What is GeoPhysics?

Welcome to the "What is Geophysics" section of the EEGS web site. The purpose of this page is to provide you with a brief introduction to the field of Environmental and Engineering Geophysics, with links to more detailed information contained within various "information repositories".

This web page provides a Near Surface Geophysics Glossary (MS Word document), some fundamental definitions, and descriptions of many of the ways in which geophysics is used - ways in which specific methods or techniques are employed to address environmental and engineering problems. It is also a resource for finding answers to the questions listed below.

  • What are the commonly used Near Surface Geophysics terms and their definitions?
  • What types of problems or applications can be addressed with geophysics?
  • What benefits or advantages are gained from using geophysical methods?
  • How do the sub-disciplines of Environmental Geophysics and Engineering Geophysics relate to each other?
  • What geophysical methods are available for various applications?
  • How are various geophysical methods applied in practice?
  • How are geophysical data interpreted and used for problem solving?
  • What are the costs of geophysics?
  • Who can provide the equipment, software, or services needed to acquire the data and information for effective site decision-making?

We hope that the "What is Geophysics" section is will be one of the most dynamically evolving parts of www.eegs.org. We suggest you book-mark this page and check back periodically to see the new information that has been added.

What are the commonly used Near Surface Geophysics terms and their definitions?

Click here to open a MS Word file to view a Near Surface Geophysics Glossary. If you have corrections, additions, or suggestions regarding the glossary, please contact Paul Wolfe at Wright State University.

What is Geophysics? A Question with Many Answers ...


A Worden gravity meter is used to
locate a buried river channel.
One of the most comprehensive and widely used definitions of geophysics is that provided by Sheriff in the Encyclopedia of Exploration Geophysics. To review this "classical" definition of geophysics, visit www.seg.org and select the "What is Geophysics" topic from the "You and Geophysics" main topic on the menubar.

For the purposes of this site, we refer more specifically to the following definitions - these focus on Environmental and Engineering Geophysics:

  1. Geophysics is: The subsurface site characterization of the geology, geological structure, groundwater, contamination, and human artifacts beneath the Earth's surface, based on the lateral and vertical mapping of physical property variations that are remotely sensed using non-invasive technologies. Many of these technologies are traditionally used for exploration of economic materials such as groundwater, metals, and hydrocarbons.
  2. Geophysics is: The non-invasive investigation of subsurface conditions in the Earth through measuring, analyzing and interpreting physical fields at the surface. Some studies are used to determine what is directly below the surface (the upper meter or so); other investigations extend to depths of 10's of meters or more.

Both of these definitions have a common component, namely that geophysics represents a class of subsurface investigations that are non-invasive (i.e. that do not require excavation or direct access to the sub-surface). The exceptions are borehole geophysical methods that expand the use of holes already drilled to access the subsurface on a very localized basis.

In addition, Definition 1. focuses on the key targets of interest (i.e. geology, geological structure, etc.) - a key consideration in understanding the realm of Environmental and Engineering Geophysics.

Definition 2. underscores the near surface aspect (i.e. in contrast to other geophysical applications, such as petroleum or mineral exploration, this type of problem solving deals with shallow depths that are most significant in terms of the lives, work and activities of the earth's human population.)

What are the Types of Problems Addressed?


The sledge hammer provides a 
source of energy for
determination of the depth
to water table and bedrock.
Generally, environmental and engineering problems fall into the following classes or types:

  • Infrastructure (highways and bridges)
  • Groundwater (exploration and contaminant mapping)
  • Geohazards (earthquake mitigation and collapse structure mapping)
  • Urban (utility mapping, underground storage tank location)
  • Geologic Mapping
  • Archeology
  • Forensics (i.e., illegal burials, etc.)
  • Civil Engineering / Non-Destructive Testing (NDT)
  • So-called "Brownfield" and Landfill Investigations
  • Unexploded Ordnance (UXO detection and characterization)
  • Dam Safety

As described above, information on each of these areas will be developed and added to www.eegs.org dynamically - so please check back periodically to see what new information is available.

How Do Environmental and Engineering Geophysics Relate to Each Other?


Magnetometers and gradiometers are used
 to acquire high resolution data for
 non-invasive archaeological investigation.
In looking at the previous topic, it may not be immediately apparent that there are distinct components of environmental and engineering sub-disciplines within each "problem" or application area.

Applications, such as Groundwater and especially contaminant plume mapping, UXO, and Archeology are closely aligned with Environmental Geophysics. Here, the focus is on human health, society and history in contrast to the engineering properties and structures on which Engineering Geophysics focuses.

Other applications that focus primarily on the Environmental sub-discipline include pre-development base line studies and re-development studies (i.e. Brownfields)

Infrastructure applications have more of an Engineering" component, that is, dealing with the detection and characterization of dangerous roadbed conditions underlying highways, for example. This type of application may involve detecting voids under roadways due to underground (i.e. mining or tunneling) excavations, or characterizing the relative integrity of reinforcing structures in bridges or other transportation structures.

Other applications that focus primarily on the Engineering sub-discipline of Environmental and Engineering Geophysics include Dam Safety, and Civil Engineering (including determining the engineering properties of rocks and soils before construction is planned).

However, it is not always possible to draw such clear distinctions between each sub-discipline. For instance, there are clearly components of both Environmental and Engineering Geophysics at work in Geohazards studies.

What are the Benefits of Geophysics?


Data from very low frequency
electromagnetics have been
converted to electrical current
density and depth. The dark blue
color indicates the core of
a leachate plume emanating from
a landfill.
Environmental and Engineering Geophysics offers a unique window into the earth as a means of detecting sub-surface conditions, and its relevancy lies in the concrete and cost-effective benefits it delivers. These include:

  • Non-destructive. It is ideal for use in populated areas, such as cities, where many of today's environmental and engineering issues arise. It also means an archeological site can be examined without destroying it in the process.
  • Efficiency. It provides a means of evaluating large areas of the subsurface rapidly.
  • Comprehensiveness. Combinations of methods (i.e. multi-disciplinary methods) provide the means of applying different techniques to solve complex problems. The more physical properties that are evaluated, the less ambiguous the interpretation becomes.
  • Cost-effective. Geophysics does not require excavation or direct access to subsurface (except in the case of borehole methods where access is typically by drilled holes). This means vast volumes of earth can be evaluated at far less cost than excavation or even grid-drilling methods.
  • Proven. The majority of techniques have been in existence for more than a half-century and are mature, yet still relatively undiscovered and underutilized by decision-makers who face complex environmental and engineering problems.

What Geophysical Methods are Available?


Horizontal loop electromagnetic
apparatus is used to locate
conductive zones that may be
leachate plumes.
As noted previously, geophysical methods as applied to Environmental and Engineering Geophysics were derived from other principal areas of subsurface investigation, including petroleum, mineral and groundwater exploration. 

With this in mind, the methods or techniques most commonly employed by practitioners include:

  • Electromagnetics
  • Gravity
  • Ground penetrating radar (GPR)
  • Magnetics
  • Resistivity (and / or induced polarization)
  • Seismic refraction (and / or near surface seismic reflection)
  • Spontaneous potential (or "SP")
  • Induced polarization (or "IP")

For a brief introduction to each of these methods, please refer to the following links that have been provided courtesy of several professional consulting groups. (The use of these links does not represent an endorsement of products or services by EEGS or its Board or Directors.)

Note that information about geophysical methods will be added in future to this site and you may want to check back periodically for the latest information.

How are Geophysical Methods Applied in Practice?

The implementation of geophysical methods is a structured process that consists of a number of key steps, including:

  • Initial evaluation of the problem at hand (i.e. what is the suspected problem, what initial information is known about the site, what additional information is required, and what are the desired outcomes)
  • Determination of which geophysical method (or combination of methods) will yield the optimal results. Not all methods will be applicable as noted in some of the links above, therefore, it is critical to carefully assess which methods are most likely to provide data and information relevant to the problem of interest. Also, while some methods may provide information, they may not be cost-effective in a particular context.
  • Identification of the scope (or size) of the required geophysical coverage.
  • Assessment of the way in which the data and information are to be acquired, interpreted and presented so as to address the issue at hand.

After these basic questions have been answered and the project approved, the geophysical work will commence.

Typically, Environmental and Engineering Geophysics consists of field surveys conducted along oriented lines (i.e. survey grids) over the desired area of interest. For more information on field surveying, you may want to refer to the links provided above in the "What Geophysical Field Methods are Available" section.

How are Data and Information Analyzed and Interpreted?


Data from a grid of ground
penetrating radar stations
is displayed in a cube.
The horizontal slices indicate
possible fractures in limestone
bedrock.
As the field of Environmental and Engineering Geophysics has matured in the last fifteen years, the nature of its application is also changing. Formerly, geophysicists were often called in to address issues that had already reached a "crisis" state. For example, a community may have discovered the presence of tainted water in their wells. Professionals were called in to map and assess the associated contaminant plumes as a means of determining the extent of the problem and develop possible remediation actions.

Now, however, the application of environmental and engineering geophysical surveys is increasingly becoming a "planned" activity that is included in budgeting (for example, in highway construction projects or dam construction).

A prerequisite for both of these types of implementations (i.e. crisis-response and planned) is the availability of data that are reliably and accurately interpreted and presented. Typically, data evolve through a "life-cycle" that consists of the following stages:

  • Data acquisition. Acquisition of digital data records according to the specifications and capabilities provided for by instrumentation manufacturers.
  • Processing. Synthesis of data and removal of cultural or other types of "noise" that might reduce the overall quality of results and ability to interpret results.
  • Visualization. Development of profile, plan and 3-dimensional maps and presentations for interpretation and presentation of results. These need to be immediately comprehensible by the non-geophysicist.
  • Modeling. Application of mathematically or computationally derived models that replicate the sub-surface geologic conditions and provide synthetic data in close agreement with actual data.
  • Interpretation. Integration of all data, visualizations and models into a final report based on the experience of the geophysical professional and conformable to geological / geophysical / geochemical constraints.

Ultimately, the objective is to develop an integrated presentation of all available data and information that can be used not only for decision-making but for other things as well, such as establishing time-indexed baseline information. Another increasingly important objective is to ensure that the data and information are archived for future access (i.e. results may be required for reference in future activities such as re-measurements to compare evolution of conditions over time, or for legislative or judicial procedures).

What are the Costs of Geophysics?

Cost is, of course, a key consideration. Most Environmental and Engineering Geophysical surveys have a cost structure that is similar to that of any licensed professional: an hourly consulting fee plus equipment rental costs. In addition, there are associated costs of mobilization (since most geophysical surveys require acquisition of data in the field), instrumentation amortization, data processing and interpretation, and report writing and presentation.

Ultimately, the application of geophysics must be assessed in terms of its projected costs and benefits as indicated above. EEGS professionals are trained to advise in developing cost and benefit assessments. It makes no sense to conduct a geophysical survey if the costs are projected to exceed any possible economic gains, or to exceed the project's operational budget. In general, however, geophysical surveys are almost always substantially less expensive than traditional non-technical means of investigation such as excavation or drilling.

Who Can Provide the Data Acquisition, Software and Services Required?

Many of the people who are available to assist in addressing Environmental and Engineering problems are members of EEGS. To find out more about who is available to address your specific issues or provide you with more information, you may want to browse our supplier links or our Board of Directors list.

When working with an EEGS member, you may want to keep in mind that our role is to:

  • Foster and encourage the application of geophysical techniques for environmental, engineering, and mining applications
  • Promote research and education in these areas
  • Provide a means for communication among researchers, practitioners, and end users of geophysics.

But, first-and-foremost, our role is to service the groups who require our assistance. This commitment is expressed in our Code of Ethics that states that Environmental and Engineering Geophysicists have a responsibility to:

  • Promote the public health, safety and welfare by applying our specialized knowledge to the evaluation of geologic, environmental and engineering problems.
  • Serve our clients and employers with honesty and integrity and place priority on quality of service.
  • Practice as faithful agents for clients and employers with loyalty that is consistent with legal obligations and ethical practice.
  • Interact with honesty and integrity toward all colleagues.
  • Advance our profession of Environmental and Engineering Geophysics.

As you browse this web site, we hope that you will gain more familiarity with the concepts, practices and benefits of Environmental and Engineering Geophysics as well as greater contact with the professional members of our community. We are ready to work with you to address the specific issues that are most important to your community, work and well being.

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