The earth has it's own physical properties such as different rock
densities and layers of strata; rocks that are conductive or radioactive
to varying degrees and local variations of the gravitational or magnetic
Mineral and hydrocarbon desposts are rare events occupying a tiny portion
of the earth's crust.
Consequently the physical properties where they are located are out of the
ordinary - referred to by the term "anomalous"
Geophysical exploration involves the search for these anomalies in the
When we measure geophysical properties the results can give us indirect
insight into the geology and potential of mineral and hydrocarbon deposits
hidden benearth the surface.
An understanding of geophysics such as seismic, gravitational, magnetic,
electrical and electromagnetic at the surface of the Earth provides us the
means to measure the physical properties of the subsurface, along with the
anomalies in those properties. Uses
Exploration geophysics is most often used to detect or infer the presence
and position of economically useful geological deposits, such as ore
minerals; fossil fuels and other hydrocarbons; geothermal reservoirs; and
Detection illuminates the target style of mineralization, via measuring
its physical properties directly.
measure the density contrasts between the dense iron ore and the
lighter silicate host rock, or you may measure the electrical
conductivity contrast between conductive sulfide minerals and the
resistive silicate host rock.
Geophysics and Subsurface Mapping
Geophysics may also used to map the subsurface structure of a region, to
elucidate the underlying structures, spatial distribution of rock units,
and to detect structures such as faults, folds and intrusive rocks.
This is an indirect method for assessing the likelihood of ore deposits or
Geophysics may also be used for:
Seismic surveys measure vibration as it passes through the Earth. This is
done using a series of geophones (sensors connected to wires) placed using
handheld tools and arranged in an array or specific pattern. This gives
information about the properties of the rocks, often down to depths of
several kilometres. The vibrations may be induced using truck-mounted
vibrating weights or small explosives. Seismic surveys are particularly
suited to specific geological forms including flat-lying sedimentary
basins. The three methods commonly used for exploration, engineering, and
environmental applications are:
Seismic refraction surveying consists of an in-line geophone array
recording the arrival time of subsurface acoustic energy head-waves.
The waves are generated from both ends of, and at intermediate
locations within the geophone array.
Common energy sources used for shallow surveys include striking a
sledgehammer against a steel plate on the ground, or black powder
blank explosives (e.g. buffalo gun) triggered in shallow holes (e.g.
For engineering surveys, between 12 and 48 geophones are commonly used
to record ground movement at surface.
Depth estimates for refracted energy arrivals can be calculated using
the measured head-wave arrival times and known geophone array
geometry. Seismic refraction is effective in areas where acoustic
energy propagation velocities increase with depth and where there is a
significant velocity contrast between upper layer material (e.g.
alluvial sediments) overlying more competent lower layer material
(e.g. bedrock, ablation till).
The primary limitation of seismic refraction is that it is dependent
upon the assumption (for a given survey area) that signal velocities
will increase with depth as head-waves propagate through the ground.
In instances where low-velocity layers are buried below high-velocity
layers (e.g. gravel deposits buried below ablation till sediments) the
method will fail to detect the low velocity layer (i.e. hidden layer)
and produce erroneous refractor arrival depth estimates.
An additional limitation is that the method requires proper coupling
between the geophones and the ground. In areas such as cobble and
boulder bars it may not be possible to obtain good ground coupling.
Over active channel areas, refraction survey data is prone to
significant noise interference from moving water and in most instances
data collection is logistically impractical to obtain.
A further requirement of refraction surveys is that the geophone array
length must at least 5 to 10 times the distance to the target depth
zone of interest. The geophone array must also be in a straight-line
to avoid errors in calculated refractor depths.
Seismic reflection techniques are the most widely used geophysical
technique in hydrocarbon exploration. They are used to map the
subsurface distribution of stratigraphy and its structure which can be
used to delineate potential hydrocarbon accumulations. Seismic
reflection maps contrasts in seismic impedance, which is the product
of seismic velocity (the speed at which seismic waves are transmitted
by the soil or rock) and density. The “reflection coefficient” – the
amount of energy reflected at a boundary – is directly proportional to
the difference in seismic impedance. Seismic reflection is much more
sensitive to subtle changes and is capable of imaging much finer
detail than seismic refraction. However, data acquisition and
processing are significantly more complex.
Multichannel Analysis of Surface Waves (MASW)
multichannel analysis of surface waves (MASW) method is one of the
seismic survey methods evaluating the elastic condition (stiffness) of
the ground for geotechnical engineering purposes. MASW first measures
seismic surface waves generated from various types of seismic
sources—such as sledge hammer—analyzes the propagation velocities of
those surface waves, and then finally deduces shear-wave velocity (Vs)
variations below the surveyed area that is most responsible for the
analyzed propagation velocity pattern of surface waves. Shear-wave
velocity (Vs) is one of the elastic constants and closely related to
Young’s modulus. Under most circumstances, Vs is a direct indicator of
the ground strength (stiffness) and therefore commonly used to derive
load-bearing capacity. After a relatively simple procedure, final Vs
information is provided in 1-D, 2-D, and 3-D formats.
Earth geophysical properties include: density, magnetic susceptibility and
permeability, seismic velocity, dielectric permittivity, electrical
conductivity/resistivity and chargeability. Ohm’s law (V=IR where V is the Voltage I is the Current and R is the
resistance), states that the current through a conductor
between two points is directly proportional to the voltage across the two
points, if you run a current through different materials ou can calculate
the electrical resistance by measuring the voltage and current applied.
In practice this means that a resistivity survey will form an electrical
circuit through the ground and take voltage and current measurements to
calculate the resistivity of the sub-surface rocks. Controlled Source Electro-Magnetics (mCSEM) can provide
pseudo-direct detection of hydrocarbons by detecting resistivity changes
over geological traps (signaled by seismic survey). Electrical resistivity tomography (ERT) surveys are relatively new
compared with GPR and seismic refraction.ERT is an advanced geophysical
technique used to determine the subsurface’s electrical resistivity
distribution by making measurements on the ground surface.
Resistivity, measured in Ωm, is the mathematical inverse of conductivity
and represents a bulk physical property that describes how difficult it is
to pass an electrical current through the material. ERT involves
introducing a DC electrical current into the ground with two electrodes
and measuring the voltage drop across the surface of the ground with two
other electrodes. Because electrical flow disperses throughout the ground,
these surface measurements provide information about the electrical
character of materials below the earth’s surface. The primary control on
the depth of investigation for a measurement is the distance between the
electrodes. ERT profiles are produced by modeling the data from a series
of measurements at different depths and locations along a survey line. ERT
data are rapidly collected with an automated multi-electrode resistivity
IP surveys induce an electric field in the ground and measure the
chargeability and resistivity of the subsurface. The technique can
identify changes in the electric currents caused by different rocks and
minerals. Readings are taken by a small crew who shift a ground array or
pattern of transmission and receiver cables.
In an IP survey, in addition to resistivity measurement, capacitive
properties of the subsurface materials are determined as well. As a
result, IP surveys provide additional information about the spatial
variation in lithology and grain-surface chemistry. IP survey can be made
in time-domain and frequency-domain mode. In time domain Induced
polarization method, voltage decay is observed as a function of time after
the injected current is switched off. In frequency-domain Induced
polarization mode, an alternating current is injected into the ground with
variable frequencies. Voltage phase-shifts are measured to evaluate
impedance spectrum at different injection frequencies, which is commonly
referred to as spectral IP
These surveys are most often used in metallic mineral exploration
Surface EM methods are based mostly on Transient EM methods using surface
loops with a surface receiver, or a downhole tool lowered into a borehole
which transects a body of mineralisation. These methods can map out
sulphide bodies within the earth in 3 dimensions, and provide information
to geologists to direct further exploratory drilling on known
mineralisation. Surface loop surveys are rarely used for regional
exploration, however in some cases such surveys can be used with success
(e.g.; SQUID surveys for nickel ore bodies).
Various potentials are produced in native ground or within the subsurface
altered by our actions. Natural potentials occur about dissimilar
materials, near varying concentrations of electrolytic solutions, and due
to the flow of fluids.
Sulfide ore bodies have been sought by the self potential generated by ore
bodies acting as batteries.
Other occurrences produce spontaneous potentials, which may be mapped to
determine the information about the subsurface.
Spontaneous potentials can be produced by mineralization differences,
electro-chemical action, geothermal activity, and bioelectric generation
Standard SP surveys utilise non-polarising, porous pot electrodes, which
have been specially adapted to minimise contact voltages. The electrodes
in contact with the ground surface should be the nonpolarizing type, also
called porous pots porous pots. Porous pots are metal electrodes suspended
in a supersaturated solution of their own salts (such as a copper
electrode suspended in copper sulfate) within a porous container. These
pots produce very low electrolytic contact potential, such that the
background voltage is as small as possible.
Readings are typically taken with one electrode fixed at a base station
and a second, mobile 'field' lectrode that is moved around the survey
area. Reading stations are spaced at regular intervals along linear
profiles, closed loops or grids depending upon the desired application.
The self potential method is traditionally used as a mineral exploration
tool and for downhole logging in the oil industry. More recently it has
been adapted for hydrogeological and water engineering applications, by
the use of more sensitive equipment and the careful application of data
EM surveys induce an electromagnetic field and measure the three
dimensional variations in conductivity within the near-surface soil and
rock. Conductive units can be studied to locate metallic minerals, and to
understand groundwater and salinity. Ground readings are taken by a small
crew who shift a ground array or pattern of transmission and receiver
EM methods generally measure the electrical resistivity of subsurface
materials. There are several categories of EM instruments. They are
differentiated from one another by either the way they generate or measure
electrical potential resulting from the interaction of EM energy surface
and subsurface materials.
The main categories include:
Time Domain, and
Very Low Frequency (FDEM, TDEM, and VLF, respectively)
Very Low Frequency (VLF) may be used to map structure when combined with
magnetics. The system typically responds to variations in overburden
conductivity, to large faults or shear zones, and to graphitic formational
conductors. The VLF signal is transmitted around the world by
governments, primarily for submerged submarine communication.
For Australia there is a transmitter at Exmouth (19.5KHz)that suited N-S
striking rocks the one at Woodside Victoria that suited E-W striking rocks
was demolished in 2015. , Monte Grande, Argentina(17.3KHz), TGolfo Nuevo,
Chubut, Argentina(12.9KHz), Plaine Chabrier, Reunion Island(12.3Khz)
Signals from these transmitters act as primary fields that are capable of
energizing conductive bodies (such as graphite, metallic minerals and
structures) in the ground. Once energized, the current within these bodies
emits a secondary field forming the basis for a geophysical exploration.
Electromagnetic (EM) surveys can be used to help detect a wide variety of
mineral deposits, especially base metal sulphides via detection of
conductivity anomalies which can be generated around sulphide bodies in
the subsurface. EM surveys are also used in diamond exploration (where the
kimberlite pipes tend to have lower resistance than enclosing rocks),
graphite exploration, palaeochannel-hosted uranium deposits (which are
associated with shallow aquifers, which often respond to EM surveys in
conductive overburden). These are indirect inferential methods of
detecting mineralisation, as the commodity being sought is not directly
conductive, or not sufficiently conductive to be measurable.
Electric-resistance methods such as induced polarization methods can be
useful for directly detecting sulfide bodies, coal and resistive rocks
such as salt and carbonates.
Ground penetrating radar is a general term to describe methods that use
radio waves (10 MHz – 1.2 GHz) to probe subsurface objects or geologic
features. GPR is a non-invasive electromagnetic (EM) geophysical technique
for subsurface exploration and characterization. GPR systems transmit
impulse electromagnetic energy (i.e. radio waves) into the ground and
detect echoes, or reflected wave front energy, at the ground surface. This
process is somewhat similar to p-wave seismic reflection methods and
theoretical similarities exist between the kinematic properties of elastic
and electromagnetic wave propagation. GPR is capable of profiling sediment
stratigraphy and bedrock surface elevations
GPR is a subsurface imaging technique that utilizes high frequency
electromagnetic energy (typically in the 50 MHz to 2 GHz range). The EM
waves are transmitted into the subsurface and a portion of them are
reflected back to a receiver each time an interface between contrasting
material types is encountered. The rapid acquisition and processing of GPR
data makes it a popular geophysical method for shallow subsurface
exploration. However, GPR signals are attenuated rapidly by soils with
high clay content thus making it more popular for site with high sand
content in the soil.
GPR is useful for a variety of applications, including:
Gravity and magnetics are also used, with considerable frequency, in oil
and gas exploration. These can be used to determine the geometry and depth
of covered geological structures including uplifts, subsiding basins,
faults, folds, igneous intrusions and salt diapirs due to their unique
density and magnetic susceptibility signatures compared to the surrounding
rocks. Remote sensing techniques, specifically hyperspectral imaging, have
been used to detect hydrocarbon microseepages using the spectral signature
of geochemically altered soils and vegetation. Specifically at sea, two
methods are used: marine seismic reflection and electromagnetic seabed
logging (SBL).Marine Magnetotellurics (mMT) or marine
To a first approximation, Earth’s magnetic field resembles a large dipolar
source with a negative pole in the northern hemisphere and a positive pole
in the southern hemisphere . The dipole is offset from the center of the
earth and also tilted.
The north magnetic pole at the surface of the earth is approximately at
The field at any location on (or above or within) the Earth are generally
described in terms described of magnitude (|B||B|), declination (DD) and
inclination (II) The magnitude of the vector representing Earth’s
magnetic field. DD: Declination is the angle that H makes with respect to
geographic north (positive angle clockwise). II: Inclination is the angle
between B and the horizontal. It can vary between -90° and +90° (positive
Most of the magnetic field comes from inside the earth and it can be from
the geomagnetic dynamo or from crustal rocks that have become magnetised.
In addition there are also magnetic fields that come from outside the
earth. The solar wind interacts with Earth’s magnetic field and creates a
magnetosphere that is “tear-dropped” shape. This diurnal variation need to
be corrected for.
Generally the easiest way to do this is to have two magnetometers - one
that is moving and doing the survey and a second that is stationary. The
stationary one records the diurnal variation which is then removed from
the moving one's data.
This results in a set of magnetic data showing only differences in the
earth's magnetic field.
Magnetic surveys measure the variations of the Earth’s magnetic field due
to the presence of magnetic minerals. Subtle variations in the abundance
of magnetic minerals are used to interpret rock types and can assist in
identifying resources. These surveys are typically undertaken by a
geophysical technician on foot carrying a magnetometer and a sensor on a
pole. They are most often used in metallic mineral exploration.
Magnetometric surveys can be useful in defining magnetic anomalies which
represent ore (direct detection), or in some cases gangue minerals
associated with ore deposits (indirect or inferential detection). The most
direct method of detection of ore via magnetism involves detecting iron
ore mineralisation via mapping magnetic anomalies associated with banded
iron formations which usually contain magnetite in some proportion. Skarn
mineralisation, which often contains magnetite, can also be detected
though the ore minerals themselves would be non-magnetic. Similarly,
magnetite, hematite and often pyrrhotite are common minerals associated
with hydrothermal alteration, and this alteration can be detected to
provide an inference that some mineralising hydrothermal event has
affected the rocks.
A gravimeter measures the gravity field to determine variations in rock
density in the Earth’s crust. Ground gravity surveys require a geophysical
technician to take gravity measurements at set intervals of distance and
record the precise height at each location. Access to the recording sites
can be by vehicle or helicopter, depending upon remoteness. These surveys
are used in mineral and energy exploration.
Contrasts in earth’s gravitational field can often be attributed to local
changes in the density of subsurface materials or the presence of
engineered structures. Sensitive gravity meters are utilized to measure
the gravitational field at designated points within the area of interest
Gravity surveying can be used to detect dense bodies of rocks within host
formations of less dense wall rocks. This can be used to directly detect
Mississippi Valley Type ore deposits, IOCG ore deposits, iron ore
deposits, skarn deposits and salt diapirs which can form oil and gas
The radiometric, or gamma-ray spectrometric method is a geophysical
process used to estimate concentrations of the radioelements potassium,
uranium and thorium by measuring the gamma-rays which the radioactive
isotopes of these elements emit during radioactive decay.
Airborne gamma-ray spectrometric surveys estimate the concentrations of
the radioelements at the Earth's surface by measuring the gamma radiation
above the ground from low-flying aircraft or helicopters.
All rocks and soils contain radioactive isotopes, and almost all the
gamma-rays detected near the Earth's surface are the result of the natural
radioactive decay of potassium, uranium and thorium. The gamma-rays are
packets of electromagnetic radiation characterised by their high frequency
and energy. They are quite penetrating, and can travel about 35
centimetres through rock and several hundred metres through the air. Each
gamma ray has a characteristic energy, and measurement of this energy
allows the specific potassium, uranium and thorium radiation to be
The gamma-ray spectrometric method has many applications but is used
primarily as a geological mapping tool. Changes in lithology, or soil
type, are often accompanied by changes in the concentrations of the
radioelements. The method is capable of directly detecting mineral
deposits. Potassium alteration, which is often associated with
hydrothermal ore deposits, can be detected using the gamma-ray
spectrometric method. It is also used for uranium and thorium exploration,
heat flow studies and environmental mapping.
Radiometric surveys measure gamma rays which are continuously being
emitted from the Earth by natural decomposition of some common radiogenic
minerals. Most gamma rays emanate from the top 30 centimetres of rock or
soil which can be detected by airborne surveys or on surface rocks using a
hand-held spectrometer. These surveys are most often used in metallic and
industrial mineral exploration.
Gamma-ray spectrometers which are designed for the detection and
measurement of low-level radiation from both naturally occurring and
man-made sources, associated with the radioactive elements; thorium,
potassium, and uranium. Gamma Ray Spectrometry provides a direct
measurement of the surface of the earth, with no significant penetration,
but permits reliable measurement of the radioactive element contacts to
the mapped bedrock and surficial geology. Potassium (K), uranium (U) and
thorium (Th) are the three most abundant, naturally occurring radioactive
K is a major constituent of most rocks and is the predominant alteration
element in most mineral deposits.
Uranium and thorium are present in trace amounts, as mobile and immobile
As the concentration of these different radioactive elements varies
between different rock types, we can use the information provided by a
gamma-ray spectrometer to map the rocks.
Where the 'normal' radioelement signature of the rocks is disrupted by a
mineralizing system, corresponding radioelement anomalies provide direct