|
|
 |
|||||||||||||||||
|
|||||||||||||||||
 |
|||||||||||||||||
| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
University of Kansas, Lawrence, USA
Near-surface geophysics is applied to a broad spectrum of problems, and new application areas continue to arise. The noninvasive tools used to examine near-surface earth materials employ electrical, electromagnetic, or mechanical energy sources, along with passive techniques that measure the physical parameters of the earth. Most advances over the past 75 years have emerged from instrument evolution and computer-processing techniques. Societal needs, such as detecting unexploded ordnance following military operations, have driven research efforts. Other compelling factors, such as the need for potable groundwater, the enactment of laws that have spurred geophysical surveying for archaeological purposes, and the necessity for better soil-physics information in geotechnical engineering and agriculture, are present worldwide.
The physical parameters measured directly during shallow surveys include elastic properties, gravitational and magnetic fields, electrical conductivity, transparency to and polarizability of electromagnetic waves, and natural gamma radiation. These in turn are used to infer the permeability, porosity, chemical constitution, stratigraphy, geologic structure, and various other properties of near-surface materials.
Although scores of present-day and potential applications of near-surface geophysical methods exist, such measurements and their geologic interpretations often are applied for the following reasons:
The geophysical methods chosen will vary according to a project's objectives, resolution requirements, available budget, and geologic conditions. For example, seismic methods are sensitive to the mechanical properties of earth materials but are relatively insensitive to their chemical makeup. In contrast, electrical and electromagnetic (EM) methods are sensitive both to contained fluids and to the presence of magnetic or electrically conductive materials. Because materials vary in both mechanical and electromagnetic properties, the use of multiple geophysical methods can provide better answers to relevant questions at most sites.
Following is a distillation of near-surface geophysical approaches as they exist today, although readers could amplify this list from the perspective of their own knowledge and experience.
 |
Accepted older methods |
|---|
 TopAbstract Accepted older methods Some newer methodsBorehole methodsThe futureSuggested reading |
|---|
The recent development of multichannel electrical resistivity systems has increased the flexibility and the rate of fieldwork characteristic of DC resistivity surveys and has facilitated the application of electrical resistivity tomography for investigation of complex subsurface environments.
The induced polarization (IP) method is similar to the resistivity method in data collection, but the IP method involves analysis of the length of time the earth remains disturbed electrically after the disturbing function has been removed. In an electronic sense, the earth's discharge rate is similar to that of a capacitor. The rate of decay of the induced voltage is dependent on ion mobility in the charged volume. The ions in clays, for example, are highly mobile. Measurements can be made either in the time domain, with voltage as a function of time, or in the frequency domain, where the phase delays of various frequencies are measured. The transmitter and receiver can be connected, or highly accurate clocks can be synchronized at the start of each day to determine the amount of delay for each frequency reaching the voltage electrodes. Frequencies commonly vary between about .05 Hz and 1 kHz. This method is used in sulfide exploration and has been used in some localities for groundwater exploration.
The spontaneous potential (SP) method provides a measure of electrochemical activity in the form of the natural voltages in the earth that rarely exceed 100 mV. Voltages usually average out to zero over distances a few times larger than any anomalies that may be present. Fluid, ions, or heat moving in the earth can generate spontaneous potentials. Because a passive technique is used to record these small voltages, the source current or configuration remains unchanged by the survey itself. Because the voltages are small, the signals are vulnerable to noise from power lines, pipelines, electrical storms, and other environmental noise sources. One problem with SP techniques is the lack of repeatability of the measurements.
Data may be interpreted by generating contour maps of voltages or by using more quantitative means involving calculations that rely on geometrical shapes similar to those used in magnetic and gravity studies. The principal use of SP methods has been to monitor subsurface water movement (i.e., observing a moving conductor in a magnetic field), although the method has been used with some success in geothermal exploration as well. In the geothermal case, in addition to the voltage from movement of geothermal fluids, mineralized waters may induce chemical reactions. Mapping the concentration gradients of chemically active leachates also may be another possible use of SP surveys.
EM methods are becoming more popular among those employing near-surface geophysical surveys. They measure the electromagnetic fields associated with the underground alternating currents induced by a primary above-ground field. In active EM surveying, the primary field is induced by passing an electrical current through a coil. This field spreads out in three dimensions and induces the flow of currents through underground conductors according to the physical laws of electromagnetic induction. A secondary electromagnetic field is then induced, which in turn distorts the primary field, and the ensuing final field is sensed by a receiving coil. The sensed field differs in intensity, phase, and direction from the primary field, thus revealing information about subsurface conductivity. EM methods do not require placing electrodes in the ground, and the surveys can sometimes be conducted from low-flying aircraft. One recent development in airborne EM offers the advantages of increased surveying speed and access to polluted, dangerous, or inaccessible areas via small (maximum dimension 1–2 m) unmanned aircraft. However, airborne surveys also have disadvantages, including limited separation between the source and receiver coils and a higher noise level caused by the movement of the coils through the earth's magnetic field.
In passive EM surveying, the earth's natural electromagnetic fields are used to provide the variations in the electric field. Among these is the audio-frequency magnetic field (AFMAG) technique, which uses electric fields generated by distant lightning flashes as a source. Another passive procedure uses very low radio frequencies (VLF). The VLF method relies upon the 15–25 kHz electric field from distant, powerful radio transmitters used to communicate with submarines.
High-resolution magnetic surveys, particularly those that employ magnetic gradiometry, are useful in shallow studies and in the search for buried metal objects such as steel drums. Gradiometry consists of taking simultaneous readings from two magnetometers spaced a few decimeters to a few meters apart and then analyzing the difference (the magnetic gradient) between simultaneous readings of the instruments. Magnetic surveys are also useful in mapping faults, locating magnetic bodies, and estimating the depth to magnetic earth materials. Such surveys are used to detect variations in the magnetite content of rocks and unconsolidated materials, so they can detect changes in some types of igneous rocks and other geologic structures. They are also used at contaminated sites to measure the perturbation of the earth's magnetic field caused by buried ferrous metal objects such as steel drums, the ferrous metal waste in landfills, and iron pipes. Although magnetic surveying is a relatively mature field, improvements in data precision and collection rates have enabled some gradiometry surveys to be conducted that produce image-like detail of such things as vehicular tracks on long-abandoned trails.
Microgravity surveys sometimes are used in shallow geophysical exploration, particularly where a large density contrast might be present. Such surveys use gravity meters with a sensitivity of 1 microGal along a profile line or a grid with typical spacing of 1–10 m. Relative to the earth's total field, the sensitivity of microgravity measurements is one part in one billion. Several geologic conditions including cavities, faults, folds, dipping layers, and lateral intralayer heterogeneity can cause microgravity anomalies, as do buried manmade features such as trenches, tunnels, disposal containers, and incipient subsidence problems. Microgravity methods cannot detect contaminants directly, although they are sensitive to fluid loss from the near-surface layers. Gravity gradiometry may increase in popularity when fast, accurate, and cheap gravity meters have been developed, particularly when used with an accurate global positioning system (GPS).
The seismic refraction method has been used in engineering and geotechnical investigations for virtually all 75 years of the SEG's existence. The reflection method, however, was not adapted for widespread use shallower than 30 m until the mid-1980s (Figure 1).
|
In future applications, combining P- and S-wave refraction methods will raise new possibilities. Among these is measuring the elastic parameters of rocks by virtue of their P-wave and S-wave velocities combined with density readings derived from gravity surveys or borehole density logs. With these three pieces of information, Poisson's ratio, Young's modulus, and the shear modulus can be computed. When these elastic constants have been measured, rock types can be identified and a preliminary determination of pore-space fluid content may be possible.
The recent advent of seismic hardware capable of collecting as well as processing high-resolution, near-surface data opens up new opportunities for three-component recording and multimode analysis. The capabilities of seismic methods involving target depths shallower than 30 m can be extended by analyzing the seismic wave types generally discarded by classical seismic reflection surveyors during the processing, analysis, and interpretation of data. Specifically, examining the near-surface broadband seismic wavefield is becoming possible by using three vector components rather than one and analyzing multiple types (modes) of seismic waves rather than P-waves alone.
Radiometric techniques measure the radiation emitted by radioactive isotopes. The methods can be used to explore for radioactive ores or to find radioactive contaminants. Spectral gamma methods are useful in identifying specific isotopes that occur within a meter or two of the earth's surface, and these methods may be useful in locating natural radioactive hazards, such as radon gas sources.
|
Some newer methods |
|---|
|
TopAbstractAccepted older methodsSome newer methodsBorehole methodsThe futureSuggested reading |
|---|
Among the more recently developed and promising methods is ground-penetrating radar. In many areas, GPR is considered the method of choice for exploring the upper few meters of the earth's subsurface by beaming a source of microwave radiation into the earth at a known time. The time required for the waves to echo back to the surface is used to calculate the depth to various layers in the earth, although velocity determination is a critical factor. In many ways, GPR is similar to seismic reflection. Data are displayed in a format that is (or can be) identical to seismic sections. The GPR method works best under dry conditions and in the absence of clays or other electrically conducting earth materials because electromagnetic radiation will not penetrate conductors.
Nuclear magnetic resonance (NMR) was introduced as a laboratory measurement to investigate molecular-scale phenomena by monitoring the change in the energy state of nuclei. A radio-frequency pulse excites the nuclei to a higher energy state. Then, their return to the original state is monitored, modeled as a sum of exponential decays, and recorded as two relaxation-time constants. The relaxation constant, T1, is associated with the longitudinal component of the magnetization, and T2 is taken from the transverse component. The NMR technique can be used to study any nuclei having an intrinsic magnetic moment, such as hydrogen or carbon 13.
Of specific interest to those in the earth sciences is proton NMR, which responds to the state of hydrogen nuclei in the ground. Referring to the near-surface geophysical use of NMR as a "permeability imaging" tool may be premature, however. The NMR effects of paramagnetic species (such as Fe3+) cause dramatic changes in T2 so that the direct link to the ratio of the surface area to volume in the earth material breaks down, thus making it much more difficult to obtain estimates of permeability. For example, two sands whose grain size, pore size, and distribution are identical could appear to have different permeabilities when one has a high Fe 3+ content and the other does not. Hence, in near-surface applications, the variation in the content of Fe3+ and other paramagnetic species could complicate or negate permeability estimates based on NMR data. Depending upon the specific location of the Fe3+ (i.e., in pore water, adsorbed to the solid phase, or in a solid mineral grain) T2 would be affected, but to a different extent.
|
Borehole methods |
|---|
|
TopAbstractAccepted older methodsSome newer methodsBorehole methodsThe futureSuggested reading |
|---|
|
The future |
|---|
|
TopAbstractAccepted older methodsSome newer methodsBorehole methodsThe futureSuggested reading |
|---|
Shallow seismic methods could benefit greatly from an increased analysis of surface waves. The petroleum industry has put great effort into removing surface waves from seismic reflection data, but relatively little effort has gone into enhancing and analyzing those waves, even though they are recorded as a normal part of the information gathered in shallow seismic reflection and refraction methods.
Every near-surface geophysical method can benefit from increased automation—from robots roving over the ground to model airplanes aloft carrying microsensing devices over hazardous or polluted areas. Even near-surface seismology may be amenable to the automated data acquisition.
At present the full waveform inversion of both seismic and GPR data is feasible only for small data sets because of the immense computational resources required. When computing costs have decreased sufficiently, these inversions may become commonplace. One caveat, however, is that the inversion process treats noise with the same reverence that it treats data. When noise is present in shallow seismic or GPR data, a data-inversion routine may produce artifacts related to its attempt to invert the noise.
Using geophysics to sense underground microbiological impacts remains a largely unexplored domain. This area may achieve greater importance when the ability to restore the underground environment using microbes has been developed sufficiently. Likewise, if underground microbial mining is realized, geophysical methods of monitoring those processes will be needed. Near-surface geophysics has the capacity to measure changes continuously, with high spatial resolution, over time, which could allow the monitoring of underground biological activity.
The areas of resolution and bandwidth in geophysical procedures must continue to be improved. Faster and more reliable methods of data processing would be useful as well, with special attention given to decreasing the ambiguities and uncertainties that now plague data interpretation. Furthermore, reducing costs will always be desirable. Cost reductions could be accomplished by combining robotics, automation, expert systems, and the use of miniature aircraft. Education concerning these methods must be improved along with the smooth transfer of technology from its developers to users and potential beneficiaries.
Lastly, near-surface geophysics can have an impact on public-policy issues, some of which bear on societal health and safety. Among these are earthquake-hazard mitigation, tsunami warning systems, pollution abatement, and even the question of global warming.
|
|
Suggested reading |
|---|
|
TopAbstractAccepted older methodsSome newer methodsBorehole methodsThe futureSuggested reading |
|---|
|
Footnotes |
|---|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
