Gravity, Electrical, Magnetic and Seismic Methods

 

Gravity Methods

The 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.

Gravity 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.

Anomalies 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 effort.

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.

High Density

Extrusive Igneous Rocks, Eg. Basalt

Metamorphic Rocks

Intrusive Igneous Rocks, e.g. Granite

Sedimentary Rocks

Low Density

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.

Density 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.

Gravity 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 geotechnical investigations.

Electrical Methods

Electrical 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.

The 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:

a)      Natural currents in rocks – Self-potential method

b)      Polarizability of rocks – Induced polarization method

c)      Electrical conductivity or resistivity of rocks – Resistivity method

d)      Induction – Electromagnetic method

Self Potential Method: 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.

Induced Polarization: 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 Method: 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 boundary.

Electromagnetic Methods: 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.

A 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.

Magnetic methods

One 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.

When 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.

Measurements 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

Magnetic 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.

It 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.

When 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:

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 refraction 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:

a)      Each layer is homogeneous and isotropic (i.e., has the same velocity in all directions)

b)      The boundaries (interfaces) between layers are nearly planar; and

c)      Each successive layer has higher velocity than the one above.

The 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 ascertained.

High-velocity 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 reflection methods:  Most 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 changes.

In 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.

The 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.

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. In most exploration programs appreciably more money is spent on seismic reflection work than on all other geophysical methods combined.

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