current state of the magnetic field is about as easy as

predicting the next bull market on Wall Street; you don’t know it’s

happening until it’s half over!

7. The magnetic field of the Earth does protect us from fast-moving

charged particles streaming from the Sun, but so does the atmosphere. It is

not clear whether or not the radiation that would make it to the Earth’s

surface during a polarity transition, when the magnetic field is relatively

weak, is sufficient to affect evolution, either directly or indirectly, and

cause extinctions, such as that of the dinosaurs. But it seems that the

radiation is probably insufficient. This conclusion is supported by the fact

that reversals happen rather frequently, every million years or so,

compared to the occurrence of mass extinctions, every hundred million

years or so. In other words, many reversals and, in fact, most reversals,

appear to be of no consequence for extinctions.

8. The USGS supports an important National Earthquake Hazards

Program. As a small part of that effort there have been studies attempting

to correlate magnetic variations, or more precisely, electro-magnetic

variations, with earthquakes. Electromagnetic variations have been

observed after earthquakes for many years now, but What is less clear is

whether or not there are detectable electro-magnetic precursors to

earthquakes. It is worth acknowledging that geophysicists would actually

dearly love to demonstrate the reality of such precursors, especially if they

could be used for reliably predicting earthquakes! Unfortunately, no

convincing evidence of electro-magnetic precursors to earthquakes has

been found, despite decades of work. And it should be emphasized that

isolated coincidences are not sufficient to demonstrate a relationship. What

is needed to confirm an extraordinary claim is, of course, an extraordinary

amount of evidence, which in this case would mean many repeated

correlations of earthquakes with specific and identifiable precusory field

variations. Such evidence simply doesn't exist.

9. Not directly, no. High-altitude pilots can experience enhanced levels of

radiation during magnetic storms, but the hazard is due to the radiation,

not the magnetic field itself. Direct effects on human health by the

magnetic field at the Earth’s surface are, quite frankly, insignificant. The

primary effects of geomagnetism are on the health of electrically-based

technological systems that are critically important to the modern

civilization of humanity, not the humans themselves.

10. Yes. There is evidence that some animals, probably most notably sea

turtles, have the ability to sense the Earth’s magnetic field (although

probably not consciously) and to use this sense, along with their several

other senses, for purposes of orientation. We acknowledge that this is an

interesting subject, and inquisitive acquaintances have posed this question

to us on many occasions. However, the issue of magnetic orientation by

animals is really more a matter of biophysics rather than geophysics, and

we will, therefore, refer the curious reader to the following authoritative

articles:

Lohmann, K. J., Hester, J. T. & Lohmann, C. M. F., 1999. Long-distance

navigation in sea turtles, Ethology Ecology & Evolution, 11, 1–23.

Skiles, D. D., 1985. The geomagnetic field: Its nature, history and

biological relevance, In Magnetite Biomineralization and

Magnetoreception by Living Organisms: A New Biomagnetism, Ed:

Kirschvink, J. L., Jones, D. S. & MacFadden, B. J., Plenum Publishing

Corporation, New York.

Walker, M. M., Dennis, T. E. & Kirschvink, J. L., 2002. The magnetic

sense and its use in long-distance navigation by animals, Current Opinion

in Neurobiology, 12, 735–744.

Wiltschko, R. & Wiltschko, W., 1995. Magnetic orientation in animals,

Zoophysiology, 33, Springer Verlag, Berlin.

11. A magnetic storm is a period of rapid magnetic field variation. The

causes of magnetic storms are explained, in general terms, in the

Introduction to Geomagnetism page given on the website

http://geomag.usgs.gov/faqs.php#qfourteen. Briefly, then, magnetic storms

have two basic causes. First of all, let us be reminded that the Sun is

always emitting a wind of charged particles that flows outward into space

away from the Sun itself. Occasionally the Sun emits a strong surge of

solar wind, something called a coronal mass ejection. When this gust of

solar wind impacts upon the outer part of the Earth’s magnetic field, the

magnetosphere, the field is disturbed and it undergoes a complex

oscillation. This causes the generation of associated electric currents in the

near-Earth space environment, which, in turn, generate additional

magnetic-field variations – all of which constitute a 'magnetic storm'. The

second cause of magnetic storms is the occasional direct linkage of the

Sun’s magnetic field with that of the Earth’s. This direct magnetic

connection is not the normal state of affairs, but when it occurs, charged

particles, traveling along magnetic-field lines, can easily enter the

magnetosphere, generate currents, and cause the magnetic field to undergo

time-dependent variation. On occasion, the Sun emits a coronal mass

ejection at a time when the magnetic-field lines of the Earth and Sun are

directly connected. Then we can experience a truly large magnetic storm.

The infrastructure and activities of our modern technologically-based

society can be adversely affected by rapid magnetic-field variations

generated by electric currents in the near-Earth space environment,

particularly in the ionosphere and magnetosphere. This is especially true

during so-called ‘magnetic storms’. Because the ionosphere is heated and

distorted during storms, long-range radio communication, which relies on

sub-ionospheric reflection, can be difficult or impossible and global-

positioning systems (GPS), which relies on radio transmission through the

ionosphere, can be degraded. Ionospheric expansion can enhance satellite

drag and thereby make their orbits difficult to control. During magnetic

storms, satellite electronics can be damaged through the build up and

subsequent discharge of static-electric charges, and astronaut and high-

altitude pilots can be subjected to increased levels of radiation. There can

even be deleterious effects on the ground: pipe-line corrosion can be

enhanced, and electric-power grids can experience voltage surges that

cause blackouts. The reason why space-based effects can have

consequences down here on the Earth’s surface is related, at least in part,

to our answer to the first question, ‘What is a magnetic field?’. Electric

currents in one place can induce electric currents in another place, this

action at a distance is accomplished via a magnetic field. So, even though

rapid magnetic-field variations are generated by currents in space, very

real effects, such as unwanted electric currents induced in electric-power

grids, can result down here on the Earth’s surface. More generally, the

hazardous effects associated with geomagnetic activity, which are

discussed more fully in the Further Reading page of this website, are one

reason why the USGS Geomagnetism Program is part of the Central

Region Geohazards Team.

Aurorae are a luminous glow of the upper atmosphere caused by energetic

particles descending from the Earth’s magnetosphere or coming directly

from the Sun. These energetic particles are mostly electrons, but protons

can also be involved, and their energetic rain into the atmosphere is

greatest during magnetic storms. As the particles descend, they collide

with molecules in the atmosphere, causing an excitation of the oxygen and

nitrogen molecular electrons. The molecules can return to their original,

unexcited state by emitting a bit of light, a photon. This light, a

photograph of which appears in the banner of this website, is the aurora

that we see. Since electrically-charged particles tend to follow magnetic-

field lines, and since magnetic-field lines are oriented in and out of the

Earth and its atmosphere, near the magnetic poles, aurorae tend to be seen

at high latitudes.

12. Both satellites and ground-based magnetometers are important for

making measurements of the Earth’s magnetic field. They are not

redundant, but are, instead, complementary. After executing several orbits

of the Earth, satellites can provide good geographical coverage for data

collection. On the other hand, ground-based magnetometers are much less

expensive than satellites, they are much easier to install and control than

satellites, and, with an array of magnetometers, they can provide coverage

from numerous locations simultaneously. Another consideration is that

satellites orbit the Earth either inside or above the ionosphere, the

electrically conducting part of the Earth’s atmosphere. Since currents in

the ionosphere contribute to the magnetic field, this means that the field

measured by a satellite is somewhat different than the field measured at

the surface. Finally, don’t forget that it is at the surface of the Earth,

where we live, that many of the effects of space weather are most

important, so measurements from ground-based observatories will always

play a critical role in space-weather studies.

Exercise 54. Which word-combination is out of place here? And why?

a) electrode spacing (the rest are current types);

b) induce (the rest have the meaning “доставлять”);

c) refraction (the rest are methods of electrical survey);

d) formation boundaries (the rest are derivatives);

e) pressure solution (the rest are frequency bands);

f) inductive (the rest are nouns that denote some notions);

g) field intensity (the rest are devices used in electrical survey);

h) flux line (the rest are different methods of electrical survey).

Exercise 57. Fill in the gaps with correct prepositions.

1. to, by, by

2. into, of, into

3. in, with

4. of, to, of, of

5. until

6. for, of

7. to, by, at, to, with

Exercise 58. Fill in the gaps with terms from the text.

1. inductive; 2. “treasure-finders”; 3. resistivity; 4. resistivity; 5. electric;

6. self-potential; 7. radio; 8. inductive.

Exercise 59. State whether the following sentences are true or false, correct

the false ones.

1. F – 3 groups

2. F – due to lack of depth penetration

3. F – resistivity and potential-drop-ratio methods

4. T

5. F – suited

6. F – applicable to depth determinations of horizontal strata and the

mapping of dipping formations

7. T

8. F – parallel with the strike

Exercise 62. Read the text and decide which of the headings (1-6) best

summarizes each part (A-D) of the text. There is one extra heading that you

do not need to use.

A – 6; B – 1; C – 3; D – 2; E – 4.

Exercise 63.



Listen to the report on the resistivity survey conducted at

Sulfurbank Mercury Mine Superfund site and identify the steps of the

investigation.

1.E, 2. B, 3. I, 4. C, 5. F, 6. A, 7. D, 8. G, 9. H

The geophysical survey determined flow paths for mercury rich acid

waters.

Electromagnetic conductivity survey was carried out over twelve

kilometer area.

Four magnetic and ore-conductive anomalies were identified.

One anomaly was chosen for detailed ground electromagnetic

conductivity survey.

The conductive zones of probable pathways for ground water flow were

detected.

The near surface areas containing high concentrations of sulfide minerals

were identified.

The samples were taken from every three meters along the constructed

lines.

There were about 1.5 million irregularly spaced samples obtained.

The wire metric mapping was done with CTAX software.

Tapescript

Electrical Resistivity Survey

Electrical resistivity surveys offer the ability to inexpensively collect vast

quantities of 3 dimensional subsurface data. Environmental sciences,

mining, archeology and many other disciplines can benefit from these

measurement techniques. However, the large irregular data sets offer

significant challenges related to analysis and visualization. CTAX EVS

and MVS excel in their ability to address these complex issues. Airborne

geophysical reconnaissance was used to identify potential flow paths for

mercury rich acid water entering clear lake at the Sulfur bank of Mercury

Mine. Magnetic and electromagnetic conductivity surveys were conducted

over its twelve square kilometer area that included the ox arm of clear

lake and Sulfur bank Mercury Mine. These surveys identified four

magnetic and ore conductive anomalies that may represent ground water

conduits towards or away from inherent impoundment. An anomaly that

extended from inherent impoundment through a waste rock down into

clear lake selected for a more detailed ground electromagnetic

conductivity survey. The combined results of the airborne and ground

surveys provided detailed lateral depiction of conductive zones which are

the most probable pathways for ground water flow. These surveys also

identified near surface areas that may contain elevated concentrations of

sulfide minerals that weathered to produce acid ground water. The Sulfur

bank Mercury Mine was an excellent example site to evaluate geophysical

techniques because the extensive network of ground water monitoring

wells had already been established there as a part of remedial

investigation and feasibility study. Full grow airborne services through the

airborne geophysical reconnaissance of the ox arm of clear lake including

SBEN on August, ninth through the seventeenth, 2 thousand. The spacing

between each flank line was roughly fifteen meters with samples every

three meters along each line and every one meter in depth. This resulted in

nearly one point five million irregularly spaced samples. CTAX software

processed this massive data set to provide a wiremetric mapping of the

resistivity data that was conformal to topography and geology. The

preliminary dataset for Sulfur bank Mercury Mine Superfund site was

provided by courtesy of Geosciences division of the water and energy

team at the US department of energies, national energy technology

laboratory. The effort was funded by ASCPA mine waste technology

program.

Exercise 64. Listen to the presentation on electromagnetic instrumentations

and match the comments on slides (1–5) with the images (A–E). One

comment is extra.

A – 6; B – 4; C – 2; D – 3; E – 1; 5 – extra

Tapescript

EM31 instrument has an approximate depth of investigation of 18 feet and

records a ground water conductivity as well as in-phase data which

respond to the presence of buried metal. EM31 survey is commonly used

to delineate and characterize land field soils and buried wastes. The

survey can also detect changes in soil conditions and lithology. This slide

shows the results of EM31 survey which was conducted to locate and

characterize buried wastes at a site in North Carolina. The top contour plot

represents the conductivity results which delineate the parameter of buried

waste in associated leachate. The middle contour plot shows the areas

containing buried metal as defined by EM31 in-phase data. Contour

shaded in orange and red in both plots identify the areas containing

highest concentration of wastes, leachate and metal. The bottom plot is a

composite map showing the limits of the landfield in the portions that

contain buried metal waste based on the survey results.

As the name implies EM61 metal detection instrument is used to locate

and delineate buried metal, tanks, drams, conduits and other metal objects.

Within the depth involved observed by EM the instrument records several

levels of sensitivity during each survey which helps in differentiating

between larger objects and smaller objects. This slide shows the results of

EM61 metal detection survey. The early time gate respond shown in the

top plot is the most sensitive component and detects all metal objects

regardless of size. The early time gate is used to locate buried conduits

and areas containing miscellaneous (смешанный, неоднородный,

разный) debris. Differential response shown in the bottom plot identifies

areas containing large metal objects such as drams and tanks and ignores

the small miscellaneous metal debris.

Exercise 65. Using the diagram describe the general principle of electrical

survey.