Brief Summary on Geophysical methods

• Geophysics and Geophysical prospecting

Collected by:Jyoti AnischitMsc in engineering GeologyTU

INTRODUCTION• Definition Geophysical prospecting is the study of the structure

of the earth’s crust by physical methods for the location and surveying of minerals and ores.

It is an integral part of geophysics.

OBJECTIVES• To determine lithology of the area.• To identify and locate exploitable reservoirs.• To detect and delineate geothermal resources.• Estimate geophysical properties of the geothermal

system.• To determine the ground water conditions in the

regions.

CONCEPT OF GEOPHYSICAL EXPLORATION

Exploration geophysics DefinitionIs an applied branch of geophysics, which uses physical methods at the surface of the Earth to measure the physical properties of the subsurface, along with the anomalies in those properties.Geophysical methods include;

Seismic, Gravitational, Magnetic, ElectricalElectromagnetic

CONCEPT OF GEOPHYSICAL EXPL CONT……These methods identify resources without the need for sampling. Usually undertaken with minimal surface disturbance. The survey used in these methods is called Geophysical survey.Geophysical surveys can be conducted either from ;The air, On the ground orDown drill holes.

CONCEPT OF GEOPHYSICAL EXPL CONT….Airborne geophysical surveysMay comprise of magnetic, radiometric, gravity

or electromagnetic methods. These surveys provide general geological

information for an area and are often used in the initial stages of exploration.

These surveys are typically undertaken using low flying helicopters or light aircraft which fly in a grid pattern.

In airborne survey, instruments may be either mounted on the aircraft or towed underneath.

The aircraft may fly between 25 and 60m above the ground, with flight lines spaced between 25 and 200m apart.

CONCEPT OF GEOPHYSICAL EXPL CONT….DOWN HOLE SURVEYS These geophysical surveys involve putting

geophysical equipment down exploration drill holes to gather magnetic, radiometric or electrical information from the rocks adjacent to the hole.

Also used to determine the exact path of the drill hole.

Occasionally tools with a small radiometric source may be used and a detailed risk assessment is required to ensure the tool is not lost down hole.

magnetic survey,Seismic surveys, Magnetic surveys, Radiometric

surveys,

Ground based surveys There are a number of types of geophysical

surveys which are undertaken on the ground.These are include;

Gravity surveys,Induced Polarization

(IP) surveys, Electromagnetic

(EM) surveys ,

• CONCEPT OF GEOPHYSICAL EXPL CONT….

Note method used for data collection in above survey is called geophysical methods.

SEISMIC METHODThe common geophysical method used in ExplorationI. SEISMIC METHODDefinitionSeismic method refer to method which use

seismic velocities to measure the elastic properties of Earth’s material.

These techniques provide detailed information about subsurface layering and subsurface mechanical properties using seismic waves.

Seismic waves propagate through a rock body at a velocity which is governed by elastic properties (stress & strain, Young’s modulus) and density of the geological formation.

SEISMIC METHOD CONT..A seismic source-such as sledgehammer is

used to generate seismic waves, sensed by receivers deployed along a present geometry (called receiver array), and then recorded by a digital device called seismograph.

Diagram showing seismograph and geophone array

Types of seismic wavesThere are two groups of seismic waves such as;

i. Body waves andii. Surface waves.

• Body waves- Are the waves that can propagate through the internal volume of an elastic solid materials.

• Body wave is divided into two types which are;

Compressional waves (Longitudinal, primary or P-wave)

Shear waves (Transverse, secondary or S-waves).

SEISMIC METHOD CONT…P-wave travel with a velocity that depends

on the elastic properties of the rock through which they travel.

P-wave has the highest velocity than all seismic waves and thus will reach all the seismograph first.

Particle motion associated with the passage of a compressional wave involves oscillation, about a fixed point, in the direction of wave propagation.

S-waves travel through material by shearing it or changing its shape in the direction perpendicular to the direction of travel.

S-waves travel slower than the p-wave as reach at the seismograph after p-waves.

SEISMIC METHOD CONT…. Surface waves are the waves which

propagate along the boundary of the solid material on the earth.

Surface behave like S-waves in that they cause up and down and side to side movement as they pass, but they travel slower than S-waves and do not travel through the body of the Earth.

These waves are of two types which are; • Love waves• Rayleigh waves

SEISMIC METHOD CONT……. Rayleigh waves propagate along a free

surface, or along the boundary between two dissimilar solid media, the associated particle motions being elliptical in a plane perpendicular to the surface and containing the direction of propagation surface waves.

Love waves are polarized shear waves with a particle motion parallel to the free surface and perpendicular to the direction of wave propagation.

SEISMIC METHOD CONT… Body wave and Surface wave with

wavelength.

SEISMIC METHOD CONT…….. SEISMIC WAVE VELOCITY Additional factors which influence

compressional and shear wave velocities are;LithologyExtent of fracturesTemperatureFluid contentSaturation and Fluid pressure in the subsurface

SEISMIC METHOD CONT……..• Typical rock velocityType of formation P-wave velocity m/s S-wave velocity m/s Density g/cc

Scree, vegetal soil 300-700 100-300 1.7-2.4

Dry sands 400-1200 100-500 1.5-1.7

Wet sands 1500-2000 400-600 1.9-2.1

Saturated shales and clays 1100-2500 200-800 2.0-2.4

Marls 2000-3000 750-1500 2.1-2.6

Porous and saturated sst 2000-3500 800-1800 2.1-2.4

Limestone 3500-6000 2000-3300 2.4-2.7

Chalk 2300-2600 1100-1300 1.8-3.1

Salt 4500-5500 2500-3100 2.1-2.3

Dolomite 3500-6500 1900-3600 2.5-2.9

Granite 4500-6000 2500-3300 2.5-2.7

SEISMIC METHOD CONT……..Type of formation P-wave velocity m/s S-wave velocity m/s Density g/ccBasalt 5000-6000 2800-3400 2.7-3.1Gneiss 4400-5200 2700-3200 2.7-3.1Coal 2200-2700 1000-1400 1.3-1.8Water 1450-1500 - 1.0Ice 3400-3800 1700-1900 0.9Oil 1200- 1250 - 0.6-0.9

Note: The Acoustic impendance is defined as the product of the density and P-wave velocity within a rock unit.When elastic wave encounters an acoustic impendance boundary, a portion of the wave energy is Reflected off of the boundary and a portion is Refracted into the second medium, according to Snell’s law.

SEISMIC METHOD CONT…….

At the critical angle for each interface (energy refracted 90 degrees), the seismic wave will travel along the interface with a velocity of the underlying layer.

Since P-waves are the fastest portion of the seismic wave, they represent the first arriving energy at each geophone (either direct or refracted).

SEISMIC METHOD CONT……. Seismic Reflection methodSeismic reflection is the primary geophysical

method used in oil and gas exploration and operated on density and elastic module of subsurface materiatials.

The seismic reflection method usually gives better resolution (i.e., makes it possible to see smaller features) than other methods, with the exception of measurements made in close proximity, as with borehole logs.

Seismic Refraction methodSeismic refraction is a method to determine

the P-wave velocity structure of the subsurface.

SEISMIC METHOD CONT…….The refraction method has been used in

mineral investigations to map low-velocity alluvial deposits such as those that may contain gold, tin or sand and gravel.

Application in geo-environmental work include studying the structure, thickness, and hydrology of tailings and extent of Acid mine drainage around mineral deposit.

Note;Reflection seismic methods provide fine

structural detail and refraction methods provide precise estimates of depth to lithologies of different Acoustic impendance.

SEISMIC METHOD CONT…….Seismic data and their acquisition, processing, and interpretation.

Digital seismic data Seismic data are physical observations,

measurements, or estimates about seismic sources, seismic waves, and their propagating media.

The form of seismic data varies, and can include analog graphs, digital time series, maps, text, or even ideas in some cases.

We focus on the analysis of data on body waves, mostly P-waves, in their transmission, reflection, diffraction, refraction, and turning processes.

SEISMIC METHOD CONT……. Relationship between data acquisition,

processing, and interpretation.

SEISMIC METHOD CONT……. Data acquisition

Seismic data acquisition in the energy industry employs a variety of acquisition geometries.

Example of the typical onshore acquisition geometry using dynamite or Vibroseis technology as sources and geophones as receivers.

SEISMIC METHOD CONT…….Panel (a) shows a split spread, using a shot

located in the middle and many receivers spread around it.

(b) shows an end-on spread, with a shot located at one end and all receivers located on one side of the shot.

Quality control, or QCThis involves checking the survey geometry,

data format, and consistency between different components of the dataset, and assuring that the quality and quantity of the dataset are satisfactory for the study objectives.

The data QC process is typically part of the pre-processing.

SEISMIC METHOD CONT……. Data processingTo identify and enhance the desired signal by

suppress various kinds of noise in the data.The goal of seismic data processing is to help

interpretation, the process of deciphering the useful information contained in the data.

Data interpretations and Seismic modelingThis is based on processed data and seismic

imaging conducted to produce various forms of imagery for the interpretation process.

Seismic modeling done after data interpretation and generate predictions to compare with the real measurements, and thus verify the interpretation.

SEISMIC METHOD CONT…….

SEISMIC METHOD CONT……. APPLICATION OF SEISMIC METHODi. Its employed for hydrocarbon exploration

By providing a high resolution map of acoustic impedance and petroleum geologists and geophysicists interpret potential petroleum reservoirs easily.

ii. Its used in mineral exploration By Identifying massive sulphide bodies since massive

sulphide bodies have higher seismic velocities and densities than their host rocks.

iii. Used to predicts the depth, thickness of geologic strata, and structural

MAGNETIC METHOD MAGNETIC METHOD Definition Magnetic method is the method used to

measure the variations of the Earth’s magnetic field due to the presence of magnetic minerals. or

It is used to investigate subsurface geology on the basis of anomalies in the Earth’s magnetic field resulting from the magnetic properties of the underlying rocks.

Earth's magnetic field refer to magnetic field that extends from the Earth's interior to where it meets the solar wind, a stream of charged particles emanating from the Sun.

Thus why the Earth’s magnetic field varies with time (it is not constant).

It believed that the earth’s magnetic field is originates from the liquid outer core of the Earth containing high concentration of iron.

As the Earth rotates, the outer layers of the ionosphere interact with the solar wind to cause minor fluctuations in the magnetic field.

• MAGNETIC METHOD CONT…….

MAGNETIC METHOD CONT……. If the rocks are magnetic (have high

susceptibility) they become magnetized, and their field adds to that of the earth. Thus the total magnetic field is stronger over magnetic rocks.

Magnetic fields are measured in Nanoteslas (nT), which used to be called gammas.

Magnetic rocks contain various combinations of induced and remanent magnetization that perturb the Earth's primary field.

The magnitudes of both induced and remanent magnetization depend on the quantity, composition, and size of magnetic-mineral grains.

MAGNETIC METHOD CONT……. In order for something to be magnetic, its

dipoles must be aligned with each other, so that they face the same direction.

The direction they face create a North end, while the opposite end creates a South end.

Some substances, known as ferromagnetic substances, have permanently aligned dipoles.

A magnetic high anomaly is where the measured field strength is higher than the value predicted by the global model, and a magnetic low is where the measured field strength is lower than the value predicted by the global model.

MAGNETIC METHOD CONT……. Anomalies in the earth's magnetic field are

caused by induced or remanent magnetism. This anomaly created when the Earth’s magnetic

field is disturbed by an object that can be magnetized.

To measure anomalies, magnetometers need a sensitivity of 10 nT or less.

Induced magnetic anomalies are the result of secondary magnetization induced in a ferrous body by the earth’s magnetic field.

MAGNETIC METHOD CONT……. Common causes of magnetic anomalies

include dykes, faults and lava flows. Where the rocks have high magnetic

susceptibility, the local magnetic field will be strong; where they have low magnetic susceptibility, it will be weaker.

Magnetic gradient anomalies generally give a better definition of shallow buried features such as buried tanks and drums, but are less useful for investigating large geological features.

Sedimentary rocks generally have a very small magnetic susceptibility compared with igneous or metamorphic rocks, which tend to have a much higher magnetite content.

MAGNETIC METHOD CONT…….• INSTRUMENTATION A magnetometer is a more complex instrument

which measures both the orientation and strength of a magnetic field.

Magnetometer surveys measure small, localised variations in the Earth's magnetic field.

Magnetometers are highly accurate instruments, allowing the local magnetic field to be measured to accuracies of 0.002%.

There are several types of instruments on the market but the common ones used for commercial applications are the Proton precession, Fluxgate, Caesium vapour and Gradiometer magnetometer systems.

MAGNETIC METHOD CONT……. Planning of magnetic survey Once you have considered all the factors as

to the type of magnetometer survey required, then you are ready to design and lay out a grid to cover the area of interest

A survey grid usually consists of a base line and one or several tie lines.

The base line serves as a zero reference line for the grid, and the tie lines serve to correct the skewness of the survey lines.

From the base line are drawn survey lines perpendicular to the base line.

MAGNETIC METHOD CONT……. The following figure below illustrates a

typical survey grid, with base lines at 0 and 40 and survey lines every two metres.

MAGNETIC METHOD CONT……. DATA ACQUISITION Ground magnetic measurements are usually

made with portable instruments at regular intervals along more or less straight and parallel lines which cover the survey area.

Often the interval between measurement locations (stations) along the lines is less than the spacing between lines.

To obtain a representative reading, the sensor should be operated well above the ground.

In rocky terrain where the rocks have some percentage of magnetite, sensor heights of up to 4 m have been used to remove near-surface effects.

MAGNETIC METHOD CONT……. All field readings should be taken twice and

the two readings should differ by no more than 1 nT.

At each station the location, time and reading must be recorded, as well as any relevant topographic or geological information and details of any visible or suspected magnetic sources.

Modern instruments can be linked to a DGPS so that map coordinates are automatically recorded against the magnetic reading.

The survey data collected should be corrected at the end of the survey day or the end of the grid.{see Data processing in detail}

MAGNETIC METHOD CONT……. DATA PROCESSING To make accurate magnetic anomaly maps,

temporal changes in the earth’s field during the period of the survey must be considered.

Normal changes during a day, sometimes called diurnal drift, are a few tens of nT but changes of hundreds or thousands of nT may occur over a few hours during magnetic storms.

Culture noises like buried metal, railroads, pipelines, bridges and iron fences result error in magnetic data.

There are three ways in which you can remove errors from magnetic data :

MAGNETIC METHOD CONT…….i. Use a base station magnetometer to

record all the changes in time and then use this data to remove the change from the readings in the field magnetometer. This is the most accurate way of doing it, but also it is more expensive, as two complete instruments are required.

If time is accurately recorded at both base site and field location, the field data can be corrected by subtraction of the variations at the base site.

MAGNETIC METHOD CONT…….ii. Use a tie-point method while doing the

total field survey. This assumes that the field is changing slowly and

evenly between the first time you measured the value at a station and the next time you check-in to that station again.

This method is not as accurate as using a base-station, but if the field is not changing rapidly, it is quite adequate to locate an anomaly.

Generally the data correction is done automatically while the survey is carried out in the tie-line mode.

MAGNETIC METHOD CONT…….iii. Perform a vertical gradient survey.

Since you are measuring the rate of change between two sensors, any changes in the background field will apply to both sensors and you will not see any of these noise effects.

This technique is quite effective for near-surface anomalies.

Gradiometers measure the magnetic field gradient rather than total field strength, which allows the removal of background noise.

MAGNETIC METHOD CONT……. After all corrections have been made, magnetic

survey data are usually displayed as individual profiles (Figure 1) or as contour maps (Figure 2). Below is a magnetic reading profile across a dyke

MAGNETIC METHOD CONT……. Magnetic Survey contour map to locate pits

containing buried metallic Containers. Figure 2.

MAGNETIC METHOD CONT……. DATA INTERPRETATION Data are usually displayed in the form of a

contour map of the magnetic field, but interpretation is often made on profiles.

From these maps and profiles geoscientists can locate magnetic bodies (even if they are not outcropping at the surface), interpret the nature of geological boundaries at depth, find faults etc.

Like all contoured maps, when the lines are close together they represent a steep gradient or change in values.

When lines are widely spaced they represent shallow gradient or slow change in value.

MAGNETIC METHOD CONT……. A modern technique is to plot the magnetic

data as a color image (red =high, blue=low and all the shades in between representing the values in between).

When interpreting the aeromagnetic image it is useful to know that magnetite is found in greater concentrations in igneous and metamorphic rocks.

Magnetite can also be weathered or leached from rocks and re-deposited in other locations, such as faults.

In a geothermal environment, this is a very useful feature as it may indicate the presence of faults, target for drilling.

MAGNETIC METHOD CONT……. Example of magnetic map intensity

MAGNETIC METHOD CONT……. In figure above shows a total field magnetic

map from an area. The magnetic survey outlined three distinct

zones where the magnetic field ranges from a low of 150 nT (blue areas) to a high of 500 nT (red areas).

The central part of the grid (blue area) has the lowest magnetic expression, whereas the areas to the grid north and grid south (yellows through reds) are more magnetic.

Generally the magnetic maps are interpreted in terms of geology therefore on correlating with magnetic map need to have or know rocks present at the area (or collect samples).

MAGNETIC METHOD CONT……. Magnetic maps are interpreted in terms of

geology. As low-susceptibility rocks such as limestones

show as areas of low and relatively uniform magnetic fields, while mafic and ultramafic rocks show as areas of higher and more variable magnetic fields.

In geophysical displays, deep blues denote the lowest values, while reds and purples identify the highest readings.

MAGNETIC METHOD CONT……. APPLICATION OF MAGNETIC METHOD There are two primary applications for

magnetic measurements:i. Locating and mapping buried ferrous

metals, and Magnetic measurements can be used for locating and mapping buried ferrous metals (e.g. waste, drums or underground structures and utilities).

ii. Mapping geologic structures-Magnetic measurements can be used for geologic mapping by responding to the magnetic susceptibility of soil and rock.

GRAVITY METHOD GRAVITY METHOD Definition Gravity Survey refer to the survey used to

measures the change of rock density by looking at changes in gravity.

Like all matters, the earth generates gravity field that can be measured by instrumentation called gravimeter.

The gravimeters are used to precisely measure variations in the gravity field at different points of the earth.

The strength of the gravitational field is directly proportional to the density of subsurface materials.

The typical units of gravity field is milligas or gravity units.

GRAVITY METHOD CONT……. Gravity measurements can be obtained either

from airborne (remote) or ground surveys. The most sensitive surveys are currently

achieved from the ground. The force of gravity is not the same all over

the world (it varies from point to point on the Earth).

Things like Mountains, Ocean trenches, tidal movements, even large buildings, Structures, and Composition of elements within the Earth’s crust all cause micro-variation in gravity all over the world.

GRAVITY METHOD CONT……. INSTRUMENTATION A gravity meter or gravimeter measures the

variations in the earth's gravitational field. Gravity instruments require careful levelling

before a reading is taken. This may have to be done manually or it may

be performed by the instrument itself. The instrument must be placed on solid

ground (or a specially designed plate) so that it does not move or sink.

The common gravimeters on the market are the Worden gravimeter, the Scintrex and the La Coste Romberg gravimeter.

GRAVITY METHOD CONT……. The Worden gravimeter is an entirely

mechanical and optical relying only on an AA battery for illuminating the crosshairs.

The Scintrex Autograv is semi-automated, but although a bit more expensive, it has been shown to have a higher stability and experience less tares (a sudden jump in a gravity reading) over long periods of time.

The Lacoste and Romberg model gravity meter has an advertised repeatability of 3 microgals (980,000,000 microgals is the Earth's gravitational field) and is one of the preferred instruments for conducting gravity surveys in industry.

GRAVITY METHOD CONT……. DATA ACQUISITION Gravity data acquisition is a relatively simple

task that can be performed by one person or two when determining location (latitude, longitude and elevation) of the gravity stations.

Surveys are conducted by taking gravity readings at regular intervals along a traverse that crosses the expected location of the target.

Its expected size will determine the distance between readings (station spacing), with larger station separations for large target and small separations for small ones.

GRAVITY METHOD CONT……. Obtaining a gravity reading, a horizontal

position and the elevation of the gravity station must be obtained.

Therefore readings are taken by placing the instrument on the ground and levelling it and this may be automatic with some instruments, as it is with the Scintrex.

The observed gravity readings obtained from the gravity survey reflect the gravitational field due to all masses in the earth and the effect of the earth’s rotation.

Thus the useful gravity values in detailed surveys is to determine the Earth tide effect as their gravitational effects may be greater than the gravity field variations due to the anomalous features being sought.

GRAVITY METHOD CONT……. DATA PROCESSING The last task of most fieldwork is to

determine the topographic changes and the effects of buildings surrounding a gravity station.

Both of these effects will be used later in processing the gravity data.

There are a number of techniques to determine the elevation changes and these usually involve a combination of recording elevation changes in the field and computer computations using digital elevation models (DEM).

GRAVITY METHOD CONT……. The most common technique is by Hammer

where one records an elevation change in quadrants at set distances (commonly from 0 to 1000 meters) from the gravity station.

To interpret gravity data, one must remove all known gravitational effects not related to the subsurface density changes.

Each reading has to be corrected for elevation, the influence of tides, latitude and, if significant local topography exists, a topographic correction.

GRAVITY METHOD CONT……. Gravity data correction Because the “weight” of a mass varies with the

square of the distance to the center of the earth, small variations in elevation of the survey station can greatly affect the measured value of gravity.

As a result, it is necessary to correct for changes in location and elevation of the survey stations.

i. Latitude correctionBecause the earth rotates, it bulges at the

equator, and flattens at the poles. Thus gravity is stronger at the poles than at the

equator and increases with increasing in Latitude.

GRAVITY METHOD CONT…….Correction is applied ONLY for relative movement

in the N-S direction. Correction is added as we move toward the

equator and subtracted as the survey station moves northward.

Latitude correction accounts for rotation and elliptical shape of the earth.

ii. Free-air correction.The free-air correction accounts for gravity

variations caused by elevation differences in the observation locations.

Accounts for the 1/r2 decrease in gravity with distance from the centre of the Earth.

GRAVITY METHOD CONT…….If a station is above the reference datum, the

correction is added to the reading; if the station is below the reference datum, then the correction is subtracted from the reading.

Free air gravity anomaly is given; gfa = gobs - gn + 0.3086h ; Where h is the elevation (in meters)iii. Bouguer correction

The difference between observed gravity (gobs) and theoretical gravity at any point on the Earth's surface after reducing the gravity readings to the geoidal surface is known as the Bouguer gravity anomaly or Bouguer gravity.

GRAVITY METHOD CONT…….This correction accounts for the attraction of

material lying between the station and the reference datum.

The Bouguer correction is {0.04192 x (density of the intervening material)} per vertical meter.

If the density of the intervening material is the average rock density of 2.67 grams per cubic centimeter (g/cc) then the Bouguer correction is 0.112 mgal/meter.

It is applied in the opposite sense to the free air correction and correction is Subtracted for stations above the datum and is added for stations below the datum.

GRAVITY METHOD CONT…….iv. Terrain correction

If the gravity reading is taken on top of a hill, then there is a deficit of mass on either side of the hill, compared to a horizontal ground surface.

The figure below need for correction for topography.

GRAVITY METHOD CONT…….The terrain correction is positive regardless of

whether the local topography consists of a mountain or a valley.

Terrain corrections are complex, and, in pre-computer days, involved enormous amounts of hand calculation.

DATA ANALYSIS AND INTERPRETATION The final gravity data are usually plotted and

contoured in the same manner as magnetic data.

The grid of gravity values, contour maps or the gravity profiles can be used to determine the lateral location of any gravity variations and thus quantify the nature (depth, geometry, density) of the subsurface feature.

GRAVITY METHOD CONT……. The map generated from gravity data and

after correction is called gravity anomaly map

A gravity anomaly map looks at the difference between the value of gravity measured at a particular place and the predicted value for that place.

Gravity anomalies are computed by subtracting a regional field from the measured field, which result in gravitational anomalies that correlate with source body density variations.

Positive gravity anomalies are associated with shallow high density bodies, whereas gravity lows are associated with shallow low density bodies.

GRAVITY METHOD CONT……. Thus, deposits of high-density chromite,

hematite, and barite yield gravity highs, whereas deposits of low-density halite, weathered kimberlite, and diatomaceous earth yield gravity lows.

Uplifts usually bring denser rocks nearer the surface and thereby create positive gravity anomalies {denser rock like-Basalt-Granite-Sandstone}.

Faults that displace rocks of different densities also can cause gravity anomalies.

Salt domes generally produce negative anomalies because salt is less dense than the surrounding rocks.

GRAVITY METHOD CONT……. Example of positive and Negative anomaly for anticline (oil) and salt dome

GRAVITY METHOD CONT…….

Contoured data with latitude, free air and Bouguer corrections.

The figure below shows a contour map of gravity data.

The data set has had Latitude, Free air and Bouguer corrections.

GRAVITY METHOD CONT……. Often gravity results such as these will show a

gradual trend across the area, which will distort local anomalies.

The gradual trend in gravity values can arise from deep seated geological variations, and can be removed from the data.

The high gravity at the west end and the low at the east end both disappear when a linear regional trend is removed.

Normally such trend removal is carried out by digital processing.

GRAVITY METHOD CONT…….

Gravity data of with linear regional trend removed from data.

GRAVITY METHOD CONT……. Sulphide bodies are usually more dense than

the host rocks, and thus yield local high-gravity anomalies.

Graphite, on the other hand, is less dense than sulphides, and close in density to typical host rocks.

If the survey area is underlain by rocks of different densities, then gravity can be used to map the distribution of each rock type.

Where positive anomalies indicate rocks with high density than crustal average.

Negative anomalies indicate rocks with low density than the crustal average.

GRAVITY METHOD CONT……. APPLICATION OF GRAVITY METHOD Typical applications for the gravity profiling

include:i. Mapping karst topography or other

subsurface cavities (natural or man-made)ii. Map regional geologic structuresiii. Map basement topography and sediment

thicknessiv. Exploration for geothermal energy including the

location of heat sources.

ELECTROMAGNETIC METHOD ELECTROMAGNETIC METHOD Definition Electromagnetic method provide a means

to measure subsurface electrical conductivity and to identify subsurface metal objects.

Electrical conductivity is a function of soil and rock type, porosity and permeability, as well as the composition of fluids that fill the pore spaces.

Electrical conductivity values are given in units of milliSiemens/meter (mS/m).

The higher the conductivity, the more current will flow in the earth for a given electrical field strength.

The higher the resistivity, the less current will flow for a given electrical field strength.

At low frequencies, conductivity and resistivity are inversely related:

SYSTEM AND OPERATION OF EM The electromagnetic method consists of

Transmitter coil and Receiver coil. An AC electric current is applied to a

transmitter coil This generates a primary electromagnetic

(EM) field in the coil or a large loop on surface.

This induces small electric currents in the ground, generating a secondary magnetic field that can be picked up by a receiver coil. {Please see the illustration below}.

The receiver measures two quantities, the in-phase component and the quadrature component of the secondary field, expressed as a percentage of the primary field at the receiver.

Anomalies from good conductors have large in-phase and small quadrature components, while weaker conductors have low in-phase and high quadrature components.

Normally the primary field is much stronger than the secondary field.

In order to detect the secondary field, a small part of the primary field is sent from the transmitter via cable to the receiver, and is used to cancel the primary field at the receiver, leaving only the secondary field to be detected.

Frequency domain electromagnetic methods (FDE)

Measure the electrical conductivity of soil and rock by measuring the magnitude and phase of an induced electromagnetic current (ASTM D6639-01).

FDE Provides measurements with depths ranging from a few feet to 200 feet.

The three common frequency domain electromagnetic instruments are EM31, EM34, and EM38 as manufactured by Geonics, Ltd.

Time domain electromagnetic (TDEM) methods

Measure the electrical conductivity of soil and rock by inducing pulsating currents in the ground with a transmitter coil and monitoring the decay of the induced current over time with a separate receiver coil (ASTMD6820-02).

The EM47 and EM57 systems manufactured by Geonics, Ltd. are common TDEM systems.

TDEM penetrate at a depth range of approximately 20 to 3,000 feet.

Very Low Frequency (VLF) measurements

These measurements are made by measuring the distortions of a VLF wave from a distant transmitter.

Distortions of the VLF wave occur due to a local increase in electrical conductivity usually found within fractures.

The increase in electrical conductivity is a function of the conductive material, such as water, clay or minerals within the fracture.

NOTE: Measurements are easily and rapidly made can provide relatively deep measurements (hundred of feet).

EM data is typically collected as point readings of ground conductivity or in-phase taken at regular intervals along a survey grid that has been set out over the site area.

The spacing of the grid-lines and reading stations is dependent upon the target size.

Generally smaller targets require closer survey lines and denser spaced readings.

PROCESSING & INTERPRETATIONThe site data is recorded on a digital data logger

for later downloading to a PC for post-survey processing and interpretation.

The most commonly used interpretation procedure is contouring, carried out with specialist interactive software to produce contour plans.

The contoured data is analyzed in detail by experts to identify anomalous features relative to the general background.

Once identified, the anomalies are correlated with local ground conditions.

APPLICATION OF ELECTROMAGNETIC METHOD1. Mineral exploration - metallic elements are

found in highly conductive massive sulfide ore bodies.

2. Groundwater investigations - groundwater contaminants such as salts and acids significantly increase the groundwater conductivity.

3. Stratigraphy mapping - rock types may have different conductivities.

4. Geothermal energy - geothermal alteration due to hot water increases the conductivity of the host rock.

5. Environmental - locate hazards such as drums and tanks.

6. Locating abandoned mineshafts, crown holes & subsidence features