Frank Morrison

Erika Gasperikova

John Washbourne

A web based course on the
principles, modeling and interpretation

used in Applied Geophysics

This is a course about the methods used to map the distribution of physical properties beneath the surface of the earth. The methods consist of measurements made on the surface of the Earth, or at selected points in boreholes, which are interpreted to yield the distribution of subsurface properties. The physical properties, which can be mapped with applied geophysics, are density, magnetic susceptibility, electrical conductivity, dielectric permittivity, and seismic velocity.

The measurements are interpreted by using numerical models or simulators of the subsurface. The models are used to simulate a realistic geological description of the ground and the parameters of the model are varied until the model ‘data’ matches the observed data. Unlike many other engineering disciplines, direct observation of the system is impossible and full-scale physical modeling is equally unrealistic. The geophysicist is ultimately left with numerical simulations of the suspected geology as the only practical means to recover quantitative values for the subsurface properties.

The physical property distribution inferred from the field measurements is, in turn, only indirectly related to the properties of interest to the geologist or subsurface engineer. In mineral exploration, rock type or mineral composition may be sought, in petroleum exploration porosity, permeability and oil saturation are the main parameters of interest, and for the geotechnical engineer water content, clay content and mechanical properties may be required. The inferred physical properties are linked to the desired properties by constitutive relations, which accurately map a desired property into the measurable property but not the other way around. The problem is that a physical property such as density depends on porosity, pore fluid density and saturation and rock mineral density, none of which can be derived directly from the bulk measured density. A fundamental aspect of Applied Geophysics is that a useful interpretation has to incorporate realistic assumptions about the parameters that are used in the simulator models.

The seismic method of applied geophysics is the most widely used and most thoroughly studied of all the methods. The interpretation is also the most direct in the sense that the field data, the travel times for wavelets reflecting off subsurface interfaces, yield a first order image of the subsurface structure. The seismic method is the least dependent on models of the subsurface for its interpretation although the difficulties of interpreting the mapped seismic velocities in terms of desired rock properties are shared with all the other techniques. Commercial and educational packages are widely available for interpreting seismic data and there is not the same need for modernizing the teaching of the method that there is in the non-seismic methods. In this version of this course we focus on the non-seismic methods.

The non-seismic methods of applied geophysics are usually grouped into the categories of potential field (gravity and magnetic) methods, electromagnetic (EM) methods, and a collection of methods that use naturally generated electric fields that are grouped in this course under the title of coupled flows. The low frequency end member of the em methods, which uses essentially direct current, is called the resistivity method. The high frequency end member, which uses radar frequencies, is called the Ground Penetrating Radar (GPR) method.

All these methods rely on the measurement of fields which are defined by classical physics. In some cases an ambient or natural field is perturbed by changes in a physical property of the subsurface. In others, one introduces fields with a controlled source and the response of the ground is measured, and in some coupled flow phenomena electric fields are actually generated by subsurface processes.

The basic principles of these methods are taught by way of models. The model is used for a conceptual description of the subsurface, usually simplified to begin with, and to calculate, or simulate, the response of a particular geophysical survey over a certain class of subsurface geology. The response of this simple model serves to illustrate the principles of the method, to design a practical field experiment or survey and ultimately, with added model complexity, to interpret the results of a survey.

This new course incorporates models as an integral part of the format. It is web based and is accessible via internet browser. It takes advantage of the hypertext features of html to present the material at several different levels, but the key new feature is the use of interactive figures to facilitate modeling. The Level 1 text is a set of compact notes at an introductory undergraduate level. The basic physics of each geophysical method is presented along with a review of the relevant physical and rock property relationships. The field techniques are presented and traditional static figures showing equipment, field layouts and the response over typical targets illustrate common applications. Some of the figures in this text can be activated by clicking on the figure. The screen is then occupied by the simulator: Menus permit parameters of the subsurface model and the configuration of the survey to be selected. The actual model is ‘drawn’ with the cursor and the results of the simulated survey are plotted above the model. Field data can be entered manually via a data entry table and displayed with the model data. The user can interpret this data by iteratively changing the model parameters, or starting a new model, until an acceptable fit is obtained to the field data. These modeling windows are used to explain the basic physics of the method, design appropriate surveys for given targets, and then interpret survey results. They capture the essence of applied geophysical methods in a way that has never been possible with traditional texts.

Level
2 contains the full theory for each method and is presented at upper division
level. Level 2 text is accessible directly or by clicking on highlighted
words or phrases in the Level 1 text. The user is required to have
prior courses or familiarity with upper division physics (particularly
electricity and magnetism and potential field theory), vector calculus,
partial differential equations, integral equations and linear algebra.
The principles of the numerical algorithms that are used in the model calculations,
product information and technical data from manufacturers of geophysical
equipment, historical footnotes and most importantly, field case histories,
are also filed in Level 2. Finally, there is provision for a Level
3 which consists of a library of important, classic, papers and articles
on the various methods. This level is again accessed directly or
reached by clicking on a referenced author in the Level 1 or Level 2 texts.

*Last update: January
19,2001**Comments or suggestions
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