Remote Sensing and geophysical Methods for Evaluation of Subsurface Conditions Matt Houston

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

Introduction Advantages and disadvantages Advantages and disadvantages There is no universally applicable geophysical method. There is no universally applicable geophysical method. Some methods are quite site specific Some methods are quite site specific It is up to the user to select a method or methods and how they apply to specific site conditions and project requirements. It is up to the user to select a method or methods and how they apply to specific site conditions and project requirements.

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

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

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

Background Site investigations have relied upon direct sampling methods such as Site investigations have relied upon direct sampling methods such as 1. Soil borings and monitoring wells for gathering hydrogeologic data and soil and water samples 2. Lab analysis of soil and water samples to provide a quantitative assessment of site conditions 3. Extensive interpolation and extrapolation from these points of data

Background This approach has evolved over many years and is commonly considered a standard. This approach has evolved over many years and is commonly considered a standard. However, there are many pitfalls associated with this approach, which can result in incomplete or even erroneous understanding of site conditions. However, there are many pitfalls associated with this approach, which can result in incomplete or even erroneous understanding of site conditions.

Background The single most critical factor we face in site evaluation work is accurately characterizing the sites hydrogeology. The single most critical factor we face in site evaluation work is accurately characterizing the sites hydrogeology. If an accurate understanding of the sites hydrogeology is obtained then the predicting the movements of contaminants, or designing a clean up operation would be easy. If an accurate understanding of the sites hydrogeology is obtained then the predicting the movements of contaminants, or designing a clean up operation would be easy.

Background If all strata where uniform and horizontal then 1 monitoring well would do, unfortunately this is not the case in most areas. If all strata where uniform and horizontal then 1 monitoring well would do, unfortunately this is not the case in most areas. In order to accurately sample an area of unknown hydrogeology one would have to reduce the area to swiss cheese, in order to find fractures, channels, sand lenses, and local permeable zones. In order to accurately sample an area of unknown hydrogeology one would have to reduce the area to swiss cheese, in order to find fractures, channels, sand lenses, and local permeable zones.

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

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

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

How Geophysical Methods are Used GPM measure a large volume of the subsurface. GPM measure a large volume of the subsurface. Anomaly detectors Anomaly detectors

How Geophysical Methods are Used Once an overall characterization of the site has been made using GPM and anomalous zones identified, a better drilling and sampling plan can be designed. Once an overall characterization of the site has been made using GPM and anomalous zones identified, a better drilling and sampling plan can be designed.

How Geophysical Methods are Used Locating soil borings and monitoring wells to provide samples that are representative of site conditions. Locating soil borings and monitoring wells to provide samples that are representative of site conditions. Minimizing the number of samples, borings, and/or monitoring wells required to accurately characterize a site. Minimizing the number of samples, borings, and/or monitoring wells required to accurately characterize a site. Reduce field investigation time and cost. Reduce field investigation time and cost. Significantly improving the accuracy of the overall investigation. Significantly improving the accuracy of the overall investigation.

How Geophysical Methods are Used Much greater confidence in the final results Much greater confidence in the final results Fewer borings and wells and lower cost. Fewer borings and wells and lower cost. Monitoring wells cost 75,000 (1987 dollars) over 30 years. Monitoring wells cost 75,000 (1987 dollars) over 30 years. Smart Holes Smart Holes

Imaging Methods Large Scale aerial photos Large Scale aerial photos Small Scale aerial photos Small Scale aerial photos SLAR (Side Looking Airborne Radar) SLAR (Side Looking Airborne Radar) Thermal Infrared** Thermal Infrared**

Thermal Infrared Measures the thermal response Measures the thermal response Earth’s surface emits radiation in the thermal infrared wavelength. Earth’s surface emits radiation in the thermal infrared wavelength. Fluctuations can help identify springs, seeps from a landfill, moist and dry areas, vegetation stress, can be used to characterize surface soil and rock Fluctuations can help identify springs, seeps from a landfill, moist and dry areas, vegetation stress, can be used to characterize surface soil and rock

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

Ground-penetrating Radar Reflection of radar waves occur when there is a change in the dielectric constant Reflection of radar waves occur when there is a change in the dielectric constant

Conductivity and Resistivity Methods Resistivity and conductivity are inverse of each other. Resistivity and conductivity are inverse of each other. They are a function of They are a function of –Type of soil and rock –Porosity –Conductivity of the fluids that fill the pore spaces

Conductivity and Resistivity Methods Anomalies can be detected Anomalies can be detected Inorganic plume Inorganic plume

Seismic Refraction and Reflection Uses seismic velocity of the rock or soil. Uses seismic velocity of the rock or soil. Some sort of sound wave is emitted and measured by a geophone as it travels back to the surface. Some sort of sound wave is emitted and measured by a geophone as it travels back to the surface.

Seismic Refraction and Reflection Can be used to Can be used to –Determine the top of bedrock –Depth of water table –Assess the continuity of geologic strata –Locate fractures, faults, and buried bedrock channels

Seismic Refraction Refraction is generally used in very shallow studies. (A few Hundred Feet) Refraction is generally used in very shallow studies. (A few Hundred Feet) With enough energy a study of few thousand feet can be conducted With enough energy a study of few thousand feet can be conducted

Seismic Refraction

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

Seismic Reflection

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

Micro Gravity

Applications of GPM

Downhole Geophysical Measurements Natural Gamma Log Natural Gamma Log Gamma-Gamma (Density) Log Gamma-Gamma (Density) Log Neutron-Neutron (Porosity) Log Neutron-Neutron (Porosity) Log Induction Log Induction Log Resistance Log Resistance Log Spontaneous-Potential Log Spontaneous-Potential Log Temperature Log Temperature Log Fluid Conductivity Log Fluid Conductivity Log Caliper Log Caliper Log

Natural Gamma Log Records the amount of natural gamma radiation emitted from a rock body Records the amount of natural gamma radiation emitted from a rock body K 30 and daughter products from U and Th decay series. K 30 and daughter products from U and Th decay series. Clays and shales concentrate these elements due to cation exchange and adsorption Clays and shales concentrate these elements due to cation exchange and adsorption

Gamma-Gamma (Density) Log Measures bulk density Measures bulk density Probe contains both a radiation source and a detector. Probe contains both a radiation source and a detector.

Neutron-Neutron (Porosity) Log Measures relative moisture content above the water-table and porosity below the water-table Measures relative moisture content above the water-table and porosity below the water-table Uses similar methods as the Gamma- Gamma (Density) Log Uses similar methods as the Gamma- Gamma (Density) Log

Induction Log An electromagnetic induction method to measure the electrical conductivity in open or PVC-case boreholes above or below the water-table An electromagnetic induction method to measure the electrical conductivity in open or PVC-case boreholes above or below the water-table The electrical conductivity is a function of soil and rock type, porosity, permeability, and specific conductance of the pore fluids The electrical conductivity is a function of soil and rock type, porosity, permeability, and specific conductance of the pore fluids

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

Resistance Log Used mainly in geologic correlation and locating fractures or washout zones. Used mainly in geologic correlation and locating fractures or washout zones. Only ran in a open hole in the saturated zone Only ran in a open hole in the saturated zone

Spontaneous-Potential Log Measures the natural potential (millivolts) developed between the borehole fluids and the surrounding rock material. Measures the natural potential (millivolts) developed between the borehole fluids and the surrounding rock material. Applications include Applications include –Characterizing lithology –Providing information on the geochemical oxidation-reduction conditions – Providing an indication of fluid flow

Temperature Log Measures Temperature of the borehole fluid. Measures Temperature of the borehole fluid. Can indicate zones of groundwater flow in an uncased borehole, by recording an increase or decrease in temperature. Can indicate zones of groundwater flow in an uncased borehole, by recording an increase or decrease in temperature.

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

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

General Characteristics and Use of Downhole Geophysical Logs

Summary As you can see there are many different types of GPM. As you can see there are many different types of GPM. Knowing which method to apply in the correct manner can lead to a more accurate understanding of the hydrogeology at a much cheaper price and with less damage to the surface. Knowing which method to apply in the correct manner can lead to a more accurate understanding of the hydrogeology at a much cheaper price and with less damage to the surface.