Physical Environment

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The Physical Environment

Geophysical Methods Lecture

This lecture looks at applications of geophysicsin environmental studies. The Main objective is
to alert you to the possibilities of using geophysical data in problem-solving.

General Scheme

Describe physical basis
Discuss simple theory
Show responses
Case history

Simple Textbooks

"Principles of Applied Geophysics" (Parasnis)
"Field Geophysics" (Milsom)
"Introduction to Geophysical Exploration" (Keary & Brooks)
"Geotechnical and Environmental Geophysics" (ed. Ward)
- or any text in the same part of the Library! (DDC 622.15 or 551.15)

What is it?

Geophysics is

"The use of observations of physical phenomena to infer the structure of the Earth."

Geophysics observes

the effects of differences in physical properties of rocks of different kinds.

Why do it?

Geophysics uses

Measurements made at, or near, the surface (or in boreholes)

Geophysics produces

Models showing physical property distributions below the surface

These distributions are interpreted in terms of other information about the soil or rock.

A simple example

Locating waste buried in steel drums
Steel affects the Earth's magnetic field in its vicinity
We can easily measure changes in the magnetic field strength at the surface
The shape and size of anomalies indicates location of drums
The example shows a map of the magnetic field in a test area, about 15 m square. The 200-litre drum is buried about 3 m below the ground surface.

Magnetic Field Mapover Steel Drum

Rock Property Examples are shown in the graphic.

Physical Fields corresponding to these properties are listed inthe next graphic.

By measuring these physical fields,Geophysics allows "remote sensing" of rocks below the surface.

Fields are usually measured at the Earth's surface, but give rise to a third-dimensionalview.

Anomalies

Geophysics is widely used in resource exploration
Effort is put into locating "anomalies" as in the drum example
Environmental studies often involve imaging "all" of the subsurface
Still a developing sub-branch of Geophysics

In this lecture, we look only at ElectricalMethods, relating to a later section in the subject, on Groundwater. Electricalmethods can give an image response over all of the subsurface, showing structuressuch as layers and changes in water properties.

But there's more
Seismic, Magnetic and Gravity fieldmeasurements can also be directly applicable, but will be described someother time! (For further introductory information, see the Website for therecently-withdrawn subject Geophysicsand Remote Sensing.)

Electrical Methods

Distinguish between materials based on their electrical properties.
Range in properties is very wide think of conductors and insulators
Variety of measurement methods also very wide
Rock, soil materials share the range of properties

Electrical properties

Soils and rocks show differences inelectrical resistivity, due mainly to their porosity and saturatingfluids.

Fluids are especially important, since these carry the electrical current.

Ohm's Law and Resistance

See the graphicfor a reminder about the definition of the Resistance of an object.

Property of Matter?

How does Ohm's Law apply to current flowingwithin an extensive body such as the Earth?

To start, take samples of rock, and make "resistors" from them.What determines the Resitance of those resistors?

The next graphicshows how experiments lead to the definition of the property "Resistivity",which is a basic physical property of materials.

Resistivity

Summarising the observations suggeststhat a single parameter is needed to define the dependence on the materialfrom which the resistor is made. The relationship becomes

Resistivity, a Property of Matter

For extended volumes (e.g. soil, rock,water) the "resistor" model is inappropriate

Resistivity is the material property. (Conductivity - the inverse - is alsoused.)

Matter Resistivities

Units of resistivity are Ohm.m

Insulators have very high resistivities

1.E+12 to 1.E+15 ohm.m is possible

Conductors have very low resistivities

1.E-08 ohm.m is possible - at ordinary temperatures

The range is extremely wide!

Fluid Resistivities

Resistivity of groundwater dependson salinity, temperature

For Salinity S in g/l
Temperature 20 C
Resistivity in ohm.m is given by:

Note that the Resistivity of hydrocarbonfluids is generally very high - they are good insulators.

Rock resistivities

The next graphicshows some general rock resistivities for orientation purposes. Note thatthe near-surface materials such as alluvium are in general less resistivethan "solid" rocks.

Real Rocks

Generally, rocks and soils in situare:

Porous, to a greater or lesser degree, and
Saturated, with a usually-saline water (pore fluid)

Archie's Law

This is based on observations of saturatedsediments.

It is observational, and empirical, butit is

Consistent with theory that current flowsthrough fluid, not "rock"

Why this form?

Matrix minerals usually insulators
Pore fluid (groundwater) usually saline, so more conductive than minerals
Electrical current then flows through fluids saturating pore spaces
- Rock and soils below the water table will almost always be saturated.

Distinguishing between earth materials

(In sediments) the dependence on porosity is both crucial and sufficient
Even if the saturating fluid is uniform, porosity differences from one geological bed to the next are likely
Resistivity changes are therefore also likely
- Note in the cartoon the representation of change of porosity at an interface

Distinguishing between earth materials

Environmental studies are often interested in water properties explicitly
Even if porosity is uniform, salinity differences in time or space can occur
Resistivity changes are therefore also likely, and can be used to image the water property changes.

Measuring Earth Resistivity Distributions:
Using four electrodes on the surface

With four electrodes, we can measure voltage difference set up by known currents. These measurements are inverted to resistivity values. Usually, the electrodes are set up in one of a number of geometrically-simple Resistivity layouts.

For example, for four electrodes in line, equally spaced (distance a) [Wenner Array], it's easy to show that

So measuring resistivity is "easy"

This technique is generally known as"Galvanic Resistivity".

Surface electrical mapping.

Resistivity of rocks-in-place can be measured with such simple experiments
Because electrical currents pass through "all" of the rocks, the result is an average resistivity.
Average is interpretable with simple analysis
- To detect variations in distribution, we observe a relationship between -

Array size and response to layers:

If Earth truly uniform, this process reports resistivity, independent of array size "a"
If Earth layered, however, response will depend on layer thicknesses and resistivities, and will change with array size
We use this to perform "Electric Drilling"
- Resistivities are measured as a function of array size.
- The measurements are interpreted in terms of a model of change of properties with depth.

Landfill study Dataexample

This shows the result of one such "depthsounding", with the interpretation on the right of the graphic. Thebackground to this experiment was as follows:

Landfill study

Client claimed the landfill base was secured by a clay seal (the "host rocks" were clay)
Resistivity experiment shows low resistivity values below landfill.
- This is consistent with, but not necessarily, clay.
Subsequent drilling showed no clay, and no seal -
- the low resistivity below the landfill results from leachate, contaminating the sandstone