INTRODUCTION
To
GEOPHYSICAL PROSPECTING
GP.1: Introduction
The extraction of at a continually increasing rate of fossil fuels and useful minerals from the Earth has raised the specter of impending shortages that could threaten the economy and way of life of the civilized world. Events of the middle 1970s have demonstrated how well founded this concern can be. The amounts of oil, gas, and metallic minerals that actually exist in the earth, both known and undiscovered, are of course limited, but the immediate problem as established reserves become scarce is o find new supplies in the Earth that will replace those which have been consumed. The exploration for energy supplies and mineral resources has become increasingly difficult as the "easy" or conventional resources are discovered and exploited.
To meet the challenge, earth scientists have made more and more sophisticated techniques of exploration. Until well into the twentieth century (20th) the search for oil and solid minerals was confined to deposits directly observable on the surface in the form of seeps and outcrops or other exposures. When all accumulations in an area that could be discovered by such simple means had been found, it was necessary to deduce the presence of buried deposits indirectly by downward projection of geological information observable on the surface. As this approach reached the point of diminishing results, new methods of studying the subsurface were needed. They did not require any geological observations, but they did involve physical measurements at the Earth's surface that would give information on the structure or composition of concealed rocks that might be useful for locating desired deposits.
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| Geophysics & Geology |
GP.2: Geophysics & Geology:
We designate the study of the earth (inner and outer) using physical measurements at or above the surface as "Geophysics". While it is not always easy to establish a meaningful border line between geology and geophysics, the difference lies primarily in the type of data with which one begins.
Geology involves the study of the earth by direct observations on rocks, either from surface exposures or boreholes, and the deduction of its structure, composition, or history by analysis of such observations.
Geophysics, on the other hand, involves the study of those parts of the earth hidden from direct view by measuring their physical properties with appropriate instruments, usually on or above the surface. It also includes interpretation of the measurements to obtain useful information on the structure and composition of the concealed zones. The distinction between two branches of Earth Sciences is not clear-cut. Well-logs, for example, are widely used in geological studies, even though they present the results of purely instrumental observations. The term "borehole geophysics" is often used to designate such measurements.
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| Borehole Geophysics |
In a broader sense, geophysics provides the tools for studying the structure and composition of Earth's interior. Virtually all of what we know about the earth below the limited depths to which boreholes or mine shafts have penetrated has come from geophysical observations. The existence and properties of the Earth's crust, mantle, and core have been determined by observations upon seismic waves from earthquakes, as well as by measurements of the earth's gravitation, magnetic, and thermal properties. The tools and techniques developed for such studies have been used in exploration for hydrocarbons and minerals. At the same time, geophysical methods devised for prospecting applications have been put to use in more academic research on the nature of the earth's interior. There is "pure" and "applied" geophysics and there are economic aspects associated with both pure & applied geophysics. Pure & applied Geophysics have so much interdependence that the separation is artificial at best.
GP.3: The Technological Challenges of Geophysics
Geophysical exploration is a relatively new area of research and technology. Ferrous minerals were sought with magnetic compasses as early as the 1600s, but only during the past century have special instruments been put to use in mining exploration. Geophysical prospecting for oil & gas is in its sixties, the first oil discovery attributable to geophysics having been made in 1924. Throughout it history, the tools and techniques of exploration geophysics have been continually improved, both in performance and economy. This progress has been in response to an unrelenting pressure to develop new capabilities after existing ones have become inadequate to find enough new deposits . Except in areas newly opened to exploration, most geophysical surveys are undertaken where previous ones have failed because the instruments, field techniques, or interpretation methods were not good enough.
The technological improvements in geophysical exploration have been of several types. In some cases, new techniques have been developed to solve problems associated with the environment where exploration is to be carried out. In offshore areas,or in deserts, Arctic tundra,or lava covered terrain, special logistics are needed. Moreover, unique types of "noise" in such areas often cause interference with desire geophysical information, and special techniques must be developed to suppress such interference. The introduction of analog computer technology in the 1950s and digital computers in the 1960s brought about new capabilities in the recording and processing of all kinds of geophysical data, making it possible to extract useful information otherwise concealed by undesired noise.
The technological revolution following World War II brought about many scientific developments which have contributed greatly to the effectiveness of geophysical exploration. Electronic computers, micro-miniature electronics, information-processing techniques, and navigation satellites to cite some examples of pertinent space-age developments, have all been put to extensive use by geophysicists searching for oil and other natural resources.
GP.4: Review of Geophysical Prospecting Methods
The geophysical techniques most widely employed for exploration work are the seismic, gravity, magnetic, electrical, and electromagnetic methods.
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| Geophysical Methods |
Less common methods involve the measurement of radioactivity and temperature at or near the Earth's surface and in the air.
Some of these methods are used almost entirely in the search for oil and gas. Others are used primarily in exploring for solid minerals. Most of them may be employed for either objective. Seismic, magnetic and gravity prospecting the chief tools for hydrocarbons' exploration; seismic and electrical methods are the two chief tools used for mineral exploration. In U.S.S.R (Union of Soviet Socialists of Russia), in former French territories, and more recently in parts of the United States electromagnetic methods have been applied routinely to the search for oil, Magnetic and electromagnetic methods are employed for both types of prospecting.
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| Seismic Method (Offshore) |
Seismic Reflection Method With this method- by far the most widely used geophysical technique- the structure of subsurface formations is mapped by measuring the times required for a seismic wave (or pulse), generated in the earth by a near surface explosion, mechanical impact, or vibration, to return to the surface after reflection from interfaces between formations having different physical properties. The reflections are recorded by detecting instruments responsive to ground motion. They are laid along the ground at distances from the point of generation, which are generally small compared with the depth of the reflector. Variations in the reflection times from place to place on the surface usually indicate structural features in the strata below. Depths to reflecting interfaces can be estimated from the recorded times and velocity information that can be obtained either from reflected signals themselves or from surveys in wells. Reflections from depths of 30,000 feet or more can normally be observed by combining the reflections from the repeated source applications, so in most areas geological structure can be determined throughout sedimentary section.
In most recent years, reflection data have also been used for identifying lithology, generally from velocity and attenuation characteristics of the transmitted and reflected seismic waves, and for detecting hydrocarbons, primarily gas, directly on the basis of reflection amplitudes and other seismic indicators. Modern reflection record sections are similar in appearance to geological cross sections, but "Geologists" must take some cautions in while interpreting the subsurface picture, so that seismic sections can't be interpreted erroneously. Under ideal conditions, structural relief can be determined with a precision of about 1/2 percent of depth below the surface. Reflection data can be used to determine the average velocities of seismic waves between the surface and the reflector.
With seismic reflection methods, one can locate and map such features as anticlines, faults, salt domes, and reefs. Many of these are associated with the accumulation of oil and gas. Major convergences caused by depositional thinning can be detected from reflection sections. The resolution (Seismic Resolution) of the method is now approaching a fineness adequate for finding stratigraphical traps such as pinch-outs as facies changes . However, successful exploration for stratigraphic oil accumulations by reflection techniques requires skillful coordination of geological and seismic information.
Seismic Refraction Method In refraction surveying, the detecting instruments record seismic signals at a distance from the shot point that is large compared with the depth of the horizon to be mapped. The seismic waves must thus travel large horizontal distances through the earth, and the times required for the travel at various source-receiver distances give information on the velocities and depths of the subsurface formations along which they propagate. Although the refraction method doesn't give as much information or as precise and unambiguous a structural picture as reflection, it provides data on the velocity of the refracting beds. The method made it possible to cover a given area more quickly and economically than with the reflection method, though with a significant loss of detail and accuracy.
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| Seismic Method (offshore) |
Suitability of Refraction Method: Refraction is particularly suitable where the structure of a high speed surface, such as the basements or the top of a limestone layer, is the target of geological interest. If the problem is to determine the depth and shape of a sedimentary basin by mapping the basement surface, and if the sedimentary rocks have a consistently lower seismic velocity than do the basement formations , refraction was in the past an effective and economical approach for achieving this objective. Airborne magnetic surveys and, to some extent, gravity have replaced seismic refraction for such purposes. Because velocities in salt and evaporites are often greater than in surrounding formations, refraction has been useful in mapping diaper features such as salt domes. Under favorable circumstances this technique has been used to detect and determine the throw of faults in high speed formations, such as dense limestone and basement materials.
Despite its advantages, refraction is now rarely employed in oil exploration because of the larger scale field operations required. Also, the reflection method has developed to the point that it can now yield nearly all of the information that refraction shooting could produce as well as relatively unambiguous and precise structural information unavailable from refracted waves.
Gravity Method In gravity prospecting, one measures minute variations in the pull of gravity from rocks within the first few miles of the earth's surface. Different types of rocks have different densities, and the denser rocks have the greater gravitational attraction. If the higher-density formations are arched upward in a structural high, such as an anticline, the earth's gravitational field will be greater over the axis of the structure than along its flanks. A salt dome, on the other hand, which is generally less dense than the rocks into which it is intruded, can be detected from the low value of gravity recorded above it compared with that measured on either side. Anomalies in gravity that are sought in oil exploration may represent only one-millionth or even one-ten-millionth of the earth's total field. For this reason, gravity instruments are designed to measure variations in the force of gravity from one place to another rather than the absolute force itself. Modern gravimeters are so sensitive that they can detect variations in gravity to within less than one-hundred millionth of the earth's total field.
The gravity method is useful wherever the formations of interest have densities that are appreciably different from those of surrounding formations. It is an effective means of mapping sedimentary basins where the basement rocks have a consistently higher density than the sediments. It is also suitable for locating and mapping salt bodies because of the generally low density of salt compared with that of surrounding formations. Occasionally it can be used for groundwater
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| Gravity Surveying |
studies and for direct detection of heavy minerals such as chromites. Recently, extremely sensitive gravimeters have been used to detect underground tunnels and the locations of burial chambers in pyramids.
Data from gravity surveys are more subject to ambiguity in interpretation than with seismic surveys, because any gravity field can be accounted for equally well by widely different mass distributions. Additional geophysical or geological information over a gravity anomaly will reduce the ambiguity and increase the usefulness of the gravity data.
Gravity measurements are routinely made in conjunction with marine seismic work and are used as a minor supplement. Gravity surveys, unaccompanied by other methods, are no longer employed in oil and gas exploration except on rare occasions.
Magnetic Methods Magnetic prospecting maps variations in the magnetic field of the earth that are attributable to changes of structure, magnetic susceptibilities, or remanence in certain near surface rocks. Sedimentary rocks generally have a very small susceptibility compared with igneous or metamorphic rocks, which tend to have a much higher magnetic content, and most magnetic surveys are designed to map structure on or inside the basement or to detect magnetic minerals directly. The magnetic method was initially used for petroleum exploration in areas where the structure in oil-bearing sedimentary layers appeared to be controlled by topographic features, such as ridges or faults, on the basement surface.
Since the development of aero-magnetic methods, most magnetic surveys undertaken for oil exploration are carried out to ascertain the thickness of the sedimentary section in areas where such information is not otherwise available (usually frontier areas). However interpretation of magnetic data is complicated.
In mining exploration, magnetic methods are employed for direct location of ores containing magnetic minerals such as magnetite . Intrusive bodies such as dikes can often be distinguished on the basis of magnetic observations alone.
Interpretation of magnetic data is subject to the same uncertainty as is found in gravity work, because of the lack of uniqueness inherent in all potential methods. Here again, the more geological information is available, the less the uncertainty in the final interpretation.
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| Electrical Methods |
Electrical Methods Electrical prospecting uses a large variety of techniques, each based on some different electrical property or characteristic of materials in the earth. The resistivity method is designed to yield information on formations or bodies having anomalous electrical conductivity.
The induced polarization method, employed in the exploration for disseminated ore bodies such as sulfides, will give diagnostic readings where ionic exchanges take place on the surfaces of metallic grains.Such effects cause perturbations in the falloff of voltage across the ore mass when current passed through the mass from surface electrodes is suddenly cut off. The resistivity method has been used for a long time to:
- Map boundaries between layers having different conductivities.
- It is employed in engineering geophysics to map bedrock
- determine salinity and depth to the water table in groundwater studies
- search for geothermal power because subterranean steam affects the resistivity of formations in a way that can often be diagnostic.
Telluric current and magnetotelluric methods use natural earth currents (the latter involving natural alternating magnetic fields as well), and anomalies are sought in the passage of such currents through earth materials. In this respect this method is different form resistivity and induced polarization, which require artificial introduction of electricity into the earth.
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| Magnetic Methods |
Magnetotelluric methods have been found to be the only effective method of oil and gas exploration in areas where seismic work is not practicable, particularly where multiple sheets of volcanic rocks overlie the sedimentary section.
The Self Potential (SP) Method is used to detect the presence of certain minerals and metallic bodies that react with electrolytes in the earth in such a way as to generate electro-chemical potentials. A sulfide body oxidized to a greater extent on its top than along its bottom will give rise to such potentials, which are detectable with electrodes at the surface.
Electromagnetic Methods detect anomalies in the inductive properties of the earth's subsurface rocks. An alternating voltage is introduced into the earth by induction from transmitting coils either on the surface or in the air, and the amplitude and phase shift of the induced potential generated in the subsurface are measured by detecting coils and recorded. Ore of base metals can often be detected by this technique.
The resistivity and magnetotelluric methods are used extensively in the U.S.S.R (Union of Soviet Socialists of Russia) for mapping sedimentary basins at the early stages of exploration for petroleum in new areas. Other electrical methods, such s the telluric have been employed by French geophysicists in Europe and Africa. Elsewhere in the world, electrical techniques have been employed for engineering purposes and in the search for solid minerals, water supplies, and geothermal energy.
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| Radioactivity Surveying |
Radioactive Methods Radioactive prospecting for minerals containing uranium has involved the use of geophysical tools (geiger counters and scintillation counters) and must therefore be looked upon as geophysical method. Much of the surface exploration for uranium is carried out by amateurs equipped with
detecting instruments. Industrial prospecting involves radioactive logging of exploratory drill holes and airborne surveys with scintillation counters.
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| Well Logging |
Well Logging Well logging involves probing the earth with instruments that give continuous readings recorded at the surface as the instruments are pulled up through the borehole. Among rock properties currently being logged with such instruments are electrical resistivity, self potential, gamma ray generation (both natural and in response to neutron bombardment), density, magnetic susceptibility, and acoustic velocity.
Although well logging is one of the most widely used of all geophysical techniques, it would require lots of scholarity notes even to introduce this "geophysical method". Well logging along with Seismic Method has extensively been used in the oil and gas exploration and production industry.
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References:
Milton B. Dobrin (Late Professor of Geology, University of Houston (USA))
Carl H. Savit (Adjunct Professor of Geology and Geophysics, Rice University;
Western Geophysical Company (retired)
Houston (United States of America)).
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© 2013
geophyx.blogspot.com
Author:
Waqas Haider
M.Sc. Geophysics (2011 - 2013)
Department of Earth Sciences,
Quaid-I-Azam University Islamabad- 44000
Pakistan.
email: geomindx@gmail.com
mobile: +923215140154
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