Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV EM Review – Some Basic Ideas Phone x 4316

The terrain conductivity method employs an alternating electromagnetic source to induce current flow in subsurface materials. The combined source or primary field and induced or secondary field is measured by a passive receiver coil. Recall that the flow of current creates a magnetic field. Current flowing in a wire as shown at right will generate a magnetic field that encircles the wire. The magnetic force is everywhere tangential to the wire and decreases in intensity with distance from the wire Remember the right hand rule!

The interaction of electric charges is defined by Coulomb’s law as shown here. F 12 Electric Force  Permitivity of free space q 1 and q 2 electric charges q1q1 q2q2 r 12 Electric field intensity

This relationship becomes complicated when we start considering the movements of these charges Electric field intensity Magnetic field intensity Velocity of charge q The bars over the letters mean that these quantities are vectors.

This relationship is known as the Lorentz relation. Terms like With the bar across the top indicate that we are considering the orientation of this quantity along with its amplitude This is a type of vector multiplication referred to as the cross product

Geometrically, the cross product is defined as shown below If v and B are perpendicular the result is just the product vB; however, if there is some angle  between these two vectors, then the result becomes vBsin .

Now consider the powerful magnet portrayed at the right. Its poles have been bent around to face each other. The field lines exit one pole (+) and enter the other (-). The drawing illustrates the basic rules for depicting field line orientations and polarity. Imagine that you throw an electron into the field between the two poles of the magnet. What will happen to it? The behavior of the electron in the magnetic field is defined by another “right hand” rule.

Connect the tale of electron velocity vector to the tale of the magnetic field vector The particle will turn and circle about the field lines. The sign of the charge is important!

The magnetic field lines are pointing into the board The electron moves along a clockwise curving path

From Halliday and Resnick What is the orientation of B? B points toward you.

Recall that the effect of current flow through a coil is to produce a magnetic field like that of a bar magnet.

You may find it useful to review basic relationships associated with the interaction of electric and magnetic fields. Recall Faraday’s Law of Induction - - and Lenz’s law, which states that “the induced current will appear in such a direction that it opposes the change that produced it.”  induced electromotive force  change in magnetic flux  t change of time

Movement of a magnetic field through a coil produces a change of flux enclosed by the coil of wire, which leads to current flow wihtin the coil. Faraday’s Law

Illustration of Lenz’sLaw The field produced by the induced current opposes the field of the magnetic field introduced into the loop

Terrain Conductivity/Resistivity Applications bedrock depth groundwater extent water table depth water salinity contamination plumes archaeological sites Determination of specific lithology is quite difficult since the conductivity/resistivity of any given lithology can vary greatly depending on porosity, fluid content and permeability, but it is certainly possible when other subsurface control such as well data is available in the survey area. after McNeill - TN5

The terrain conductivity method offers certain benefits over the alternative electrical resistivity method. The terrain conductivity meter evaluates subsurface electrical properties without the necessity of making direct contact with the ground surface. Its effectiveness is not limited by the presence of highly resistive layers in the near surface. The terrain conductivity survey can be conducted much more rapidly. BENEFITS LIMITATIONS It is difficult to collect extensive sounding-type data because of the limited range of intercoil spacings available on conventional terrain conductivity meters.

We begin by covering some basic definitions Ohm’s Law V is potential difference i current R resistance Resistance, defined as opposition to direct current flow, is not a fundamental physical attribute of materials since it varies depending on the conductor geometry. The geometrical influences are evident in this relationship where  is the resistivity, l the conductor length, and A the cross-sectional area of the conductor

The resistivity  represents a fundamental physical property of the conductor, and this or its inverse  (the conductivity) are the parameters we wish to measure. In general - Resistivity is the property of a material which resists current flow. A l  or 

Units - The unit of resistance is the ohm Balancing units in the definitional formula for resistivity, we see that resistivity has units of ohm-meters or  -m. Conductance is the reciprocal of resistance and has units of ohm -1. Thus conductivity (  ) has units of ohm -1 /m or mho/meter

The reciprocal of a resistivity of 1  -m corresponds to a conductivity of 1 mho/meter 1 mho/meter = 1000 millimhos/meter 1 millimho/meter =0.001 mho/meters The reciprocal of mho/meters is 1000  -m Working back and forth between units of conductivity and resistivity Conductance (1/R) is often measured in units of Seimens (S) which are equivalent to mhos. 1 S = 1 mho

In general when given a resistance the equivalent conductivity in millimhos/meter is obtained by taking the inverse of the resistivity and multiplying by The same applies to the computation of resistivity when given the conductivity. 100  -m = _____ millimhos/meter 20 millimhos/meter = _____  -m 10 50

Factors Affecting Terrain Conductivity 1. Porosity: shape and size of pores, number 2. Permeability: size and shape of interconnecting passages 3. The extent to which pores are filled by water, i.e. the moisture content 4. Concentration of dissolved electrolytes 5. Temperature and phase state of the pore water 6. Amount and composition of colloids

Clay particles are a source of loosely held cations

Cation clouds provide a source of electrolytes, they can also form a partial barrier to current flow through small pores. In this case their effect is similar to that of a capacitor.

Readings from the text * –* Introduction to Applied geophysics Burger, Sheehan and Jones Chapter 1 Approaching the Subsurface1-6 Electromagnetic terrain conductivity measurements at low induction numbers J. D. McNeill, Technical Note TN-6: Geonics LTD. Reading Assignment