Remote Sensing and Remote Sensing and geophysical Methods geophysical Methods for Evaluation of for Evaluation of Subsurface ConditionsSubsurface Conditions

Matt HoustonMatt Houston

IntroductionIntroduction

Remote sensing and geophysical Remote sensing and geophysical methods encompass a wide range methods encompass a wide range of airborne, surface, and of airborne, surface, and downhole tools which provide a downhole tools which provide a means of investigating means of investigating hydrogeologic conditions and hydrogeologic conditions and locating buried waste materials. locating buried waste materials.

IntroductionIntroduction

Advantages and disadvantagesAdvantages and disadvantages There is no universally applicable There is no universally applicable

geophysical method. geophysical method. Some methods are quite site Some methods are quite site

specificspecific It is up to the user to select a It is up to the user to select a

method or methods and how they method or methods and how they apply to specific site conditions apply to specific site conditions and project requirements. and project requirements.

AdvantageAdvantage

Unlike other methods, the Unlike other methods, the geophysical methods provide geophysical methods provide nondestructive, insitu nondestructive, insitu measurements of physical, measurements of physical, electrical, and geochemical electrical, and geochemical properties of the natural or properties of the natural or contaminated soil and rock. contaminated soil and rock.

DisadvantageDisadvantage

The success of a geophysical The success of a geophysical method depends on the existence method depends on the existence of a sufficient contrast between of a sufficient contrast between the measured properties of the the measured properties of the target and background target and background conditions.conditions.

BackgroundBackground

Traditional approaches to Traditional approaches to subsurface field investigations at subsurface field investigations at hazardous waste disposal sites hazardous waste disposal sites have often been inadequate.have often been inadequate.

BackgroundBackground

Site investigations have relied Site investigations have relied upon direct sampling methods upon direct sampling methods such assuch as

1.1. Soil borings and monitoring wells for Soil borings and monitoring wells for gathering hydrogeologic data and soil gathering hydrogeologic data and soil and water samplesand water samples

2.2. Lab analysis of soil and water samples Lab analysis of soil and water samples to provide a quantitative assessment of to provide a quantitative assessment of site conditionssite conditions

3.3. Extensive interpolation and Extensive interpolation and extrapolation from these points of dataextrapolation from these points of data

BackgroundBackground

This approach has evolved over This approach has evolved over many years and is commonly many years and is commonly considered a standard.considered a standard.

However, there are many pitfalls However, there are many pitfalls associated with this approach, associated with this approach, which can result in incomplete or which can result in incomplete or even erroneous understanding of even erroneous understanding of site conditions.site conditions.

BackgroundBackground

The single most critical factor we The single most critical factor we face in site evaluation work is face in site evaluation work is accurately characterizing the sites accurately characterizing the sites hydrogeology.hydrogeology.

If an accurate understanding of the If an accurate understanding of the sites hydrogeology is obtained then sites hydrogeology is obtained then the predicting the movements of the predicting the movements of contaminants, or designing a clean contaminants, or designing a clean up operation would be easy.up operation would be easy.

BackgroundBackground

If all strata where uniform and horizontal If all strata where uniform and horizontal then 1 monitoring well would do, then 1 monitoring well would do, unfortunately this is not the case in unfortunately this is not the case in most areas.most areas.

In order to accurately sample an area of In order to accurately sample an area of unknown hydrogeology one would have unknown hydrogeology one would have to reduce the area to swiss cheese, in to reduce the area to swiss cheese, in order to find fractures, channels, sand order to find fractures, channels, sand lenses, and local permeable zones.lenses, and local permeable zones.

Sample DensitySample Density

The area of The area of the target the target (At) is 1/10 (At) is 1/10 of the entire of the entire area (As). So area (As). So As/At=10As/At=10

Sample DensitySample Density

With a 100% With a 100% probability of probability of detection one detection one would have to would have to have 16 have 16 monitoring monitoring wells. As As/At wells. As As/At get larger the get larger the amount of wells amount of wells needed goes up needed goes up drastically. drastically.

Sample DensitySample Density

This is the primary reason for the This is the primary reason for the application of geophysical application of geophysical methods.methods.

How Geophysical How Geophysical Methods are UsedMethods are Used GPM GPM

measure a measure a large large volume of volume of the the subsurface.subsurface.

Anomaly Anomaly detectorsdetectors

How Geophysical How Geophysical Methods are UsedMethods are Used Once an overall characterization Once an overall characterization

of the site has been made using of the site has been made using GPM and anomalous zones GPM and anomalous zones identified, a better drilling and identified, a better drilling and sampling plan can be designed.sampling plan can be designed.

How Geophysical How Geophysical Methods are UsedMethods are Used Locating soil borings and monitoring Locating soil borings and monitoring

wells to provide samples that are wells to provide samples that are representative of site conditions.representative of site conditions.

Minimizing the number of samples, Minimizing the number of samples, borings, and/or monitoring wells required borings, and/or monitoring wells required to accurately characterize a site.to accurately characterize a site.

Reduce field investigation time and cost.Reduce field investigation time and cost. Significantly improving the accuracy of Significantly improving the accuracy of

the overall investigation.the overall investigation.

How Geophysical How Geophysical Methods are UsedMethods are Used Much greater confidence in the Much greater confidence in the

final resultsfinal results Fewer borings and wells and Fewer borings and wells and

lower cost.lower cost. Monitoring wells cost 75,000 Monitoring wells cost 75,000

(1987 dollars) over 30 years.(1987 dollars) over 30 years. Smart HolesSmart Holes

Imaging MethodsImaging Methods

Large Scale aerial photosLarge Scale aerial photos Small Scale aerial photosSmall Scale aerial photos SLAR (Side Looking Airborne SLAR (Side Looking Airborne

Radar)Radar) Thermal Infrared**Thermal Infrared**

Thermal InfraredThermal Infrared

Measures the thermal responseMeasures the thermal response Earth’s surface emits radiation in Earth’s surface emits radiation in

the thermal infrared wavelength.the thermal infrared wavelength. Fluctuations can help identify Fluctuations can help identify

springs, seeps from a landfill, springs, seeps from a landfill, moist and dry areas, vegetation moist and dry areas, vegetation stress, can be used to stress, can be used to characterize surface soil and rockcharacterize surface soil and rock

Nonimaging MethodsNonimaging Methods

Do not result in a picture, but Do not result in a picture, but provide a measurement of some provide a measurement of some parameter along the flight path.parameter along the flight path.– Electromagnetic measurementsElectromagnetic measurements– Magnetic measurementsMagnetic measurements– Radiometric measurementsRadiometric measurements– Ground-penetrating radarGround-penetrating radar

Ground-penetrating Ground-penetrating RadarRadar Reflection Reflection

of radar of radar waves waves occur occur when when there is a there is a change in change in the the dielectric dielectric constantconstant

Conductivity and Conductivity and Resistivity Methods Resistivity Methods Resistivity and conductivity are Resistivity and conductivity are

inverse of each other.inverse of each other. They are a function ofThey are a function of

– Type of soil and rockType of soil and rock– PorosityPorosity– Conductivity of the fluids that fill the Conductivity of the fluids that fill the

pore spacespore spaces

Conductivity and Conductivity and Resistivity MethodsResistivity Methods AnomalieAnomalie

s can be s can be detecteddetected

Inorganic Inorganic plumeplume

Seismic Refraction and Seismic Refraction and ReflectionReflection Uses seismic velocity of the rock Uses seismic velocity of the rock

or soil.or soil. Some sort of sound wave is Some sort of sound wave is

emitted and measured by a emitted and measured by a geophone as it travels back to the geophone as it travels back to the surface. surface.

Seismic Refraction and Seismic Refraction and ReflectionReflection Can be used toCan be used to

– Determine the top of bedrockDetermine the top of bedrock– Depth of water tableDepth of water table– Assess the continuity of geologic Assess the continuity of geologic

stratastrata– Locate fractures, faults, and buried Locate fractures, faults, and buried

bedrock channelsbedrock channels

Seismic RefractionSeismic Refraction

Refraction is generally used in Refraction is generally used in very shallow studies. (A few very shallow studies. (A few Hundred Feet)Hundred Feet)

With enough energy a study of With enough energy a study of few thousand feet can be few thousand feet can be conductedconducted

Seismic RefractionSeismic Refraction

Seismic ReflectionSeismic Reflection

Same technique as refraction but Same technique as refraction but can measure much deeper.can measure much deeper.

Seismic ReflectionSeismic Reflection

Micro GravityMicro Gravity

Measures the change in earth’s Measures the change in earth’s gravitational field caused by gravitational field caused by changes in density in the soil or changes in density in the soil or rock.rock.

Micro GravityMicro Gravity

ApplicatioApplicationsnsof GPMof GPM

Applications of GPMApplications of GPM

Downhole Geophysical Downhole Geophysical MeasurementsMeasurements Natural Gamma LogNatural Gamma Log Gamma-Gamma (Density) LogGamma-Gamma (Density) Log Neutron-Neutron (Porosity) LogNeutron-Neutron (Porosity) Log Induction LogInduction Log Resistance LogResistance Log Spontaneous-Potential LogSpontaneous-Potential Log Temperature LogTemperature Log Fluid Conductivity LogFluid Conductivity Log Caliper Log Caliper Log

Natural Gamma LogNatural Gamma Log

Records the amount of natural Records the amount of natural gamma radiation emitted from a gamma radiation emitted from a rock bodyrock body

K 30 and daughter products from K 30 and daughter products from U and Th decay series.U and Th decay series.

Clays and shales concentrate Clays and shales concentrate these elements due to cation these elements due to cation exchange and adsorptionexchange and adsorption

Gamma-Gamma Gamma-Gamma (Density) Log(Density) Log Measures bulk densityMeasures bulk density Probe contains both a radiation Probe contains both a radiation

source and a detector.source and a detector.

Neutron-Neutron Neutron-Neutron (Porosity) Log(Porosity) Log Measures relative moisture Measures relative moisture

content above the water-table content above the water-table and porosity below the water-and porosity below the water-tabletable

Uses similar methods as the Uses similar methods as the Gamma-Gamma (Density) LogGamma-Gamma (Density) Log

Induction LogInduction Log

An electromagnetic induction An electromagnetic induction method to measure the electrical method to measure the electrical conductivity in open or PVC-case conductivity in open or PVC-case boreholes above or below the water-boreholes above or below the water-tabletable

The electrical conductivity is a The electrical conductivity is a function of soil and rock type, function of soil and rock type, porosity, permeability, and specific porosity, permeability, and specific conductance of the pore fluidsconductance of the pore fluids

Induction LogInduction Log

Because the response of the log Because the response of the log will be a function of the specific will be a function of the specific conductance of the pore fluids, it conductance of the pore fluids, it can be used to determine the can be used to determine the presence of inorganic presence of inorganic contaminationcontamination

Resistance LogResistance Log

Used mainly in geologic Used mainly in geologic correlation and locating fractures correlation and locating fractures or washout zones.or washout zones.

Only ran in a open hole in the Only ran in a open hole in the saturated zone saturated zone

Spontaneous-Potential Spontaneous-Potential LogLog Measures the natural potential Measures the natural potential

(millivolts) developed between the (millivolts) developed between the borehole fluids and the surrounding borehole fluids and the surrounding rock material.rock material.

Applications includeApplications include– Characterizing lithologyCharacterizing lithology– Providing information on the geochemical Providing information on the geochemical

oxidation-reduction conditionsoxidation-reduction conditions– Providing an indication of fluid flowProviding an indication of fluid flow

Temperature LogTemperature Log

Measures Temperature of the Measures Temperature of the borehole fluid.borehole fluid.

Can indicate zones of Can indicate zones of groundwater flow in an uncased groundwater flow in an uncased borehole, by recording an borehole, by recording an increase or decrease in increase or decrease in temperature.temperature.

Fluid Conductivity LogFluid Conductivity Log

Measures specific conductance of Measures specific conductance of the borehole fluid. A temperature the borehole fluid. A temperature log must also be run to correct log must also be run to correct values.values.

Caliper LogCaliper Log

This log simply provides a This log simply provides a borehole measurement. It will borehole measurement. It will indicate any problems in the indicate any problems in the borehole diameter.borehole diameter.

General Characteristics and General Characteristics and Use of Downhole Use of Downhole Geophysical LogsGeophysical Logs

SummarySummary

As you can see there are many As you can see there are many different types of GPM.different types of GPM.

Knowing which method to apply Knowing which method to apply in the correct manner can lead to in the correct manner can lead to a more accurate understanding of a more accurate understanding of the hydrogeology at a much the hydrogeology at a much cheaper price and with less cheaper price and with less damage to the surface.damage to the surface.