Gravity, Electrical, Magnetic and Seismic Methods
gravity field of the Earth can be measured by timing the free fall of an object
in a vacuum, by measuring the period of a pendulum, or in various other ways.
Today almost all gravity surveying is done with gravimeters. Such an instrument
typically consists of a weight attached to a spring that stretches or contracts
corresponding to an increase or decrease in gravity. It is designed to measure
differences in gravity accelerations rather than absolute magnitudes.
Gravimeters used in geophysical surveys have an accuracy of about 0.01 milligal
(mgal; 1 mgal = 0.001 centimetre per second per second). That is to say, they
are capable of detecting differences in the Earth's gravitational field as small
as one part in 100,000,000.
differences over the earth’s surface occur because of local density
differences between adjacent rocks. The
variations in the density of the crust and cover are presented on a 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 form a
pattern, which may be mapped as an image or by contours. The wavelength and
amplitude of the gravity anomalies gives geoscientists an idea of the size and
depth of the geological structures causing these anomalies.
Deposits of very dense and heavy minerals will also affect gravity at a
given point and produce an anomaly above normal background levels.
of exploration interest are often about 0.2 mgal. Data have to be corrected for
variations due to elevation (one metre is equivalent to about 0.2 mgal),
latitude (100 metres are equivalent to about 0.08 mgal), and other factors.
Gravity surveys on land often involve meter readings every kilometre along
traverse loops a few kilometres across. It takes only a few minutes to read a
gravimeter, but determining location and elevation accurately requires much
Gravity measurements can be obtained either from airborne (remote) or ground surveys. The most sensitive surveys are currently achieved from the ground. Variations of gravity are due to local changes in rock density and therefore depend on the type of rocks beneath the surface. Sedimentary rocks are, for example, less dense than granite, which is in turn less dense than basalt.
In most cases, the density of sedimentary rocks increases with depth because increasing pressure reduces porosity. Uplifts usually bring denser rocks nearer the surface and thereby create positive gravity anomalies. 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. Such faults, folds, and salt domes trap oil, and so the detection of gravity anomalies associated with them are crucial in petroleum exploration. Moreover, gravity measurements are occasionally used to evaluate the amount of high-density mineral present in an ore body. They also provide a means of locating hidden caverns, old mine workings, and other subterranean cavities.
contrasts of different materials are also controlled by a number of other
factors. The most important are the
grain density of the particles forming the material, the porosity of the
material, and the interstitial fluids within the material. Generally, specific
gravities of soil and shale range from 1.7 to 2.2. Massive limestone averages
2.7. While this range of values may appear to be fairly large, local contrasts
will be only a fraction of this range. A common order of magnitude for local
density contrasts is 0.25.
surveys provide an inexpensive method of determining regional structures that
may be associated with groundwater aquifers or petroleum traps. Gravity surveys
have been one of the principal exploration tools in regional petroleum
exploration surveys. Gravity surveys have somewhat limited applications in
methods are used to map variations in electrical properties of the subsurface.
The main physical property involved is electrical conductivity, which is a
measure of how easily electrical current can pass through a material. Subsurface
materials exhibit a very large range of electrical conductivity values. Fresh
rock is generally a poor conductor of electricity, but a select group of
metallic minerals containing iron, copper or nickel are very good conductors.
Layers of graphite are also very good conductors.
examples of good conductors mentioned above are quite rare. For most rocks, the
electrical conductivity is governed to a large degree by the amount of water
filling the pore spaces and the amount of salt dissolved in this water. Pure
water has a very low electrical conductivity. On the other hand, seawater, which
contains high levels of dissolved salts such as NaCl, is a relatively good
conductor of electrical current. Groundwater can vary in salt content from fresh
through brackish (slightly salty) to saline (similar in salt content to
seawater) through to hyper-saline (more salty than seawater).
Electrical conductivity of
rocks is not the only attribute which is of value to exploration geologists.
A number of different electrical properties of rocks are measured and
interpreted in mineral exploration. They depend on:
Some materials tend to become natural batteries that generate natural electric
currents whose effects can be measured. The self-potential method relies on the
oxidation of the upper surface of metallic sulfide minerals by
downward-percolating groundwater to become a natural battery; current flows
through the ore body and back through the surrounding groundwater, which acts as
the electrolyte. Measuring the natural voltage differences - usually 50-400
millivolts (mV), permits the detection of metallic sulfide bodies that lie above
the water table. Other mineral deposits that can generate self-potentials are
graphite, magnetite, anthracite, and pyritized rocks.
The passage of an electric current across an interface where conduction changes
from ionic to electronic results in a charge buildup at the interface. This
charge builds up shortly after current flow begins, and it takes a short time to
decay after the current circuit is broken. Such an effect is measured in
induced-polarization methods and is used to detect sulfide ore bodies.
Resistivity methods involve passing a current from a generator or other electric
power source between a pair of current electrodes and measuring potential
differences with another pair of electrodes. Various electrode configurations
are used to determine the apparent resistivity from the voltage/current ratio.
The resistivity of most rocks varies with porosity, the salinity of the
interstitial fluid, and certain other factors. Rocks containing appreciable clay
usually have low resistivity. The resistivity of rocks containing conducting
minerals such as sulfide ores and graphitized or pyritized rocks depends on the
connectivity of the minerals present. Resistivity methods also are used in
engineering and groundwater surveys, because resistivity often changes markedly
at soil/bedrock interfaces, at the water table, and at a fresh/saline water
The passage of current in the general frequency range of 500-5,000 hertz (Hz)
induces in the Earth electromagnetic waves of long wavelength, which have
considerable penetration into the Earth's interior. The effective penetration
can be changed by altering the frequency. Eddy currents are induced where
conductors are present, and these currents generate an alternating magnetic
field, which induces in a receiving coil a secondary voltage that is out of
phase with the primary voltage. Electromagnetic methods involve measuring this
out-of-phase component or other effects, which makes it possible to locate low-resistivity
ore bodies wherein the eddy currents are generated.
number of electrical methods described above are used in boreholes. The
self-potential (SP) log indicates mainly clay (shale) content, because an
electrochemical cell is established at the shale boundary when the salinity of
the borehole (drilling) fluid differs from that of the water in the rock.
Resistivity measurements are made by using several electrode configurations and
also by induction. Borehole methods are used to identify the rocks penetrated by
a borehole and to determine their properties, especially their porosity and the
nature of their interstitial fluids.
of the most important tools in modern mineral exploration methods is magnetic
survey. Magnetic surveys are fast,
provide a great deal of information for the cost and can provide information
about the distribution of rocks occurring under thin layers of sedimentary rocks
- useful when trying to locate orebodies.
the Earth's magnetic field interacts with a magnetic mineral contained in a
rock, the rock becomes magnetic. This is called induced magnetism. However, a
rock may itself be magnetic if at least one of the minerals it is composed of is
magnetic. The strength of a rock's
magnetism is related not only to the amount of magnetic minerals it contains but
also to the physical properties, such as grain size, of those minerals. The main
magnetic mineral is magnetite (Fe3O4) - a common mineral
found disseminated through most rocks in differing concentrations.
of the Earth's total magnetic field or of any of its various components can be
made. The oldest magnetic prospecting instrument is the magnetic compass, which
measures the field direction. Other instruments, which are appreciably more
accurate include magnetic balances, fluxgate magnetometers, proton-precession
and optical-pumping magnetometers
effects result primarily from the magnetization induced in susceptible rocks by
the Earth's magnetic field. Most sedimentary rocks have very low susceptibility
and thus are nearly transparent to magnetism. Accordingly, in petroleum
exploration magnetic surveys are used negatively - magnetic anomalies indicate
the absence of explorable sedimentary rocks. Magnetic surveys are used for
mapping features in igneous and metamorphic rocks, possibly faults, dikes, or
other features that are associated with mineral concentrations. Data are usually
displayed in the form of a contour map of the magnetic field, but interpretation
is often made on profiles.
must be remembered that rocks cannot retain magnetism when the temperature is
above the Curie point (»
500oC for most magnetic materials), and this restricts magnetic rocks
to the upper 40 kilometres of the Earth's interior.
exploring for petroleum, magnetic surveys are usually made with magnetometers
borne by aircraft flying in parallel lines spaced two to four kilometres apart
at an elevation of about 500 metres. When
searching for mineral deposits, the flight lines are spaced 0.5 to 1.0 kilometre
apart at an elevation of roughly 200 metres above the ground. Ground surveys are
conducted to follow up magnetic anomalies identified through aerial surveys.
Such surveys may involve stations spaced only 50 metres apart. A ground
monitor is usually used to measure the natural fluctuations of the Earth's field
over time so that corrections can be made. Surveying is generally suspended
during periods of large magnetic fluctuation (magnetic storms).
Seismic methods are based on measurements of the time interval between initiation of a seismic (elastic) wave and its arrival at detectors. The seismic wave may be generated by an explosion, a dropped weight, a mechanical vibrator, a bubble of high-pressure air injected into water, or other sources. The seismic wave is detected by a Geophone on land or by a hydrophone in water. An electromagnetic Geophone generates a voltage when a seismic wave produces relative motion of a wire coil in the field of a magnet, whereas a ceramic hydrophone generates a voltage when deformed by passage of a seismic wave. Data are usually recorded on magnetic tape for subsequent processing and display. Seismic methods are of two kinds - Refraction methods and Reflection methods
Seismic energy travels from source to detector by many paths. When near
the source, the initial seismic energy generally travels by the shortest path,
but as source to geophone distances become greater, seismic waves travelling by
longer paths through rocks of higher seismic velocity may arrive earlier. Such
waves are called head waves, and the refraction method involves their
interpretation. From a plot of travel time as a function of source to geophone
distance, the number, thicknesses, and velocities of rock layers present can be
determined for simple situations. The assumptions usually made are that:
velocity values determined from time-distance plots depend also on the dip
(slope) of interfaces, apparent velocities increasing when the geophones are
updip from the source and decreasing when downdip. By measuring in both
directions the dip and rock velocity, each can be determined. With sufficient
measurements, relief on the interfaces separating the layers also can be
bodies of local extent can be located by fan shooting. Travel times are measured
along different azimuths from a source, and an abnormally early arrival time
indicates that a high-velocity body was encountered at that azimuth. This method
has been used to detect salt domes, reefs, and intrusive bodies that are
characterized by higher seismic velocity than the surrounding rock.
Seismic waves may be used for various other purposes. They are employed,
for example, to detect faults that may disrupt a coal seam or fractures that may
allow water penetration into a tunnel.
seismic work utilizes reflection techniques. Sources and geophones are
essentially the same as those used in refraction methods. The concept is similar
to echo sounding - seismic waves are reflected at interfaces where rock
properties change. The round-trip
travel time, together with velocity information, gives the distance to the
interface. The relief on the interface can be determined by mapping the
reflection at many locations. For simple situations the velocity can be
determined from the change in arrival time as source to geophone distance
practice, the seismic reflection method is much more complicated. Reflections
from most of the many interfaces within the Earth are very weak and so do not
stand out against background noise. The reflections from closely spaced
interfaces interfere with each other. Reflections from interfaces with different
dips, seismic waves that bounce repeatedly between interfaces
("multiples"), converted waves, and waves travelling by other modes
interfere with desired reflections. Also, velocity irregularities bend seismic
rays in ways that are sometimes complicated.
objective of most seismic work is to map geologic structure by determining the
arrival time of reflectors. Changes in the amplitude and waveshape, however,
contain information about stratigraphic changes and occasionally hydrocarbon
accumulations. In some cases, seismic patterns can be identified with
depositional systems, unconformities, channels, and other features.
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. In most exploration
programs appreciably more money is spent on seismic reflection work than on all
other geophysical methods combined.
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