Geophysics HM

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MAG

Magnetic surveys are a geophysical method to image anomalies in the earth’s magnetic field caused by source bodies within the sub-surface. Oil and gas exploration use magnetic anomalies to detect faults and igneous intrusions. Magnetics are commonly used with gravity as a low cost way to understand the structure of the subsurface during the beginning phases of exploration. Both gravity and magnetics are potential fields, meaning that they are the spatial derivatives of their respected fields. Gravity and magnetics are also low resolution and non-unique, meaning that multiple geologic models can fit the data. The more that is known about the sub-surface, the more the geologic model can be narrowed down. Other uses of magnetics include detecting pipes, buried objects, and archaeological sites (https://wiki.seg.org/wiki/Magnetic_methods).

Magnetic surveys are carried out using a man-portable instrument with readings taken on a regular grid or along selected traverse lines. The equipment functions by measuring the Earth’s magnetic field to a very high precision at each survey station. Ferrous materials in the subsurface have an induced magnetic field that is superimposed on the Earth’s field at that location creating a magnetic anomaly. The spacing of survey stations depends on the width of the expected anomaly, which broadens with the size, and depth of burial of the targeted feature. Continuous profiling methods may be used for a high-resolution dataset.

Transient Electromagnetic

Transient electromagnetics is a geophysical exploration technique in which electric and magnetic fields are induced by transient pulses of electric current and the subsequent decay response measured. TEM methods are generally able to determine subsurface electrical properties, but are also sensitive to subsurface magnetic properties in applications like UXO detection and characterization. The surveys are a very common surface EM technique for mineral exploration, groundwater exploration, and for environmental mapping, used throughout the world in both onshore and offshore applications. Two fundamental electromagnetic principles are required to derive the physics behind TEM surveys: Faraday's law of induction and Lenz's Law. A loop of wire is generally energized by a direct current. At some time (t0) the current is cut off as quickly as possible.

Faraday's law dictates that a nearly identical current is induced in the subsurface to preserve the magnetic field produced by the original current (eddy currents). Due to ohmic losses, the induced surface currents dissipate—this causes a change in the magnetic field, which induces subsequent eddy currents. The net result is a downward and outward diffusion of currents in the subsurface which appear as an expanding smoke ring [1] when the current density is contoured. When conductive bodies are present, the diffusion of the transients is changed. In addition, transients are induced in the conductive bodies as well. This is only the most basic overview. The paper by McNeill is freely available from the Geonics website[4] explaining the basics of the method (https://en.wikipedia.org/wiki/Transient_electromagnetics).

Magnetotellurics

Magnetotellurics (MT) is an electromagnetic geophysical method for inferring the earth's subsurface electrical conductivity from measurements of natural geomagnetic and geoelectric field variation at the Earth's surface. Investigation depth ranges from 300 m below ground by recording higher frequencies down to 10,000 m or deeper with long-period soundings. Proposed in Japan in the 1940s, and France and the USSR during the early 1950s, MT is now an international academic discipline and is used in exploration surveys around the world. Commercial uses include hydrocarbon (oil and gas) exploration, geothermal exploration, carbon sequestration, mining exploration, as well as hydrocarbon and groundwater monitoring. Research applications include experimentation to further develop the MT technique, long-period deep crustal exploration, deep mantle probing, and earthquake precursor prediction research.

The Magnetotelluric technique was introduced independently by Japanese scientists in the 1940s (Hirayama, Rikitake), Russian geophysicist Andrey Nikolayevich Tikhonov in 1950[1] and the French geophysicist Louis Cagniard.[2] With advances in instrumentation, processing and modelling, MT has become one of the most important tools in deep Earth research. Since first being created in the 1950s, magnetotelluric sensors, receivers and data processing techniques have followed the general trends in electronics, becoming less expensive and more capable with each generation. Major advances in MT instrumentation and technique include the shift from analog to digital hardware, the advent of remote referencing, GPS time-based synchronization, and 3D data acquisition and processing. (https://en.wikipedia.org/wiki/Magnetotellurics)

Nuclear Magnetic Resonance Sounding (NMRS)

The principles of the NMRS method are based on the phenomena of Nuclear Magnetic Resonance commonly used in the fields of medicine as well as in Nuclear Spectroscopy, chemistry and biophysics. Usually for the execution of a common NMR sounding a short pulse of electromagnetic field generated by an alternating current transmitter with a frequency equal to the Larmor precession of hydrogen protons is induced onto the subsurface soils and rocks through a wire loop laid out in the terrain surface.

During the electromagnetic field transmission the hydrogen protons will break the equilibrium with the earth magnetic field causing a precession of the protons. After the pulse is terminated the hydrogen protons will freely precess back to equilibrium inducing a decay voltage or signal measurable in the surface by the NMRS equipment. The NMRS survey technique is used for porosity calculation, water content and depth of water table estimations.

Self Potential

Self potential or spontaneous potential (SP) methods measure the electrical potential field caused by ambient DC electrical currents in the earth. Electrical currents generated by underground chemical reactions and by flow of chemicals to a permeable medium can occur nearly everywhere in the earth due to:

Oxidation-reduction reactions around metals or metal ores (e.g. corrosion)
Movement of ionic fluids across soil or rock contacts (e.g. saltwater intrusion)
Underground temperature gradients (e.g. near mine fires or geothermal zones)
Underground chemical gradients (e.g. near sulfides or decomposing petroleum)
Movement of subsurface water (i.e. seepage, leakage, flow pathways)

Where there are contaminant plumes or other subsurface phenomena which contain decomposing or reacting chemicals of all kinds, the release or capture of charged ions creates currents detectable by SP surveys. Acid mine drainage generation zones, as well as underground fires in mines or organic waste masses in particular produce strong SP anomalies.

Electric Resistivity Tomography

Electrical resistivity tomography (ERT) or electrical resistivity imaging (ERI) is a geophysical technique for imaging sub-surface structures from electrical resistivity measurements made at the surface, or by electrodes in one or more boreholes. If the electrodes are suspended in the boreholes, deeper sections can be investigated. It is closely related to the medical imaging technique electrical impedance tomography (EIT), and mathematically is the same inverse problem. In contrast to medical EIT, however, ERT is essentially a direct current method. A related geophysical method, induced polarization (or spectral induced polarization), measures the transient response and aims to determine the subsurface chargeability properties.

The technique evolved from techniques of electrical prospecting that predate digital computers, where layers or anomalies were sought rather than images. Early work on the mathematical problem in the 1930s assumed a layered medium (see for example Langer, Slichter). Andrey Nikolayevich Tikhonov who is best known for his work on regularization of inverse problems also worked on this problem. He explains in detail how to solve the ERT problem in a simple case of 2-layered medium. During the 1940s, he collaborated with geophysicists and without the aid of computers they discovered large deposits of copper. As a result, they were awarded a State Prize of Soviet Union (https://en.wikipedia.org/wiki/Electrical_resistivity_tomography).

Radiometry

Radiometry is a set of techniques for measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which characterize the light's interaction with the human eye. The fundamental difference between radiometry and photometry is that radiometry gives the entire optical radiation spectrum, while photometry is limited to the visible spectrum. Radiometry is distinct from quantum techniques such as photon counting.

The use of radiometers to determine the temperature of objects and gases by measuring radiation flux is called pyrometry. Handheld pyrometer devices are often marketed as infrared thermometers. Radiometry is important in astronomy, especially radio astronomy, and plays a significant role in Earth remote sensing. The measurement techniques categorized as radiometry in optics are called photometry in some astronomical applications, contrary to the optics usage of the term. Spectro-radiometry is the measurement of absolute radiometric quantities in narrow bands of wavelength (https://en.wikipedia.org/wiki/Radiometry).

Mise-A-La-Masse

The Mise-Á-La-Masse method is normally used for mapping ore deposits with high electrical conductivities such as sulfide bodies. For geothermal exploration the Mise-Á-La-Masse technique can be used to help define the ground fluids that flow into a well. It can also be useful for mapping faults and fractures in a geothermal system and ultimately helps in defining the boundaries of a reservoir. This method is a DC resistivity technique in which the charged current electrode (C1) is connected to a conductive structure that goes deep into the surface. The return current electrode (C2) is placed far from the survey area, essentially at infinity. Traditionally this method is used in sulfide ore mining and the positive electrode (C1) is connected to an outcrop of the ore body or an existing borehole within the ore body.

In geothermal exploration this method is usually performed by connecting the positive electrode (C1) directly to the casing of an existing well. When a voltage is applied potentials develop which can be measured on the surface and mapped. Electrical potentials are measured using the potential electrode (P1); moving it at incremental distances and radially around the borehole. The fixed potential electrode (P2) is normally placed far from positive current electrode (C1) and usually opposite of the negative current electrode (C2). The shape and size of localized conductive deposits is reflected by the pattern of electrical potentials measured on the surface (https://openei.org/wiki/DC_Resistivity_Survey_(Mise-A-La-Masse).

Ground Resistance Probing

The first soil resistance measuring instrument was invented in the 1950s by Evershed & Vignoles Meggers who made the first insulation and earth resistance testers. One of the most used analog grounding testers in USSR were М416. From the 21st century several companies produced digital earth resistance meters and testers. The main purpose of the instrument[4] is to determine the adequacy of the grounding of an electrical system. By a standard of the National Electrical Code[5] the resistance of the soil should be less than 25 Ohms to reliably and efficiently ground the installation.

When measuring earth resistance with an instrument, it is important to know some of its basic characteristics in order to accurately measure the soil resistance and to properly size the grounding installation. Most importantly, the range of resistance the device measures. Usually the range is three or four degrees. The soil moisture at which the appliance operates is another important parameter. If the instrument cannot operate at a certain humidity, then the measurement may differ significantly from the real value of soil resistance. Comparison analog and digital grounding resistance testers https://en.wikipedia.org/wiki/Grounding_resistance_tester).

Gravimetry

Gravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used when either the magnitude of a gravitational field or the properties of matter responsible for its creation are of interest. The modern gravimeter was developed by Lucien LaCoste and Arnold Romberg in 1936. They also invented most subsequent refinements, including the ship-mounted gravimeter, in 1965, temperature-resistant instruments for deep boreholes, and lightweight hand-carried instruments. Most of their designs remain in use with refinements in data collection and data processing. An instrument used to measure gravity is known as a gravimeter. For a small body, general relativity predicts gravitational effects indistinguishable from the effects of acceleration by the equivalence principle. Thus, gravimeters can be regarded as special-purpose accelerometers. Many weighing scales may be regarded as simple gravimeters.

In one common form, a spring is used to counteract the force of gravity pulling on an object. The change in length of the spring may be calibrated to the force required to balance the gravitational pull. The resulting measurement may be made in units of force (such as the newton), but is more commonly made in units of gals. Microgravimetry is a rising and important branch developed on the foundation of classical gravimetry. Microgravity surveys are carried out in order to solve various problems in geology and geotechnical investigations mainly location of voids and their monitoring. Very detailed measurements of high accuracy can indicate voids of any origin, provided the size and depth are large enough to produce gravity effect stronger than is the level of confidence of relevant gravity signal. (https://en.wikipedia.org/wiki/Gravimetry).

Infrared Thermal Imaging

Infrared thermography (IRT), thermal imaging, and thermal video are examples of infrared imaging science. Thermographic cameras usually detect radiation in the long-infrared range of the electromagnetic spectrum (roughly 9,000–14,000 nanometers or 9–14 µm) and produce images of that radiation, called thermograms. Since infrared radiation is emitted by all objects with a temperature above absolute zero according to the black body radiation law, thermography makes it possible to see one's environment with or without visible illumination.

The amount of radiation emitted by an object increases with temperature; therefore, thermography allows one to see variations in temperature. When viewed through a thermal imaging camera, warm objects stand out well against cooler backgrounds; humans and other warm-blooded animals become easily visible against the environment, day or night. As a result, thermography is particularly useful to the military and other users of surveillance cameras. (https://en.wikipedia.org/wiki/Thermography).