*SUMMARY ON AGP202*

 _Gravity Methods, Solar system and Earthquake (Seismic)_ 

Magnetic Methods and Polar wandering later

 Topic 1️⃣

  *GRAVITY METHODS*

 *Theory of Gravitation*

Newton's law of gravity, which quantifies the mutual attraction between two masses m1 and m2 separated by a distance r given by:-

F = Gm1m2/R²  (N)

G= Universal gravitational constant = 6.67 × 10-¹¹Nm²kg-²

 *NOTE:-* The attraction that is exerted by the Earth (mass M) on any mass m outside of it in the surface of the Earth is directed towards acceleration. - This is because a point mass and a spherical symmetrical body of the same total mass have exactly the same gravitational field

g = GM/R²

p=5520kgm-³ - This is greater than the densities of rocks (2500-3000) indicates that the Earth's density increases towards the centre of the Earth.

The net gravitational force at any point on the surface is the resultant of forces due to internal and centrifugal action.

 *IGF - International Gravity Formula* is a best fitting value st the sea level surface for an idealised surface, however, is affected by inhomogeneities of density beneath the surface and therefore it departs from the theoretical 

1mgal -10-³cms-²  = 10gu

Milligal (Galileo Galilei) is an Italian Physicist.

Instrumental sensitivity of the order of 0.01mgal is used to measure accurately small changes in gravity caused by subsurface rocks.

 *INSTRUMENTATION*

The instrument used to rapidly measure gravity is *GRAVIMETER* It records the direct effects of the pull if gravity on a weight suspended by a delicate spring.

The extension of the string is proportional to the pull according to Hooke's law :- ds = dg (m/k)

A type of Gravimeter in which the spring functions to support the mass and also as the measuring device is *STABLE/STATIC GRAVIMETER* and is not very sensitive. Hence, optical, mechanical or electronic amplification of the extension is used

Both *LA Coste and Romberg* Gravimeter is enclosed in a Thermos bottle housing in order to reduce or remove the effect of temperature variations on the spring of the gravimeter.

 *Normal Gravimeter* can be used for marine surveying by lowering them in a waterproof housing to the sea bed, their use is limited.

 ```ROCK DENSITIES```

Gravity anomalies result from the difference in Density or density contrast between a body of rock and it's surroundings.

∆p = p1 -p2

∆p = density contrast

Rock densities are dependent on composition and porosity.

Sedimentary rocks tend to become denser with depth, due to compaction and with age due to cementation.

 *IGSN - International Gravity Standardization Network*

 *LATITUDE CORRECTION*

Dg only due to difference in latitude 

Dg = gs - gp

The latitudinal effect can by removed by applying IGF which shoes that there is a N-S effect gravity gradient at latitude.

 *ELEVATION CORRECTIONs*

 *1. Free-air correction (DgFA) :-* corrects for the decrease in gravity with height in free air resulting from increase distance from the earth's centre.

The FA correction will be positive for an observation point above the reference station to correct for the decrease in gravity with elevation and negative for an observation point below the reference station

2. Bouguer Correction:- ∆gbp

The size of the Bouquer correction depends on the density of the rock to an infinite horizontal slab with a thickness equal to the elevation of the site above the reference station.

∆gb = 0.4191phgU

3. *Topographic correction*  is rarely the case that Bouquer correction assumes that topography around the observation site is flat.

Earth's tides Re caused by the gravitational effects of the sun and moon.

```Eotvos correction```

Is applied to gravity measurements taken in moving vehicles such as ship/aircraft

 *QUANTITATIVE INTERPRETATION*

1. Separation of Anomalies

2.Residual Anomaly interpretation

Several analytical methods if separation of regional and residual anomalies are available such as :-

A. Least squares fitting

B. Polynomial curve

C. Second derivative

D. 5 point center ring

E. Downward and upward continuation

 *APPLICATIONS*

1. Defining the size and extent of ore bodies (+ve anomalies)

2. Defining buried river channels

3. Demarcating faults (changing anomaly shape)

4. Delineating intrusive bodies 

5. Concept of isostacy

 Topic 2️⃣

*SOLAR SYSTEM🪐* 

This is a group of stars and planets which consists of the sun and *8 planets*

The luminous and non- luminous bodies are STARS and *PLANETS* respectively except Venus and Uranus, the other six planets rotate in the same direction and revolve around the sun🌞

Sun and Earth distance = 1.496×10^8km

Earth distance = about 12742km

Earth average density = 5.52g/cm³

PLUTO was discovered in 1930 as DWARF planet

 *PLANETS OF SOLAR SYSTEM*

1. Terrestrial planet (inner circle) - MVEM.  1st 4 planets

2. Giant planet (outer circle) - JSUN last 4 planets

 *GALAXY* is small part of the system of stars. The diameter is 10^5 light years

The planets were originally gaseous but were subsequently cooled down into liquid and ultimately to solid state

 *INTERIOR OF THE EARTH* 

3 LAYERS

Crust, Mantle and Core

Crust (sial). - lithosphere - conraid discontinuity

Upper Mantle - Astenosphere - Moho Discontinuity

Lower mantle - Mesosphere - Repetti Discontinuity

Outer core - Gutenberg discontinuity

Baryshpere

Inner core - Lehman discontinuity 

 Topic 3️⃣

 *SEISMICITY AND EARTHQUAKE ZONES*

Earthquake is the shaking of the Earth's crust that is which may cause destruction to buildings and results from the sudden release of tectonic stress along a fault line or volcanic activity.

 *SEISMIC (EARTHQUAKE)*

 *SEISMICITY (EARTHQUAKE FREQUENCY)*

 *TECTONIC STRESS* (related to causes of Earthquake)

 *Other types of Natural disasters*

1. Volcanoes

2. Landslides

3. Famines and Droughts

4. Extreme temperature

5. Extreme precipitation and flooding

6. Wildfires

7. Hurricanes, tornadoes and cyclones

 *CAUSES OF EARTHQUAKE* 

Usually caused when rock underground suddenly breaks along a fault. The sudden release of energy causes the seismic waves that make the ground shake.

When rock break, the earthquake occurs.

The spot underground where the rock breaks is called *FOCUS* of the Earthquake.

Earthquake-like seismic waves can also be caused by explosions underground.

The place right above the focus is Epicenter of the Earthquake

 

 *EARTHQUAKES STUDIES*

 *Seismologists* study earthquakes by using *seismographs*

 *A seismograph* is an instrument that records the shaking of the Earth's surface caused by seismic waves

 *Seismometer* is used to refer to some devices......

1st seismograph (132 A.D) by Chinese Astronomer Mathematician Chang Heng

8 dragons had a bronze ball I it's mouth.

In 136 A.D, a Chinese Choke updated this meter and called it *SEISMOSCOPE*

 

 *Microseisms* can be caused by heavy traffic near the seismograph waves hitting a beach, the wind and any number of other ordinary things.

The *P waves* which is bigger than the rest of the little ones (microseisms)

P waves are fastest seismic waves

 *S waves* is next and is usually bigger than P waves

Swaves can't travel through liquid layers of the Earth, so these waves never made it to your seismograph. They have lower frequency which means that waves are more spread out.

  *Read on Richter scale (Charles F Richter) 1934*

*And Mercalli scale* 

 *WHY DO EARTHQUAKE HAPPEN?*

Along plate edges, oceanic and continental plates. Earthquake usually occur where 2 plates are running into each other/sliding past each other..

Along faults, faults are cracks in The Earth where sections of a plate moving in different directions

 

 Faults are caused by all bumping or sliding plates do.

 *TYPES OF FAULTS*

1. *Normal fault:-* (sliding downward and away from another block of rock). Pulling away from each other.

A normal fault is defined by the hanging wall moving down relative to the footwall which is moving up.

2. *Reverse fault:-* (pushing into another plate). Compressed by another plate pushing against it.

A reverse fault is defined by hanging wall moving up relative to the footwall which is moving down.

3. *Strike slip fault:-* (sliding past each other)

Read on Earthquake zones in India

 *WHAT ARE SEISMIC WAVES?*

1. *Primary/longitudinal wave (P wave) :* analogous to sound wave

2. *Secondary/Transverse wave:* (S wave) analogous to light waves

3. *Surface/Long period wave:* originated when Pwave hits the surface

Brought to you by *FLASHPEE EDUCATIONAL TEAM*