Engineering geophysics

Engineering geophysics involves the application of earth science to problems of interest to the engineering and groundwater geologist, including site evaluation, resource exploration, and pollution monitoring. Research methods in the field are focused on discovering shallow targets by seismic, electrical, magnetic, electromagnetic, gravity, and radar surveys.

Seismic Refraction and Reflection

The methods of engineering geophysics include a number of surveying techniques that detect the physical characteristics of materials and objects in the earth's subsurface. The technique may detect such phenomena as rock and soil layers, the thickness of glaciers, disturbed or disrupted soils, buried walls or foundations of archaeological interest, buried metal drums, buried sand and gravel deposits, and aquifers. Each of these targets has a distinct set of physical properties that may be sensed from the surface with the right kind of survey. The selection of a particular geophysical surveying instrument and method depends upon the size, depth, and other characteristics of the target, as well as upon the nature of the surrounding materials. The most commonly used methods are seismic refraction and reflection, ground-penetrating radar, magnetic and gravity surveys, and electrical and electromagnetic techniques.

The seismic refraction method relies on the different wave velocities of rocks and soils and the principles of refraction, which govern all wave motion. A seismic survey is accomplished using a number of seismic sensors, or “geophones,” that are arrayed in a straight line and are usually spaced at regular intervals. Geophones consist of a spring-mounted magnet moving within a wire coil to generate an electrical signal. Recent designs use microelectromechanical systems technology to generate an electrical response to ground motion. These phones will detect the arrival of seismic waves from an artificial energy source such as a hammer blow, a dropped weight, or an explosive charge. The additional equipment includes a digital recording system that measures the interval between the time that the energy is put into the ground and the time that it is picked up at the geophones. The velocities of the seismic waves are then calculated from the distance between the energy source and a particular geophone and the time elapsed for the wave to reach that geophone. The data are simplest to interpret where layers are flat and of constant thickness; more complex equations allow interpretation where layers are tilted and of variable thickness.

Seismic reflection profiling is used to detect surfaces at depth. In general, the seismic array of the energy source and geophones is kept under 10 meters in total length because the waves that are detected are traveling along nearly vertical paths. The elapsed time for the wave to reach a geophone is a function of the velocities of the subsurface layers and their depths. The applications of this method for shallow targets are more limited than seismic refraction because of surface noise and the difficulties in identifying individual reflections.

Ground-Penetrating Radar

Ground-penetrating radar, or GPR, works on the same principles as seismic reflection but does not suffer from the same noise problems for shallow investigations. This technique measures the time necessary for a controlled pulse of radar energy to return from a reflecting surface. The equipment involves a transmitter and receiver that may be towed across an area to detect buried objects and soil layers.

While radar waves can travel great distances in air or space, they are rapidly absorbed by rocks, soils, and especially water. For this reason, GPR is used for depths of 10-15 meters. The depths of radar penetration are even less in areas underlain by clayey soils or shale, both of which contain large amounts of water in the crystal structure, or where the water table is high. For most engineering purposes, GPR is a fast and efficient survey method in relatively smooth terrain.

Magnetic and Gravity Surveys

Magnetic and gravity methods involve the detection of variations in the natural magnetic and gravity fields of the earth. Changes in the magnetic field are caused by the presence of materials that have a high magnetic susceptibility. These substances include very magnetic materials, such as metallic iron and steel, but also may include rocks and soils that have elevated quantities of the mineral magnetite. Either the very magnetic metals or the slightly magnetic rocks and soils may be detected by surveying the area of interest using a magnetometer. Magnetic surveys are often used to locate metallic targets that are quite small.

A gravity survey detects variations in the gravitational field as a result of changes in density of subsurface materials. Denser materials will cause a slight increase in gravity, while less dense objects cause a small decrease in gravity. A small object may be detected only if it has large contrast in density. If the density contrast of the target is small, as it is in most engineering applications, the object must be large to be detectable. Gravity surveys are usually employed to find large bodies of rock or soil as opposed to smaller objects. The survey techniques perhaps used most in engineering geophysics detect the electrical properties of subsurface materials. The contrasts in electrical properties of natural and human-made materials are very large, so surface surveys can detect anomalous regions easily.

Electrical and Electromagnetic Techniques

Electrical-resistivity surveys involve an array of electrodes planted in the ground in one of a number of patterns. Generally, two electrodes are used to introduce a current into the ground, while other electrodes measure the voltage drop between two points in the array. The pattern of the array and the spacing of the electrodes are chosen based on the character of the target and whether the investigator wishes to see a single depth or different depths. Measurements are interpreted by a series of complex equations that model the depths, thicknesses, and resistivities of the soil and rock layers.

Terrain conductivity is an electromagnetic method that measures the electrical properties in the subsurface without the necessity of placing electrodes. The method uses a pair of electrical coils, one to “broadcast” and the second to receive. The first coil is energized with an alternating current, creating an alternating magnetic field that penetrates into the earth. Where this magnetic field encounters a good conductor in the subsurface, a secondary electrical current is generated or induced. The secondary electrical current is accompanied by a secondary magnetic field that is detected at the surface by the receiving coil. The depths of penetration of the conductivity method are related to the spacing of the two coils and to the power of the electrical system.

All the methods used in engineering geophysics involve time and expense. Nevertheless, they are used because they may provide additional data on the continuity of conditions discovered from outcrops, drill holes, or excavations. In addition, anomalous regions within the site may become targets for more detailed study by drilling or coring.

Site Evaluation

Geophysical surveys are used to detect potential problems and provide additional information on a site undergoing evaluation. Engineering geology involves, for example, site evaluations for buildings, highways, well fields, dams, and sanitary or hazardous waste sites. Engineering geologists also deal with questions of slope stability, where data on the thickness of the unstable or moving part of the slide may be needed to assess the feasibility of construction or to develop plans for slope stabilization. In all these cases, information on the depth to bedrock, the lateral continuity of soil or rock layers, and the existence and orientation of surfaces within the rocks may be needed. Additionally, the geologist may have to detect human-made objects within an area. These include buried or abandoned gas and power conduits, storage tanks, and metal drums. One of the major problems requiring engineering geology today is the monitoring and correction of contaminant leakage from hazardous waste disposal sites across the country. In these situations, data are required to define geological conditions around the site or to map the extent and location of the plume of the contaminants that may be moving off the site.

For each of these needs, a geophysical survey may be useful. The first step in the selection of a specific technique depends on the identification of the target. Those physical properties of the target that contrast most with the properties of the surrounding rock and soil will indicate which geophysical technique may be most valuable. Next, the expected size, shape, orientation, and depth of the target will be guides in the selection of the best survey methods. Finally, the character of the terrain (whether it is dry or wet and boggy, topographically smooth or irregular) will be considered. Three examples may serve to elucidate this kind of planning.

Landfill Application

One hypothetical application of engineering geophysics and landfills involves a site at which a large number of 55-gallon steel drums containing toxic chemicals were buried many years ago. In this case, the drums need to be located and removed from the site. The task must be accomplished carefully, for it is essential that none of the drums be split or broken. Knowledge of the exact location of the drums would greatly lessen the chances of spilling toxic waste during the process of removal. A number of geophysical methods might apply, but the geologist wishes to choose the surest method. The geologist begins by considering the physical properties and sizes of the bodies in an effort to eliminate some methods. While the density of the drums is likely to be slightly different from that of the surroundings, the drums are too small to result in a measurable gravity anomaly. Similarly, a seismic refraction survey is not likely to detect individual objects as small as the drums even if there is a large contrast in velocity between the metal drums and the other material on the site.

Each of the remaining methods reviewed earlier might be useful. Seismic reflection and GPR would be able to detect reflections off the drums. In this situation, however, the drums are probably too shallow to use the seismic method, since surface-wave noise will be high. GPR, in contrast, would do the job very well and has worked in other areas. The only weakness of GPR is that the survey line must cross directly over a drum to detect it. To ensure finding all the drums, a very close, and therefore expensive, set of GPR survey lines would be needed.

Electrical methods could detect the large contrast in the resistivity of the metal drums. Either a resistivity or a terrain-conductivity survey could sense the drums. Because the electrical or electromagnetic field extends out beyond the direct survey line, either method could locate drums slightly off the direct line of the survey. A drawback to the method, however, would be the possible existence of salty water or other electrolytes in the landfill. If these fluids are present, their high electrical conductivities could mask the presence of the drums. Because the drums are steel, they will cause a magnetic anomaly, and a magnetic survey will be able to detect even those drums off of the survey line. Although the magnetometer will detect other iron objects in the landfill, the method may be the fastest and least expensive method to survey the site for this particular target and should result in locating all the drums.

Landslide Application

In the second example, the geologist needs to determine the depths to the slip-surface, or base, of a landslide. In this case, the target is too deep for GPR, and the variations in density and magnetic susceptibility are expected to be too small to produce a significant anomaly. Previous studies, however, have shown that seismic refraction and electrical surveys can be quite successful in solving this kind of problem. The upper, mobile, and generally disrupted part of the landslide will have a different seismic velocity from that of the lower, undisturbed rocks or sediments. While the thickness of the upper layer is likely to be variable and the slip-surface complex, the seismic method has been useful. Electrical methods will depend either on a difference in conductivity of the disrupted upper layer and the undisturbed rock beneath or on the conductivity of the slip-surface itself. The exact nature of the conductivity contrast will depend on the local geology, the soil moisture, and the groundwater conditions.

Water Supply Application

The third example involves a common problem with water supplies in coastal areas where fresh groundwater floats typically above a deeper zone of saltwater. In this case, withdrawal of freshwater from wells leads to a drawdown of the water table and also to intrusion from the sides and from below of saltwater from the ocean. In many places, the supply of potable water is very limited by the occurrence of salty water beneath the thin lens of freshwater. The depth to and migration of saline water can be monitored by wells at critical places, but to obtain a more continuous set of data across a region, a geophysical survey is desirable. The greatest contrast in physical property will be the high electrical conductivity of the saltwater versus the low values of the freshwater. Surface electrical surveys have been successfully used to estimate the thickness of the freshwater layer above saltwater and to monitor the extent of the migration of saltwater laterally into freshwater aquifers. Similar applications of electrical techniques have been used to detect acid mine drainage or to monitor movement of electrically conductive pollutant plumes from industrial or waste-disposal sites.

Data Interpretation

Engineering geophysics involves the application of general geophysical techniques for shallow investigations. The methods are appropriate for depths from a few meters to a few hundred meters. Applications of geophysics to shallow targets often involve greater complexity in interpretation than for deep targets because the roughness and irregularities of buried objects or surfaces are large compared to the depth. Similarly, many of the targets of interest to the engineer are not simple in shape. Surfaces such as bedrock buried beneath soil or recent sediments or the fracture patterns in bedrock are inherently complex. Furthermore, the transitional character of physical properties in near-surface environments may be difficult to detect or model. Examples of such transitions include velocity changes caused by gradations from unweathered to weathered bedrock and density and electrical property changes caused by variation in the percentages of clay or sand in surficial gravels.

While geophysics is often used in a qualitative fashion, in many areas, mathematical modeling allows a quantitative measure of the depth, size, shape, and composition of the target. Seismic refraction and reflection and GPR generally give the most unequivocal information about the subsurface. Magnetic and gravity surveys yield information on the depths, sizes, and character of subsurface objects, but usually more than one interpretation will fit the data. Electrical and electromagnetic methods also supply data that can be interpreted in a number of ways. Thus, some geophysical methods are less exact than others, and all involve uncertainties in the interpretation. For these reasons, it is important to gather geophysical information in areas where other kinds of geological data are available from surface mapping, drilling, or excavation. The combination of geophysical and geological data provides an excellent basis for the evaluation of sites of engineering interest, for estimation of water or other resources, and for the monitoring of hazards.

Geological and Value

The power of geophysics lies in its use to detect anomalous regions and to evaluate the lateral continuity from one point to another across an area of interest. In many cases, the existence of an anomaly can guide the engineering geologist to the critical area where drilling or excavation will be used to acquire data. In other cases, extensive drilling and sampling in an area may tell much about the region, but information is needed between the sample sites. In these cases, geophysical surveys may be used to show the extent to which the regions between sample sites are the same as the sample sites themselves. The nature of the array will allow the scientist to look to different depths in a single area or to improve the accuracy and precision of the data.

The use of geophysics to solve problems in geological and civil engineering has become more common as the need for more detailed information onsite assessment and evaluation has become apparent. While nothing can match the factual nature of soil and rock sampling by drilling, excavation, or examination of outcrops, the need to test the continuity from one sample site to another requires additional data. Geophysical methods can supply that additional information through detection of the bulk physical properties of materials in the subsurface or the interfaces between the different materials.

The bulk physical properties that can be detected include density, magnetic susceptibility, seismic-wave velocity, and electrical conductivity or resistivity. The surfaces separating different bodies in the subsurface may be thick enough to have their own bulk properties, but generally they act as reflectors of seismic or radar wave energy.

Principal Terms

geophysical survey array: a description of the orientation and spacing of sensors and energy sources relative to one another for a geophysical survey

geophysical target: the object or surface that one wishes to detect by means of a geophysical survey; knowledge of the target is essential to selection of a survey type

physical property contrast: the difference in a characteristic (density, velocity) between the object of interest and its surroundings

site evaluation: a process whereby a site is selected or rejected as a location for a particular use such as construction or mining

survey line: a usually straight line along which points are located where geophysical measurements will be taken

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