Nonintrusive investigation techniques are geophysical in nature. Geophysical techniques measure contrast in material properties in the subsurface and include the following:
- Seismic methods
- Electrical and electromagnetic procedures
- Gravitational field techniques
- Magnetic field techniques
- Ground Penetrating Radar
Geophysical investigations can be a timesaving and cost-effective method for providing qualitative subsurface information for a site. They can be used for screening large areas for potential buried wastes, for focusing resources for intrusive investigation activities on the anomalous areas, and for identifying or confirming the presence and extent of landfills and/or burn dumps. Buried solid waste and metals will (most likely) exhibit different bulk material properties than the surrounding native soil. This will typically allow geophysical instruments to distinguish the waste form the soil.
The results obtained from a geophysical investigation are subjective and rely on geologic interpretation. Geophysical techniques do not directly measure the parameter needed to solve the problem but instead measure contrasts in material properties. For example, seismic methods measure velocities of seismic waves through the subsurface material and electrical methods measure the conductivity (or it’s inverse, resistivity) of a material.
The interpretation of geophysical contacts is based on geologic assumptions: (1) earthen materials have distinct subsurface boundaries, (2) a material is homogeneous (material properties are the same throughout) and (3) the unit is isotropic (material properties are the same in all directions). Since these conditions rarely occur in nature, and almost never occur in solid waste or burn ash, geophysical methods are most often used in conjunction with other intrusive methods in order to more correctly assess the site.
Although geophysical interpretations are not always perfectly accurate, geophysical equipment is very precise. That is to say that the measurements obtained from non-intrusive geophysical techniques are very exact. The raw data is good data. The problem resides in the geophysical interpretation of the data, which are often educated estimations and/or calculated correlations and can lead to inaccuracies. However, when the appropriate geophysical technique is coupled with an intrusive investigation, large volumes of material can be explored accurately and cost-effectively.
Non-intrusive geophysical methods can be utilized as preliminary screening before performing intrusive investigations, they may be implemented as the primary investigative technique, they may be used in combination with intrusive investigation methods such as bore holes or test pits, or they can be used in combination with other non-intrusive geophysical methods. Understanding the specific strengths and weaknesses of each method will allow the investigator to decide how to best utilize geophysical investigation.
The following tables compare various methods and point out strengths and weaknesses:
Advantages and disadvantages of seismic methods
Applications of Geophysical Methods
A variety of non-intrusive investigative techniques can be used to study environmental issues. The location and boundaries of buried wastes can be best approximated using Ground Penetrating Radar, Electromagnetics, and/or Resistivity. The following are site-specific examples of successful use of geophysical investigative techniques.
- Landfill Resistivity Survey at Brownfield Site in Arizona
- Electromagnetic Survey Used to Map Sulfides and Acid Sulfate Ground Waters at the Abandoned Mine: USGS
- Yucca Mountain (high-level nuclear waste repository) Geophysical Studies:: USGS
- Landfill Study for a Planned Residential Development in Southern California
- Geophysical Investigation of Chemical Waste Landfill in Northwestern Arkansas: USGS
- Geophysical Characterization of Landfill Flow Path, Winthrop, Maine: USGS
- Surface-geophysical investigation of the University of Connecticut landfill, Storrs, Connecticut: USGS
Site Investigation Reports
A table listing site investigations completed by the CIWMB is available. The investigations are listed alphabetically by site name and include: Solid Waste Information System (SWIS) number, type of plan or report produced, investigation type, types of operations, and whether the site is located in a rural or urban setting. The table identifies site investigations during which geophysical survey operations were utilized.
Burger, H.R., Exploration Geophysics of the Shallow Subsurface, 1992. [with] accompanying Macintosh computer software by Douglas C. Burger and H. Robert Burger. Englewood Cliffs, N.J.: Prentice Hall, c1992.
Fowler, C.M.R., 1995. The Solid Earth: An Introduction to Global Geophysics. Cambridge University Press. New York. 76-96.
Seismic geophysical methods use a seismic source and receivers to “see” the subsurface using compressional waves. The velocity of the seismic waves are recorded by the receivers, called geophones (Spring-mounted electric coils moving within a magnetic field, which generate electric currents in response to ground motion.), and correlated to the material properties of the subsurface. Compressional (P-waves) waves are generated by a hammer and propagate down into the earth. Geophones “listen” for the waves to return to the earth’s surface. Careful analysis can tell us whether it is a direct surface wave, one reflected from a subsurface geologic interface, or a wave refracted along the top of a geologic interface.
Seismic refraction measures the seismic velocity of the subsurface material, which is related to density and elastic properties and therefore can be correlated to a material type. It is more commonly used for shallow subsurface investigations than seismic reflection.
More information on seismic refraction
Seismic reflection measures the seismic velocity of the subsurface material, which is related to density and elastic properties and therefore can be correlated to a material type. It is different from seismic refraction in that it records the reflected seismic waves. Seismic reflection is commonly used in oil exploration and for deep subsurface exploration.
More information on seismic reflection
Electrical and Electromagnetic Procedures
Geological materials have different electrical properties. The variations in these properties are useful geophysical parameters for characterizing earthen materials. Subsurface variations in electrical conductivity (or its inverse, resistivity) typically correlate with variations in water content, fluid conductivity, porosity, permeability, and the presence of metal. These variations may be used to locate subsurface features whose electrical properties contrast with the surrounding earth. For example, decaying solid waste and metal have a higher electrical conductivity than most soil and therefore produce anomalous readings in measured conductivity readings.
The methods used to measure the properties of geologic materials can be divided into two types: methods using applied currents and those using naturally occurring currents. Those methods that used applied currents include electrical resistivity, induced polarization, and electromagnetic surveying. Methods that use naturally occurring current flow include telluric surveying, magnetotelluric surveying, and the self-potential method.
Two of the more commercially utilized techniques are the resistivity method and the electromagnetic (EM) method. Resistivity can provide better vertical resolution and is generally less sensitive to interfering noise such as fences, buildings and overhead power lines. EM requires no direct contact with the ground surface, so the data can be acquired more quickly than with resistivity. For more specific information on all the previously mentioned methods, visit the Southwest Geophysics website.
More on electromagnetic surveys
Example of an electromagnetic subsurface image:
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More on electrical resistivity
Gravitational Field Technologies
Gravity is defined as the force of attraction between two masses. The most commonly understood gravitational force is between the sun and the earth. However, lateral density changes in the earth’s subsurface cause a change in the force of gravity at the surface of the earth. A subsurface body of a different density from its surroundings will attract a mass on the surface to a greater or lesser extent than the surrounding earth. By analyzing the change in gravitational attraction along the surface of the earth these subsurface anomalies can be detected.
Geophysicists can use gravity measurements to help them understand the internal structure of the earth while an environmental scientist can use gravity measurements to locate underground gravitational anomalies. A gravimeter is an instrument designed to measure spatial variations in gravitational acceleration. There are various types of gravimeters in existence today. The most common gravimeter used in surveys is based on a simple mass-spring system.
Gravity measurements alone are very difficult to analyze, there are many solutions and interpretations to the observed measurements. Gravity surveys and measurements are commonly used in conjunction with other studies to confirm theories drawn by geologists. Mathematical corrections to measured anomalies are often necessary and should be applied with the discretion of a geologist or geophysicist.
More information on gravity and gravitational surveys
Defining Landfill Geometry Using a Gravimetric Survey
Roberts et al.* performed a gravity survey of a municipal solid waste landfill in order to investigate the effectiveness of the method in establishing the lateral boundaries and the vertical extent of the landfill. The survey consisted of approximately 200 gravity stations spaced at 5-meter to 10-meter intervals. Bouguer and terrain corrections were made to the raw data. The information gathered from the gravity survey was compared to data from boreholes and pre-landfill and post-landfill topographic maps. The results of the survey closely correlated with the topographic maps and borehole data.
To illustrate the importance of obtaining independent geologic information about the site prior to performing a gravity survey, Roberts stated “the gravity method depends on precise gravity and surface elevation measurements, careful computations, and constraining information obtained from collateral geologic and geophysical studies for the interpretations to be valid.” (Roberts et al p. 259).
* Roberts, R.L., Hinze, W.J., and Leap, D.I., 1990, Application of the gravity methods to investigation of a landfill in the glaciated mid-continent, USA in Ward, Stanley., ed., Geotechnical and environmental geophysics; Volume 2: Environmental and groundwater: Society of Exploration Geophysicists Investigations in Geophysics No. 5, p. 253-266.
Magnetic Field Techniques
Whether on the surface or below, iron objects or minerals cause local distortions or anomalies in the earth’s magnetic field. Magnetometers measure these variations in the magnetic field. Magnetometers were originally designed for mineral exploration, but are now used in the environmental field for locating buried steel drums, tanks, pipes, and iron debris in trenches and landfills. A magnetometer detects local buried iron objects because the object causes the (locally) uniform magnetic field of the Earth to strengthen or weaken depending on the size, orientation, and magnetic characteristics of the object.
The magnetometer can only sense ferrous materials such as iron and steel. Other metals like copper, tin, aluminum, and brass are not ferromagnetic and cannot be located with a magnetometer (but may be found with a metal detector). An object with weaker magnetic characteristics may be detected at a maximum depth of about 5-10 feet. On the other hand, large masses of drums may be detected easily to depths of 10-40 feet.
Example of a magnetic survey image
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More information on magnetic surveys
Ground Penetrating Radar
Ground penetrating radar is a geophysical method that generates a continuous profile of the subsurface. GPR’s radiate a very short burst of radio-frequency energy into the ground to detect discontinuities. A transmitter antenna generates high frequency radio waves that propagate into the earth in a broad beam. An echo from various subsurface interfaces is reflected back to the observer from a remote target. The strength of the echo is dependent on the absorption of the signal on the path to and from the target, the size and shape of the target, and the degree of discontinuity at the reflecting boundary. GPR’s detect a boundary between rock and air (a cave or cavity) or between one type of soil and another (for example undisturbed soil to waste). The dielectric properties of the subsurface materials correlate with many of the mechanical and geologic parameters of these materials. The performance GPR is limited by attenuation. The signals in moist soils, especially soils having high clay content, can be significantly attenuated. GPR is most useful in detecting changes in the geometry of subsurface interfaces.