Болсуновская Л.М., Абрамова Р.Н., Матвеенко И.А. (ред.) Petroleum engineering. Teachers book. Нефтегазовое дело. Книга для учителя

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sphere. The following equation expresses the gravitational attraction that

two spherical objects have on one another:

F = G * M1 * M2 / R

R is the distance separating the two objects.

G is a constant that is 6.67259x10

–11

kg.

M1 and M2 are the two masses that are attracting each other.

F is the force of attraction between them.

Assume that Earth is one of the masses (M1) and a 1-kg sphere is the

other (M2). The force between them is 9.8 kg*m/s

– we can calculate this

force by dropping the 1-kg sphere and measuring the acceleration that the

Earth's gravitational field applies to it (9.8 m/s

The radius of the Earth is 6,400,000 meters (6,999,125 yards). If you plug

all of these values in and solve for M1, you find that the mass of the Earth

is 6,000,000,000,000,000,000,000,000 kilograms (6E+24 kilograms /

1.3E+25 pounds).

It is "more proper" to ask about mass rather than weight because weight is a force

that requires a gravitational field to determine. You can take a bowling ball and weigh

it on the Earth and on the moon. The weight on the moon will be one-sixth that on the

Earth, but the amount of mass is the same in both places. To weigh the Earth, we

would need to know in which object's gravitational field we want to calculate the

weight. The mass of the Earth, on the other hand, is a constant.

5. Differences in gravity can be mapped by tracking the movement of two

orbiting satellites. As the Earth’s gravitational pull increases and

decreases, the position of the satellites also changes.

For example, if two satellites were flying above a large mountain, the

scientists explain, the first satellite would be pulled closer to the

mountain. The mountain has more mass, so there is a stronger

gravitational pull. As the first satellite is pulled toward the mountain, it

moves farther away from the second satellite – the distance between the

satellites increases. As the second satellite approaches the mountain it is

pulled in and the distance between the satellites decreases again. The

changes in distance between the satellites can be translated into a map of

the Earth's gravitational field.

Exercise 30. Work in groups and enumerate all the possible solutions to the

problem. Discuss and give the explanations.

Two theories have been proposed to account for this anomaly. But before we

go over them, it's important to first consider what creates gravity. At a basic

level, gravity is proportional to mass. So when the mass of an area is

somehow made smaller, gravity is made smaller. Gravity can vary on

different parts of the Earth. Although we usually think of it as a ball, the

Earth actually bulges at the Equator and gets flatter at the poles due to its

rotation. The Earth's mass is not spread out proportionally, and it can shift

position over time. So scientists proposed two theories to explain how the

mass of the Hudson Bay area had decreased and contributed to the area's

lower gravity.

One theory centers on a process known as convection occurring in the Earth's

mantle. The mantle is a layer of molten rock called magma and exists

between 60 and 124 miles (100 to 200 km) below the surface of the Earth .

Magma is extremely hot and constantly whirling and shifting, rising and

falling, to create convection currents. Convection drags the Earth's

continental plates down, which decreases the mass in that area and decreases

the gravity.

A new theory to account for the Hudson Bay area's missing gravity concerns

the Laurentide Ice Sheet, which covered much of present-day Canada and the

northern United States. This ice sheet was almost 2 miles (3.2 km) thick in

most sections, and in two areas of Hudson Bay, it was 2.3 miles (3.7 km)

thick. It was also very heavy and weighed down the Earth. Over a period of

10,000 years, the Laurentide Ice Sheet melted, finally disappearing 10,000

years ago. It left a deep indentation in the Earth.

To get a better idea of what happened, think about what happens when you

lightly press your finger into the surface of a cake or a piece of really springy

bread. Some of it moves to the sides and there's an indentation. But when you

remove your finger, it bounces back to normal. A similar thing happened

with the Laurentide Ice Sheet, the theory proposes – except the Earth isn't so

much "bouncing" back as it is rebounding very slowly (less than half an inch

per year). In the meantime, the area around Hudson Bay has less mass

because some of the Earth has been pushed to the sides by the ice sheet. Less

mass means less gravity

Exercise 35. Which words from your active vocabulary are given here?

coincide; spontaneous; induce; arisefrom; supplement; disposition;

eliminate; superimposed

Exercise 36. Choose the suitable word from the given variants

1. required

2. assume

3. arise from

4. supplemented by

5. affect

6. are subject to

7. supplement

8. intensity

9. superimposed

10. induce

Exercise 37. Compile sentences, add words if necessary.

1. Magnetic content, heat, mechanical stresses, disintegration are factors,

which affect magnetic characteristics of rocks.

2. Structural forces may alter the disposition of magnetic formations.

3. Correction for temperature of instrument arises from the fact that

magnets lose their strength with an increase in temperature.

4. Results of magnetic exploration are generally represented in the form

of lines and curves.

Exercise 39. Change the sentence using the given word. The meaning of a

new sentence should be approximately the same.

1. An important factor when we interpret magnetic methods is that rock

magnetism is of bipolar nature.

2. Fields of geologic bodies are superimposed upon a normal field of the

Earth.

3. Magnetic prospecting requires less accurate measurements.

4. The general expression for magnetic field is gauss.

5. The vertical component exhibits the clearest relation between the

magnetic anomaly and the place where the geologic body is located.

6. In magnetic prospecting we prefer to measure magnetic vertical

intensity.

7. Magnetic anomalies of geologic bodies depend on their magnetic

susceptibility and remanent magnetism.

8. Such factors as magnetic content, grain size, mechanical stress and

others influence\determine magnetic characteristics of rocks.

Exercise 40. Complete the given sentences.

1. (utilize natural and spontaneous fields of force);

2. (to magnetic bodies but not to geologic bodies);

3. (to change with latitude);

4. (is of bipolar nature);

5. (into its vertical and horizontal components);

6. (measurements of the magnetic vertical intensity);

7. (gauss(line per centimeter in the air));

8. (one thousandth of this unit and is used for convenience);

9. (less than in gravity survey);

10. (igneous and iron ores that are strongly magnetic and sedimentary

rocks that are weak in magnetization).

Exercise 41. Place the sentences according to the order of their appearance

in the text.

12; 7; 1; 4; 9; 5; 10; 2; 6; 3; 8; 11.

Exercise 42. State whether these sentences are true or false. Correct the

false sentences. Give additional information on the subject.

1. F – magnetometers. There are different types of magnetometers such

as Schmidt, horizontal, vertical. The former measures horizontal

intensity and later the vertical component of the magnetc field.

2. F – Magnetic survey requires accuracy but it is less than in

gravitational survey Besides, magnetometers give accurate enough

information. We shouldn’t forget about corrections which are applied

such as ……

3. F – Correction for temperature of an instrument arises from the fact

that magnets lose their strength when temperature increases\ or

planetary correction eliminates the variation of earth’s magnetic field

with latitude.

4. F – We can carry out magnetic survey far from such natural handicaps

as fences, bridges and pipelines as it is difficult to eliminate their

influence on magnetic field variations.

5. F – Results of magnetic survey are registered in the form of lines –

isanomalic lines or in the form of curves for profiles.

6. F – The scientists give quantitative and qualitative interpretation of

magnetic anomalies. In the qualitative interpretation fifty to several

hundred gammas are associated with some topographical features or

compositions of igneous rocks. In quantitative interpretation the

magnetic effect of the assumed body is calculated according to field

curves and are based on some assumptions about the reasons of such

magnetic anomalies.

Exercise 44.



Listen to the interview with Chris Leech, geophysicist and

director of Geomatrix Earth Sciences Ltd, who is talking about

archeological geophysics. Answer the following questions by saying

whether each of the following statements are 1 – True (T) or 2 – False (F).

1. F, 2. F, 3. T, 4 F, 5. T, 6. F, 7. F, 8. T, 9. T, 10. T.

Tapescript

Seeing Secrets Underground

Chris L: Geophysics is really the science of trying to find what is down

below the ground without actually having to dig a hole. We’re looking for

changes in the physical properties of the ground. If you’ve got a 10

hectare site that you’re interested in and you think there might be

archaeology underneath it you’re going to have to dig an awful lot of

holes in the ground if you don’t do geophysics first. With the geophysics

it allows you to very rapidly, relatively cost-effectively narrow-down the

area which is of great interest to the real archaeologists.

Interviewer: What are the techniques used?

Chris L: There are several techniques which are useful for archaeological

geophysics. We tend to look for the magnetic signatures of the ground.

We tend to look for the electrical properties of the ground. Also we use

the growing technique of ground-penetrating radar therefore looking at the

electromagnetic properties of the ground.

Interviewer: The equipment that you have strapped to yourself here is

basically a horizontal pole with two vertical poles attached at either side

and a rucksack that’s strapping you into it, really. Which technique does

this piece of equipment use?

Chris L: This is a magnetometer. Therefore we are actually measuring

precisely changes in the Earth’s magnetic field at this point. The two

vertical poles are actually two separate magnetometers so you can

measure the Earth’s magnetic field in two places at the same time. The

horizontal pole you described feeds the data back into a small data logger.

By having two magnetometers we can speed up our operation. We can

acquire two lines of data right at the same time.

Interviewer: How does a magnetometer actually measure the magnetic

field beneath it?

Chris L: Each of the tubes which we mentioned earlier contains two

fluxgate sensors. These measure very precisely the Earth’s magnetic field

in the vertical component. By measuring it in two different places we can

get a measure of the change in the magnetic field over that distance which

is 1m between each sensor. We’re measuring the vertical gradient of the

Earth’s magnetic field in two places simultaneously.

Interviewer: How does this then translate to tell you where things are

located?

Chris L: All materials have got a magnetic signature be it different types

of soils or manmade objects such as bricks or artefacts of some

description. If we can measure the magnetic field with sufficient accuracy

and sufficient resolution as we go over the ground we can see very small

changes in the Earth’s magnetic field.

Exercise 45.



Listen to the interview with Dr. Michio Kaku, Professor of

Theoretical Physics telling about leaks in the Earth’s magnetosphere.

Choose the correct answer to the given questions.

1. C; 2. A; 3. A; 4. B; 5. B; 6. C

Tapescript

Leaks in the Earth’s Magnetic Field

TV Presenter: They could be bigger and more immediate than the threat

of global warming they say. Earth’s magnetic field which acts as our

protective shield in space has holes in it and that could put a lot of our

earthly functions at risk. Dr. Michio Kaku, Professor of the theoretical

physics is back with us. Professor, how are you? Good morning to you.

Professor: Glad to be on the show.

TV Presenter: What is this, solar shield?

Professor: Well, every eleven years the sun has scintillation and releases

a shock wave. Tsunami or radiation can wipe our communication, weather

satellites, GPS, space satellites, you name it. It’s up there in outer space.

TV Presenter: You say, you say my blackberry (беспроводное

мобильное устройство) also?

Professor: The internet box, television, our cables, satellite TV, all of it

could get wiped at around twenty twelve that’s when we have the peak of

the sunspot cycle. That’s when the sun’s magnetic field flips North Pole

and South Pole flip releasing a shock wave radiation which then hits the

Earth minutes later potentially wiping out good chunks of our satellite

communications. We’re watching it very carefully now.

TV Presenter: This happens how often you say?

Professor: Every eleven years.

TV Presenter: Why in eleven year cycle?

Professor: It takes eleven years for the magnetic field of the sun to build

up enough intensity to begin the flip process because it slowly revolves in

a circle like winding up the windings of the clock. If you wind up the

main spring of the clock too much, its boi-on-on comes out of control.

Same things with the sun’s magnetic field. It literally flips every eleven

years releasing a shock wave or radiation and that means we have to start

to think about reinforcing satellites, building redundant systems cause that

means GPS, the powered grid, weather satellites, communication

satellites, satellite television, all of that could get disrupted peaking

around twenty twelve.

TV Presenter: So, your point is, we can’t do anything in space, but we

can do something here on Earth: backup systems our companies’re really

doing that?

Professor: Well, they gonna have to do it because we scientists made a

mistake. We found the next cycle was gonna be quiet.

Well, some of our data was off by a factor of twenty and that’s why we’re

issuing this alert now. We made a mistake the next cycle peaking around

twenty twelve will be much more serious than we previously thought. In

the past we didn’t lodge the bullet because we didn’t have that many

satellites up there than eleven twenty years ago. Back in nineteen ninety

we did not have that many space satellites

TV Presenter: Well, that makes sense, but listen, aren’t they just a tactic

to put a lot of fear in lot of people? That’s really no danger in the end?

Professor: Let’s hope that it doesn’t, let’s hope that nothing happens.

However, what if our communication systems are wiped out, billions of

economic activity could be stopped.

TV Presenter: Could you think it could be taken seriously?

Professor: I think we’re gonna have to. The physicists are now sounding

alert. We made a mistake. We made the wrong projection. The next solar

cycle will be more intense than previously thought. An ounce of

prevention is a pound of cure and now it’s time to begin to create

redundant systems, reinforce the satellites, build backup systems.

TV Presenter: Thank you, professor. Nice to know.

Exercise 47. Give the explanation to the following diagram.

Suggested answer

Different rocks produce local variations in Earth’s magnetic field and so

yield different readings from a magnetometer. In the picture we can see

that country rocks produce regional magnetism, topsoil produce

background magnetism, ores just below the surface and deeply buried ore

are relatively strong in magnetism. So interpretation of magnetic survey

results can indicate certain types of subsurface rocks, for example, buried

iron ores. Because iron is often found with sulfides, magnetometers may

lead indirectly to non-ferrous metals, too.

Exercise 49. Work in groups. Think over, give your idea and justify your

opinion.

1. In a sense, yes. You probably know that the Earth is stratified. In radius

it is composed of layers having different chemical composition and

different physical properties. The crust of the Earth has some permanent

magnetization, and the core of the Earth, the outer part of which is liquid

iron and the inner of which is solid iron, generates its own magnetic field,

sustaining the main part of the field we measure at the surface. So we

could say that the Earth is, therefore, a ‘magnet’. But there is no giant bar

magnet near the Earth’s center, despite the depictions you may have seen

in elementary textbooks on geology and geophysics. Permanent

magnetization cannot occur at high temperatures, like temperatures above

650 degrees centigrade or so, when the thermal motion of atoms becomes

sufficiently vigorous to destroy the ordered orientations needed to

establish permanent magnetization. The core of the Earth has a

temperature of several thousand degrees, and so, even though the core is

the source of most of the geomagnetic field, it is not, itself, permanently

magnetized.

2. This is explained, in general terms, in the Introduction to Geomagnetism

page given on the website http://geomag.usgs.gov/intro.php. Briefly, then,

as the result of radioactive heating and chemical differentiation, the outer

core is in a state of turbulent convection. This sets up a process that is a bit

like a naturally occurring electrical generator, where the convective kinetic

energy is converted to electrical and magnetic energy. Basically, the

motion of the electrically conducting iron in the presence of the Earth's

magnetic field induces electric currents. Those electric currents generate

their own magnetic field, and, as the result of this internal feedback, the

process is self-sustaining, so long as there is an energy source sufficient to

maintain convection. The depiction of the geodynamo shown here is only

schematic; in fact, the fluid motion and the form of the magnetic field

inside the core are still the subject of intensive research.

3. Models and charts of the magnetic field at the Earth’s surface need to be

periodically updated because the field is constantly changing in time. The

same fluid motion in the Earth’s core that sustains the main part of the

magnetic field also causes the field to slowly change in spatial form, a

time-dependence known as ‘secular variation’. This variation can be seen

in all vectorial parts of the magnetic field, but it was first noticed in

declination several hundred years ago, since it is that quantity that is so

important for navigation. In fact, the demands of navigators helped to

motivate, centuries ago, some of the original studies of the Earth's

magnetic field. On average the declination at the Earth’s surface changes

by about a fifth of a degree per year.

4. Yes. We know this from an examination of the geological record. When

lavas are deposited on the Earth’s surface, and subsequently freeze, and

when sediments are deposited on ocean and lake bottoms, and

subsequently solidify, they often preserve a signature of the ambient

magnetic field at the time of deposition. This type of magnetization is

known as 'paleomagnetism'. Careful measurements of oriented samples of

faintly magnetized rocks taken from many geographical sites allow

scientists to work out the geological history of the magnetic field. We can

tell, for example, that the Earth has had a magnetic field for at least 3.5

billion years, and that the field has always exhibited a certain amount of

time-dependence, part of which is normal secular variation, like that which

we observe today, and part of which is an occasional reversal of polarity.

Incredible as it may seem, the magnetic field occasionally flips over! The

geomagnetic poles are currently roughly coincident with the geographic

poles, because the rotation of the Earth is an important dynamical force in

the core, where the main part of the field is generated. Occasionally,

however, the secular variation becomes sufficiently large such that the

magnetic poles end up being located rather distantly from the geographic

poles; we say that the poles have undergone an ‘excursion’ from their

preferred state. Now, we know from physics that the Earth’s dynamo is

just as capable of generating a magnetic field with a polarity like that

which we have today as it is capable of generating a field with the opposite

polarity. The dynamo has no preference for a particular polarity.

Therefore, after an excursional period of enhanced secular variation, the

magnetic field, upon returning to its usual state of rough alignment with

the Earth’s rotational axis, could just as easily have one polarity as

another. The consequences of polarity reversals for the compass are

dramatic. Nowadays, the compass points roughly north, or, more precisely,

the north end of the compass points roughly north at most geographical

locations. However, before the last reversal, which was about 780,000

years ago, the polarity was reversed compared to today's, and the compass

would have pointed roughly south, and before that reversed state the

polarity was like that which we have today, and the compass would have

pointed roughly north, and so on. The timings of reversals forms the so-

called 'geomagnetic polarity timescale', shown here at the right. During a

reversal, between polarities, the geometry of the magnetic field is much

more complicated than it is now, and a compass could point in almost any

direction depending on one’s location on the Earth and the exact form of

the mid-transitional magnetic field. One of the things that is interesting

about reversals is that there is no apparent periodicity to their occurrence.