midterm 2 GPR and Resistivity

GPR systsem used for geological investigations… including

detection of natural cavities and fissures

containment plume mapping

road pavement analysis

archaeological investigation

forensic science

GPR (ground pentrating radar) uses…

electromagnetic wave propagation and scattering to image, locate and quantitatively identify changes in electrical and magnetic properties in the ground.

GPR has the highest frequency and …

the highest resolution on subsurface imaging of any geophysical method, approaching centimeters under the right conditions

In the GPR survey

the amplitude and arrival time of the reflected EM pulse are measured

GPR uses electromagnetic waves in the 100-1000MHZ range to…

image the shallow subsurface. It measures travel time of reflected radar waves velocity controlled by the dielectric constant

GPR Fundamentals

Difficult to apply seismics to the study of shallow structure on the scale of 10’s of meters as high frequency signals are needed

GPR usually limited to depths of <20 meters, depending on the conductivity of the subsurface materials.

GPR can investigate >50 meters, for ice in polar regions

GPR Fundamentals (work goes back to 1930s) (QUIZ)

some work goes back to the 1930s but was generally held to be useless for geological applications due to the poor depth of penetration of high frequency radar signals

1970s: GPR was used for limited applications, such as cavity and pipeline dection

1980s-1990s: GPR in archeaological, engineering, and geological problems

• improvements in electrical hardware (antennas, amplifiers, fiber optics)

• improvements in micro processors to enhance week radar signals

Electromagnetic Spectrum for GPR range

typical frequencies used for GPR range from 25MHZ to 1GHz

Ground Penetrating Radar - Principle

• uses short pulses which are transmitted into the ground

• Reflections and diffractions of electromagnetic waves occur at boundaries between rock state and objects that have differences in electrical properties

• Electric permittivity (dielectric constant), electric conductivity determine the reflectivity of layer boundaries and penetration depth

Propogation of EM Waves

• EM waves of GPR are made up of two orthogional vector components

• 1) an electrical component (E)

• 2) a magnetic component (H)

• In a plane perpendicular to the direction of travel

GPR measurements

Physical Properties

• Dielectric Permittivity

  • considered the diagnostic physical property for GPR

  • impacts velocity and refelction/refraction of radio waves

  • significantly impacted by water content

• Electrical Conductivity

  • impacts attenuation (amplitude loss) of GPR signals

• Magnetic Permeability

  • only important if things are very susceptible (generally ignored)

Dielectric Properties

D=eE

water is one of the most polar molecules since the hydrogen atoms develop a positive charge and the oxygen develops a negarive charge

most materials are mixtures so need to consider a combination of properties

Wave Velocity

radar versus siemisic waves

• travel times associated with radar waves are shorter than seismic waves

• the wavelengths of radar waves are usually short compared to seismic wavelengths

  • higher resolution of GPR studies

Attenuation constant depends on the physical properties of the media

skin depth- distance at which a wave is reduced to 37% of its original amplitude

skin depth is smaller if the radiowave frequency is higher

skin depth is larger in materials with lower conductivities

skin depth is larger in materials with higher dialectric permittivity

GPR signal contains a range of frequencies

• important that they all behave in the same way

• othersie the characteristics of the reflected pulses will be affected, making them hard top recognize

Reflection and Transmission of Radiowaves

reflection at conductive object boundaries

• many compact objects relevant to GPR applications are highly conductive

• when radiowave cannot propogate through object

• because of this, when radiowaves reach the interface between the earth and a highly conductive obkective, the wave is completely reflected

Diffraction Hyperbola

• A point discontinuity will act as a point source and produce a diffraction that, like a reflection, will produce a hyperbolic event (seismic or GPR)

• Radar waves go out in all direction so the strongest reflections may not be directly below the radar antennas

  • examples include boulders, voids, pipes/drums

  • anomalous zone produces a diffraction hyperbola

Radar Velocities

• in seismic reflection, velocity is found during stacking from the moveout patterns

• In GPR, we only have one receiver

  • fixed offset survey

  • common midpoint survey

GPR vs Seismic

GPR waves

• electromagnetic waves, microwaves/radio waves

• depends on EM properties of medium

• Frequency = 10-2000Mhz

• Wavelength (200 Mhz, granite) 0.65m

Seismic Surveys (Refraction/Refelcttion)

• seismic waves

• velocity depends on mechanical properties of medium

• frequency= 0.1-100Hz

• Depends on the source

• Wavelength (100 HZ, granite) 50m

What causes a Reflection?

GPR

• change in relative permittivity

Seismic Surveys (Refraction/Reflection)

• change in acoustic impedance

Transmitter/Receiver vs Geophones

Seismic Refelction

• one source

• lots of geophones

• stacking

• moveout

GPR

• one source (transmitter)

• one receiver

  • both are antennas

• stacking performed by repeating the EM pulse

• Moveout

Principles of Operation

• radar systems comprise: a signsl generator , transmitting and receiving atennae, a reciever that can record and/or output the signal graphhically

• Tx produces a short pulse of waves lasting a few ns. Which travels through the subsurface. At interfaces where there is a change in the electromagnetic impednance the waves are reflected back to the surface and recorfed by the reciever

GPR Equipment

Two types of GPR system

• Monostatic: one antyennae is used both transmitter and receiver

• Bistatic: separate transmitter and receiver antennae are employed

Frequency range: 10Mhz and 2GHz

Wave speed in vacuum (air): 300,000 km/s or 0.3 m/ns

Travel time of radio waves is of the order of a few 10s to a few 1000s nanoseconds

Monostatic advantagesvs disadvantages

Advantages

• single antenna

• simple position determination

Disadvantages

• limited aperture

Bi-static system advantages vs disadvantages

advantages

• seprate antenna positioning

• large potential aperture

• multiple angles of reception

disadvantages

• accurate relative position, information required

• accurate time base recquired

• relative motion between

Data Acquisition and Processing

• GPR systems operate at different frequencies as stated above

• some antenna can be used as both transmitters and receivers - known as monostatic/trancievers mode or seperate transmitters and receivers can be used - bistatic mode.

Data Acquisition and Processing (A-Reflection Profiling)

• source and reciever antennae moved overground simutaneously at fixed seperation constant offset profiling

• several shots are summed at each surface position to improve signal to noise ratio

• succesive radar recordings displayed side by side at serperation corresponding to distance moved by antennae

• radar reflections displayed on a vertical axis corresponding to time, can be converted to depth if subsurface veolocities known. Sections can be corrected for surface elevations

Data Acquisition and Processing (B-Wide Angle Radar reflection (WARR))

• The TX is stationary and measurements are made while the RX is gradually moved away, equivalent to seismic reflection survey methods

• This type of survey mode is used to obtain an estimate of the radar signal velocity versus depth in the ground by varrying the antenna spacing at a fixed location measuring the change in travel time to the reflector.

• Reflection wave velocity can be determined from the x²-t² method

Data Acquisition and Processing (C-Common Midpoint or Common depth)

• The Tx-Rx spacing is increased and reflections from a specific common depth point are gathered, equivalent to seismic reflection survey methods.

• objective is to sample each subusrface point sevral times

• radar traces are grouped into CDP gathers on the basis of shared source-receiver midpoint bins

Data Acqisition and Processing (D- Transilllumination (radar topography))

• borehole radar surveys - one antenna is fixed to a borehole while the other antenna is moved past the stationary antenna in another borehole

• radar topography can be performed in boreholes or mines

Resolution

• Vertical Resolution: a measure of the ability to recognize individual, closely-spaced reflectors and is determined by the pulse length on the GPR section

• rule of thumb is ¼ the wavelength

• the wavelength is the velocity of propogation in the material divided by the frequency of the propagation wave

Waves with higher frequencies

higher resolution, attenuate more rapidly than longer period waves

smaller depth penetration

resolution and depth of penetration

wavelength and vertical resolution of 1.5Ghz frequency atenna according to various dielectric constants of the concrete

vertical resolution decreases as a function of depth since deeper travelling EM waves tend to have a lower dominant frequency due to the progressive loss opf higher frequencies by absorption.

energy loss and attenuation

• antenna transmission loss

• ground coupling effects

• reflection and transmission effects

• scattering

• geometric spreading

• absorption and attenuation

GPR limitations

• geological materials not well-suited for GPR surveying

  • any geologic model with high electrical conductivity (or low resistivity)

  • conductive materials attenuate EM waves

  • Clay (mud), metallic ores, salt water

• geologic materials well suited for GPR surveying

  • any geological material with low electrical conductivity (or high resistivity)

  • sand (quartz), nearly all crystalline rocks, freshwater, ice, concrete

Probing Distance Vs. Resolution

• higher frequencies give better resolution

• lower frequencied give larger probing distance

common sources of noise

• many possible signals and paths and the objective is to maximize the target response and mnimize others.

• to limit the effects of external sources, the transmiter and receivers are frequently protected by a sheild

Resistivity surverying investigates

variations of electrical resistance, by causing an electrical current to flow through the subsurfae using wires (electrodes) connected to the ground

Two Categories of Electrical Methods

Active and Passive

Active Method

current is applied to the earth, electrical resisitivity (er), induced potential (IP) and electromagnetic (EM)

Passive Method

use naturually occuring energy sources, telluric, magnetotelluric (MT), and self potential or spontaneous potential (SP)

Electrical Methods Applied Current Methods

DC Electrical Resistivity, Induced Polarization, Electromagnetic Induction, Magnetometric Resisitivity

they can use artificial or natural sources, most DC methods utilize artificial sources, electrical methods are probably the most widely used near surface geophysical techniques, for environmental investigations

DC Electrical Resistivity

potential difference (voltage) is measured at various points (direct current)

Induced Polarization (IP)

rise and fall time of the electric potential is measured

Electromagnetic Induction (EM)

Applies AC with a coil and the resulting magnetic feild is measured with another coil

Magnetometric Resisitivity (MMR)

mafnetometer surveys to detect DC magnetic fields produced by DC current flow

Electrical Methods Usage

Petroleum Companies, Mining Companies, Environmental, Groundwater and Egineering Studies

Petroleum Companies

well-logging, but uses of surface-based methods are minor

Mining Companies

heavy users of some types of electrical methods (detections of ore bodies, or environments that commonly host ore bodies)

Environmental, GroundWater and Engineering Studies

commonly used electrical methods (depth to bedrock, depth to water table, clay or sand content, direction of pollution brines)

Primary Objectives of Electrical Methods

• determine electrical properties of the subsurface (identify subsurface materials or to determine qualities of the groundwater)

• interpret the electrical structure at depth, which is usually assumed to be horizontally layered and to asses its geologic significance

• map lateral variations in electrical properties and to asses their geologica significance

Electrical Resistivity Basics

• a battery acts as an energy supply, pushing electrons around the circuit

• A resistor (R) resists the flow of current I=current put in by the transmitter, R=resistance of the circuit V=measured voltage

• For Earth System

  • inject current into the ground

  • measure potentials between electrodes

  • potentials depend upon distribution of subsurface resistivity

ER Method

measures potential difference at points on the earth’s surface by directing electrical current to flow through the subsurface using electrodes connected to the ground

ER method (current)

the current is injected into the earth through current electrodes, and the potential difference is measured between a pair of ptential electrodes. The current and potential electrodes are generally arranged in a linear array

To get current to flow, you must provide a push

the push is a potential difference or voltage, the flow is called the current

Resistance

the amount of potential difference recquired to push a given current is directly proportional to the resistance

Ohms Law

Resistance R=V/I

resistance depends on

the naterial properties such as resistivity… quanitifies how strongly a given material opposes the flow of electric current

the shape of the material that has a current flowing through it R=p I/a or P=Ra/I

Resistance is higher when current is forced through a small area long length

How do we measure resistivity?

apply a known potential difference to a circuit with a resistive material of known length and cross sectional area, then measure the current (w ammeter), this gives the resistance, R use the length and cross-sectional area to calculate

Electrical Methods

resistivity is one of the most variable physical properties

A resisitivity meter

consits of both a voltmeter and a current meter (ammeter).

most systems report the ratio V/I instead of each one separately

it gives the resistance, the resistance can then be converted into resistivity using geometrical parameters based on the type of array

Resisitivity of Geologic Materials

resistivity if the subsurface depends upon:

• the presence of certain metallic ores

• the temperature of the subsurface

  • geothermal energy

• the presence of archeological features

  • graves, fire pits, pot holes

• amount of groundwater present

  • amount of dissolved salts

  • presence of contaniments

  • % of porosity and permeability

Rock Types and Resisitivty

igneous rocks → higher resistivity

• why? only a minor component of pore water

Sedimentary rocks → tend to be the most conductive

• why? abundant pore space and fluids

Metamorphic rocks → have intermediate but overlapping resistivity

• why? hydrous minerals and fabrics

Rock Types and Resisitivity cont

rocks are usually porous and pores are filled with fluids, mainly water. As the result, rocks are electrolytic conductors.

electrical current is carried through a rock mainly by the passage of ions in pore waters

Electrical Conduction in the Earth

for earth materials, three main types of conduction: electronic resistivity, electrolytic/ionic resistivity, and dielectric permittivity

Electronic Resisitivity (1)

movement of electrons in metals semiconductors like silicate minerals (temperature dependent)

Electrolytic/Ionic Resistivity (2)

movement of cations (+) and anions (-) in opposite directions, salts dissacoiate into ions in solution and move

Dielectric Permitivity (3)

occurs in insulators, which contain no free electrons. displacement of protons/electrons within their orbitial shells. Not important at low frequency (to DC) but important at high frequency for GPR

Rocks and Mineral Resisitivity

resistivity spans several orders of magnitude, making it one of the most widely varying properties of geology

Rocks and Mineral Resistivity Cont

1. Naturally Occuring Metallic Conductors and Semi-Conductors: graphite, native metals, and many types of sulfides have very low p.

2. Soils and unconsolidated overburden: p ranging from 10 (wet clays) to >1000 ohms (sands, dry soils)

3. Sedimentary Rocks: Shale: vary from 10"‘s to a few 100”s of ohms Sndstone: a few 100’s to >1000 ohms Limestone: fresh >1000 ohms

4. Igenous and Metamorphic Rocks: commonly >1000 ohms, except where heavily fractured and weathered where they commonly drop to several 100’s ohms or less

Factors Influencing Electrical Resisitivity in Rocks

• porosoity

• pore saturation (% air or gas)

• Hydrocarbon Fluid Saturation

• Water Salinity (Total Dissolved Solids)

• Clay Content

• Metallic Sulfide Mineral Content

• Fluid Temperature

• Rock Matric intrinsic resisitvity

Effects of Clay Minerals

• extra water can be tied up in clays minerals

• clay minerals supply ions to the groundwater

• clay minerals abosrb a layer of highly exchangable cations (enhanced conduction along surface of clay grains) diffused layer further out. Not strongly bound, free to move in E-Field

• All rocks containing (wet) clay minerals exhbiti low resistivity

Archies Law

empirical relationship definifng bulk resisitivity of a saturated porous rock

Factors that will DECREASE the resistivity of a rock

1. add more pore fluid

2. Increase the salinity of the pore fluid - more ions to conduct electricity

3. Fracture rock to create extra pathways for current flow

4. Add clay minerals

5. Keep fluid content constant but improve interconnection between pores

Factors that will INCREASE the resistivity of a rock

1. Remove pore fluid

2. Lower salinity of pore fluid

3. Compaction-less pathways for electric current flow

4. Lithification - block pores by deposition of minerals

5. Keep fluid content constant but decrease connection between pores

How is current distributed?

the fraction of the total current (if) penetrating depth z for an electrode seperation of d is …

the greater the electrode seperation, the greater the depth to which a given percentage of current penetrates

Subsurface Current Paths

• current penetration depends upon current electrode seperation

• about 70% of the current applied by two electrodes at the surface stays within a depth equal to the seperation of the electrodes

• Typically electrode spacing is 2x your target depth but depends on electrode spread (array)

Electricity follows the path or area of least resistance

Fundamental principle (conservation of charge): all the current that goes into a body must come out. there are no sources or sinks of current anywhere, except at the current electrode itself

Depth of Current Penetration

By increasing the electrode spacing, more of the injected current will flow to greater depths

Current Flow - Two Layer Medium

When p2>p1, the current flow lines are more closely spaced in layer 1. current density is higher in layer 1

when p1>p2, the current flow lines are more closely spaced in layer 2. current density is higher in layer 2

Apparent Resistivity

in a heterogenous medium, the measured resistivity is apparent resistivity which is a function of the form of the inhomegeneity and of the electrode spacing and surface location.

it is dependent on the resistivity along the entire path that the current traverses.

Electrode Arrays/Spreads

Electrode Array- consists of two electrodes at which DC current flows into and out of the ground plus two electrodes between which the potential difference at the surface is measured,

The apparent resistivity measured by different arrays is not the same because the geometric factor K is different

Choice of the best Array depends on

1. the type of structure to be mapped

2. sensitivity to the resistivity meter

3. background noise level

Choice of the best array things to be considered

1. depth of investigation

2. sensitivity of the array to vertical and horizontal structures

3. horizontal data coverage

4. signal strentgth

Two main types of resisitivity survey methods

A- Vertical Electrical Sounding (Sounding Mode)

B- Constant Seperation Traversing (Profiling Mode)

A: Vetrtical Electrical Sounding

expanding spread traversing

depth profiling

electrical drilling

resistivity sounding

B: Constant Seperation Traversing (profiling mode)

constant spread traversing

electrical mapping

resistivity profiling

Vertical Electrical Sounding (VES)

measurements are repeated as array is expanded about a fixed point, mantaining the relative spacing of the electrode

used to determine mainly the depth variation in resistivity

used to find overburden thickness, aquifers, and other horizontal structures

Wenner Array

All four electrodes have to be moved for wach measurement

Dipole-Dipole Array

rarely used for VES surveys

Schlumberger Array

potential electrodes are kept fixed until measured voltage decreases to low values as potential gradient in ground falls with increasing current electrode separation. then potential electrodes are moved and the process is repeated

Examples of Resistivity Data - VES

apparent resistivity usually plotted on logarithmic scale against electrode seperation

Constant Seperation (spread) Traversing (CST)

measurements are repeated as array is moved along a profile with electrodes mantained at fixed distances

used to determine lateral variations in resistivity

used to detect shear zones, faults and other vertical boundaries

acqusition can be simplified by laying out more than four electrodes and using a subset for reading

Examples of Resisitivity Data - CST

resistivity values are plottedon linear scale against locatiion of centre of array along profile

Sources of Noise in the Data

• Instrument Noise

• Cultural Features

• Telluric Currents - naturually occurriing earth currents

  • self potentials - generally caused by either geochemical reactions or greater than normal subsurface fluid flow

  • magnetolleuric currents - electromagnetically induced by naturally occurring or man-made magnetic fields

  • may be unavoidable

Current Induction in Measurement Cables (noise in data)

An AC current flowing through the cables connecting the current source to the current electrodes can produce an induced current in the cables connecting the voltmeter to the voltage electrodes, thereby generating a spurious voltage reading

can be reduced by using arrays where current and measurement wires dont overlap

Geologic Noise

Near surface variations: can dominate response thus masking signature of deeper targets

Topography: currents will be focused under valleys and dispersed under hills, thus causing perturbations in measured voltages. can think of in terms of charge build up on topographic interfaces

Field Procedures

Survey Design - depends on the specific characteristics of the site and the objective of the survey

Common Modes of Resistivity Survey:

• Sounding Mode: to map the depths and thickness of stratigraphic units

• Profiling Mode: lateral electrical resistivity contrast, such as lithogic contrast

• Sounding Profiling Mode: electrical resistivity varies both vertically and horizontally, such as in contaminant plume mapping

Application to Location of Permafrost

Permafrost represents significantly difficulty to construction projects due to exacavattion problems and thawing under construction. Ice has high resistivity og 1-120 ohm. For example, in Fair banks Alaska, they need to identify pemafrost prior to construction of road cutting