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#### 9.5 Option - Geophysics: 2. Remote sensing

Syllabus reference (October 2002 version)
2. Some physical phenomena such as gravitation and radiation provide information about the Earth at a distance from it
Students learn to: Students:

Extract from Physics Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.

Prior learning: Preliminary modules 8.2, 8.3, 8.4.

Background: Most if not all, large mineral deposits now found in the world are found by the utilising processes of remote sensing. The ability of remote sensing techniques to provide baseline data for widespread environmental management programs is immense. Without remote sensing it is unlikely that many of the mineral deposits recently found would have been found. The techniques are relatively non invasive and hence cause little or no damage to the areas under study.

#### perform a first-hand investigation to gather data to demonstrate the relationship between the nature of a surface and the radiation reflected from it

• To do this activity you need to determine the nature of reflected light from a surface and view that light through some sort of filter device to make comparisons of the differences between the surface and the nature of the light reflected. This type of investigation could be as simple as viewing coloured photographs through different coloured filters to observe how the appearance of the images in the photographs appear to vary.
• Draw up a table to record the data you collect.

#### describe how absorption and reflection of radiation can provide information about the reflecting surface

• Some surfaces will reflect or absorb particular wavelengths preferentially. Observing that effect can indicate the nature of the surface. Geophysicists using remote sensing can use this property to make observations and draw conclusions about the nature of a surface without the need to actually go and have a look other than to do spot checks to determine they are correct in their conclusions.

#### explain how remote sensing techniques can be used to monitor climate, vegetation and pollution

• In explaining how remote sensing can be used in monitoring you would need to refer to at least the following:
• Aerial and satellite surveys can provide baseline data for the monitoring of the climate, vegetation and pollution.
• Weather satellites are perhaps the most spectacular success story known in satellite monitoring. Weather photographs of the Earth or particular regions are an invaluable and everyday feature of almost all television news reports. At least one pay television channel broadcasts the satellite photos of weather over Australia regularly.
• Satellites monitoring vegetation rely on the different absorption and reflectance patterns of particular bands of the electromagnetic spectrum to enable vegetation monitoring. Changes in vegetation cover can be noted through variations in the strength of reflectance patterns over time. Once a baseline pattern is established it can be compared to future reflectance patterns to ascertain change.
• Pollution, be it air or water will affect the reflectance and absorption of certain bands of the electromagnetic spectrum observed by satellites. These changes in reflectance or absorption serve as a proxy for the level of pollution. In the case of water pollution it may be possible to take photographs of the pollution. In a similar manner it may be possible to photograph visual air pollution such as smoke.

• In addition to these points it may be necessary for you to demonstrate an understanding of how a difference in reflectance of certain wavelengths might indicate certain ground cover conditions.

#### identify two uses of the remote sensing of radiation in mineral exploration

• To complete the requirements for this dot point from the syllabus you would need to refer to things similar to the following: Different minerals may absorb and reflect different bands of the electromagnetic spectrum differently. In this way and using systems such as the AVARIS satellite borne spectrometer it may be possible to detect concentrations of different minerals.

• In addition to that it is possible to use airborne surveys with extremely sensitive radiation detectors to do regional geological surveys to produce geological maps based on the difference in the gamma radiation emitted by varying concentrations of radioactive elements in different minerals. Such surveys are also capable of detecting near surface deposits of uranium in situ.

#### process information to describe the significance of Jean Richer's experiments with the pendulum in disproving the spherical Earth hypothesis

• In 1671 Jean Richer determined that as he approached the Equator the period of his survey pendulum clocks was wrong. Since the length of the pendulum was a constant the only explanation was that the gravity became stronger approaching the Equator because of the greater distance of the Earth's surface from the centre of the Earth.

#### solve problems and analyse information to calculate the mass of the Earth given g and the diameter of the Earth

• The surface gravity (g) of a body depends on the mass (M) and the radius (r) of the given body. These variables are related by the formula: where g is called the Gravitational constant (G = 6.67 X 10-11 Nm2kg2) and r is the radius of the Earth at that point.

• Example problem
• Find the mass of the Earth if the radius r of the Earth is 6378 km or 6378 000 m. The surface gravity on Earth at the Equator location is measured at 9.79000 ms-2.

Note that the gravity difference around the world at different locations would result in differences in the calculation of the mass of the Earth.

#### outline reasons why the gravitational field of the Earth varies at different points on its surface

• Earth's gravity varies from place to place because of variation in the distance to the centre of the Earth from a point on the surface or because of the difference in the density of the material beneath the point of the surface where measurements are taken. If the point of measurement is beside a mountain range such as the Himalayas then a gravitational attraction will occur between the mountain range and a body on the surface of the Earth. The resulting gravity at a point on the Earth's surface is the vector sum of the gravitational attractions experienced by the body at that point.

#### describe how the paths of satellites are used to study the Earth's gravity

• Satellites are equipped with sensitive altimeters and have two components of motion. One is acceleration toward the centre of the Earth produced by gravity and the other a sideways motion that prevents the falling satellite from hitting the Earth. As they fly vertically over different parts of the Earth where the gravity varies they will accelerate at slightly different rates resulting in their altitude relative to the Earth's surface either increasing or decreasing. The relative change in altitude is therefore a proxy for gravity strength.

• Sophisticated satellite systems such as GRACE use two satellites that are accurately positioned relative to one another by sensors. As one satellite passes a point where it accelerates faster or slower the relative difference in the position of one satellite compared to the other can be used to determine the gravity at that point.

• To see a website that describes how the GRACE satellite system works to determine gravity over the surface of the Earth see GRACE University of Texas, USA

#### solve problems and analyse information to calculate the mass of the Earth given the period and the altitude of a satellite:

• In order to do this type of calculation you must be aware that G represents the Universal gravitation constant 6.67 X 10-11 Nm2kg2 and that the period and radius of the satellites orbit from the centre of the Earth must be known.

Consider this example:

For a geostationary satellite the period of orbit is 23 hours and 56 minutes or 86160 seconds. The height of the orbit is 35 787 km which is 42 164 km from the centre of the Earth. The mass of the Earth is therefore

#### outline the structure and function of a gravimeter

• This is best done with the aid of a schematic diagram rather than with words alone.

• A gravity meter or gravimeter is designed to measure gravity to a high order of accuracy in a specific location. Mostly a gravimeter is used to make direct comparisons of the gravity in one location compared to another nearby location.

• The simplest type of gravimeter is a mass-spring system. Its operation based on simple harmonic motion in a spring. If you hang a mass on a spring the weight force (m g) acting on the mass causes the spring to stretch a certain distance (x) from its unstretched length. The length the spring stretches is dependent on a characteristic of the spring called the spring constant (k) and the size of the attached mass.

• The relationship that describes this stretching is . In other words the stretching is proportional to the weight force acting on the spring. If the mass and spring are kept the same but moved to a number of different locations where the g varies then the length of stretching due to the constant mass hanging off the end of the spring will vary as the weight force varies. The change in the relative stretching of the spring (change in x) is a direct result of any changes in g.

• To see a site that shows and describes an absolute gravimeter go to Gravimetry: Absolute Gravity National Geodetic Survey, National Oceanic and Atmospheric Administration, Department of Commerce, USA

#### describe the purpose of data reduction in gravity surveys

• When a gravity survey is done the geophysicists doing the survey are looking for very small variations in the gravity in an area due to a change in the relative density of underlying rock layers so they might find an orebody or an oil well. Absolute gravity measurements are too difficult to obtain so the geophysicist is looking for comparison of gravity at locations in the survey area.

• The purpose of gravity reduction then is to remove from data the gravity effects that are not due to subsurface geology. There are basically three types of correction: latitude, topographic, and the Eötvös correction (for a moving measuring platform such as a ship or helicopter). For the syllabus dot point we need only worry about the first two types of correction.

Remember that gravity anomalies are measured in milliGals (mGal or mgal) or gravity units (g.u.)

1mGal = 10 g.u. = 1 x 10-5ms-2 (approximately 1 x 10-6 g)

#### process information from secondary sources to reduce collected gravity data

• Due to the lack of availability of suitable raw data, simulated data have been provided.

• The data are representative of actual field data, however, it is simulated and not produced by forward modelling and as such does not claim to be correct in all aspects.

• The following spreadsheet contains simulated data as would be collected using a LaCoste and Romberg Model G gravity meter along a north – south trending survey line. The target is a massive sulphide orebody with a depth of burial of approximately 100m, a cross-section of 100m x 100m and a density of around 3.5 - 4 g/cc. The orebody has an excess mass of around 20 million tonnes.

• The data can be processed as suggested in the instructions below to satisfy this outcome dot point.
CLIENT: XYZ MINERALS
GRAVITY METER : Gxyz
Station Field Reading Time / Elapsed time (minutes) Drift corrected reading Meter scale constant Relative gravity Elevation
(m)
Latitude correction Bouguer Gravity
Easting Northing
1000 0000 2360.62 0942 (0) 2360.62 1.00799 0.00 0.00
1000 0100 2364.00 0947 (5) 2364.00 1.00799 3.41 1.00
2364.00 0949(7) 2364.00 1.00799 3.41 1.00
1000 0200 2366.10 0953(11) 2366.09 1.00799 5.51 2.00
2366.12 0955 (13) 2366.11 1.00799 5.53 2.00
1000 0300 2368.00 0959 (17) 2367.99 1.00799 7.43 3.00
2368.00 1001 (19) 2367.98 1.00799 7.42 3.00
1000 0400 2369.00 1005 (23) 2368.98 1.00799 8.43 4.00
2369.02 1007 (25) 2369.00 1.00799 8.45 4.00
1000 0500 2370.82 1011 (29) 2370.80 1.00799 10.26 5.00
2370.84 1013 (31) 2370.81 1.00799 10.27 5.00
1000 0600 2371.62 1017 (35) 2371.59 1.00799 11.05 5.50
2371.62 1020 (38) 2371.59 1.00799 11.05 5.50
1000 0700 2370.82 1024 (42) 2370.79 1.00799 10.25 5.00
2370.84 1026 (44) 2370.80 1.00799 10.26 5.00
1000 0800 2369.00 1030 (48) 2368.96 1.00799 8.41 4.00
2369.02 1032 (50) 2368.98 1.00799 8.43 4.00
2368.98 1035 (53) 2368.94 1.00799 8.39 4.00
1000 0900 2368.00 1039 (57) 2367.95 1.00799 7.39 3.00
2368.00 1041 (59) 2367.95 1.00799 7.39 3.00
1000 1000 2366.10 1045 (63) 2366.05 1.00799 5.47 2.50
2366.12 1047 (65) 2366.07 1.00799 5.49 2.50
1000 1100 2364.00 1050 (68) 2363.94 1.00799 3.35 2.00
2364.00 1053 (71) 2363.94 1.00799 3.35 2.00
1000 1200 2362.58 1115 (93) 2362.50 1.00799 1.90 1.50
2362.58 1117 (95) 2362.50 1.00799 1.90 1.50
1000 0000 2360.72 1142 (120) 2360.62 1.00799   0.00
• The first step in the reduction of the data is to remove temporal variations due to instrument drift and earth tidal variations. Normally tidal variations are removed by applying corrections calculated by a computer program such as ertide. In the case of these data where we do not have such values available, the tidal effect can be considered to be linear for periods of less than two hours. The period of the sinusoidal tidal curve is approximately 12 hours and as such can be removed together with the linear instrument drift in one calculation.

The combined drift correction =

When

f b= final base reading, f i = initial base reading, T total = elapsed time for loop in minutes and

T elapsed = elapsed time in minutes for reading being processed.

From the above data: loop time = 120 minutes (09:42 to 11:42)
Final base reading – initial base reading = 2360.72 – 2360.62 = 0.10
Therefore drift correction = -0.10/120 = -.000833 meter units / minute

The drift corrected value for station 1000E/0900N is then:

2368.00 – 08 x 57 = 2363.44 for reading taken at 1039

• The rest of the readings are processed in the same manner. Where more than one reading is taken at a station, each reading is processed and the average reduced value is used.

• Next the meter calibration constant for the range of readings is selected from a table of meter constants provided in the meter manual and the relative gravity is determined at each station assuming the value at the base station ( 0000N/1000E) is 0.00. The readings at each station are determined by subtracting the drift corrected base reading from the drift corrected station reading and multiplying the result by the meter calibration constant.

• A correction for latitude has to be applied next.
g = 978.0327(1 + 0.0053024sin2 θ - 0.0000058 sin22 θ in Gals
Differentiating we obtain
ΔgL = 0.813 sin2θ - 1.78 x 10-3 sin 4 θ mGal/km

For latitude 30 degrees south this gives a value of .704 mGal/km (note that this correction is negative with increasing latitude) and hence for the line surveyed 0.007 mGal has to be added for each 100m North of the base station.

• The Free-air and Bouguer corrections are normally combined and applied as a single elevation correction given by:-

Elevation Correction = + (0.3086 – 0.0419ρ)h
where ρ is the mean density of the near surface rocks in g.cm-3 and h is the elevation of the gravity station in metres relative to a datum level for the survey.

In these data the elevation is relative to the base station and the local near surface rock density is close to 2.2g.cm-3. (Note. Local density can be estimated by a method called Nettledon Profiling where the density that results in the reduced data bearing least resemblance to the topographic profile, yields the most correct density)

• The gravity values resulting from these corrections are Bouguer values and are all relative to each other. They can be converted to absolute by tying them to the gravity station network or by determining the absolute value of one or more of the station covered by the survey.

#### recount the steps involved in gravity data reduction - latitude correction, free air correction and Bouguer correction

• Remove all corrections using the formula .
This will enable you to determine the terrain corrected Bouguer anomaly. Once the free-air correction, latitude correction, Bouguer slab correction, and terrain corrections have accurately accounted for the variations in gravitational acceleration they produce, the remaining variations in g are associated with the terrain corrected Bouguer gravity. This is interpreted to be caused by a real variation in the geological structure beneath the survey site.

is the g value expected at the latitude at which the survey is conducted. The free air correction is Fayles correction. A high station above the base station has gravity that is too low because it is farther from centre of Earth, so g is lower. A station lower than the base station is closer to the centre of the Earth so has a g that is too high.

Latitude correction – Due to its shape (an oblate spheroid), and the rotational motion of the Earth, gravity increases as one moves from the equator to the poles. As a result, a correction must be made along a survey if the latitude changes – added as we move north in Australia and subtracted if we move south.

The symbol for the latitude correction is gø and the correction is given by

= 0.811 sin 2θ mGal km-1

where θ is the latitude. Note that the correction is a maximum at latitude 450 where it is a change of about 0.01mGal every 13m. Near Sydney the correction is about 0.07mGal/100m.

Topographic corrections

There are three types: Free-air correction, Bouguer correction and Terrain correction. Only the free-air and Bouguer corrections are dealt with here but in areas of high relief, topographic corrections are needed to take into account the relative lateral attraction of hills and valleys.

Free-Air Correction. - Gravitational attraction decreases as elevation increases. To account for this, a correction needs to be made relative to a specific altitude. This altitude (the datum level) may be sea level or the height of a station chosen by the geophysicist. If g0 is the observed gravitational attraction at the datum level then at a height h above it the observed gravitational attraction g is given by

where R is the radius of the Earth.

For small changes the free-air correction (CF) is given by

C F = 0.3086h mGal

where h is the elevation change from the datum level in metres. Note that this is a relatively large correction. It means that a geophysicist needs to know the elevation to within about 30cm if the gravity measurement is to be accurate to within 0.1mGal.

Bouguer Correction. - If a measurement is made above a datum level the effect of the rock material between the measuring station and the datum level must be taken into account. It makes a positive contribution to the gravity reading.

The Bouguer correction (CB) is given by

C B= 0.04192 X 10-3 mGal

where h is the height above the datum level in metres and ρ is the density of the rock material in kg m-3. The correction is named after Pierre Bouguer who first dealt with the effect in his measurements in the Andes in 1735. To see how the three corrections work together, consider the following diagram showing a cross section along a gravity survey line.

Location 1 is our datum level and the traverse is from south (S) to north (N).

The latitude correction is added to take account of the increasing Earth radius as we move north.

The free-air correction is added at station 2 because it is above the datum level to take the increased elevation into account. The free-air correction is subtracted at station 3 because it is at a lower elevation.

The Bouguer correction(CB) is subtracted at station 2 to remove the effect of the rock between the station and the datum level. This correction is added at station 3 to compensate for the ‘missing’ mass of the valley.

The calculations in the diagram are called simple Bouguer anomalies (Bouguer anomalies contain a terrain correction). A free-air anomaly is one which involves only the free-air and latitude corrections. This type of correction is used at sea or on flat land.

As a final note it should be recognised that in gravity surveys a drift correction is also made to account for changes in the gravity meter and tidal effects.

#### identify and describe the uses of gravity methods in resource exploration

• Gravity methods are commonly used in regional surveys particularly to identify ore bodies with high density that produce a gravity high and in surveys looking for oil traps such as low density salt domes that produce gravity lows.

• During gravity surveys all gravity readings are made relative to a selected location called a base station which is near to the area being surveyed. The aim of the survey is to determine differences in gravity over relatively short distances. Areas of equal gravity are linked with contour lines on a map to show areas of equal gravity. The map thus produced indicated the relative subsurface of the area.

• This information can be used as a proxy for rock type or geological features. It cannot tell you what rock is beneath the surface but can assist with the delineation of subsurface structures.

A good site showing photographs and fact sheets for a variety of geophysical equipment that is actually used in the field in geophysical studies and how it is used can be found at Gisco Geophysical Instruments Minneapolis, Minnesota, USA.