10.4 Environmental geophysics and site characterization
4 min read•august 14, 2024
Environmental geophysics plays a crucial role in site characterization, helping us understand what's beneath our feet. By using methods like and , we can map subsurface structures, detect buried objects, and track contaminants.
Interpreting this data gives us a clearer picture of what's happening underground. By combining geophysical, geotechnical, and geochemical data, we can create comprehensive site models. These models are invaluable for planning remediation efforts and monitoring their progress over time.
Geophysical Methods for Site Characterization
Electrical Resistivity Methods
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- Electrical resistivity methods measure the resistance of subsurface materials to the flow of electric current, providing information about lithology (e.g., clay, sand, bedrock), , and fluid content (e.g., fresh water, saline water, contaminants)
- The direct current (DC) resistivity method injects current into the ground through electrodes and measures the resulting potential difference to determine the resistivity distribution
- The induced polarization (IP) method measures the voltage decay after current injection, indicating the presence of polarizable materials such as clay or metallic minerals (e.g., pyrite, galena)
Ground-Penetrating Radar (GPR)
- Ground-penetrating radar (GPR) uses high-frequency electromagnetic waves to image subsurface structures and detect buried objects (e.g., pipes, tanks, foundations)
- are generated by contrasts in dielectric properties, which are influenced by water content, lithology, and the presence of man-made objects
- The depth of penetration and resolution of GPR depend on the antenna frequency and the electrical conductivity of the subsurface materials
- Other geophysical methods used in environmental site characterization include and reflection (for bedrock depth and layering), (for mapping conductivity variations), and (for detecting metallic objects)
- The choice of geophysical methods depends on the target depth, resolution requirements, site conditions (e.g., terrain, vegetation, noise sources), and the physical properties of the materials of interest
Interpreting Geophysical Data
Electrical Resistivity and IP Data Interpretation
- Electrical resistivity data can be inverted to create 2D or 3D models of the subsurface resistivity distribution, revealing lithological boundaries, fracture zones, and groundwater salinity variations
- IP data can indicate the presence of clay layers or contamination plumes containing polarizable materials, such as organic pollutants (e.g., hydrocarbons) or heavy metals (e.g., lead, chromium)
GPR and Seismic Data Interpretation
- GPR profiles show the travel times of reflected waves, which can be converted to depth using the velocity of the subsurface materials
- GPR reflections can delineate stratigraphic layers, buried pipes, tanks, and other man-made objects
- The can indicate the presence of electrically conductive materials, such as clay or contaminated groundwater
- Seismic data can reveal the depth and geometry of bedrock, sedimentary layers, and the water table, as well as the presence of voids, fractures, and low-velocity zones (e.g., saturated sediments, unconsolidated fill)
Electromagnetic and Magnetic Data Interpretation
- Electromagnetic induction and magnetic data can detect the presence of metallic objects, such as buried drums or steel pipes, and map the extent of conductive contamination plumes (e.g., leachate, saline intrusion)
Integrating Geophysical Data for Site Assessment
Data Integration Techniques
- Geophysical data provide spatial information about subsurface structures and properties, while geotechnical data (e.g., borehole logs, soil properties) offer ground-truth constraints and detailed local information
- Geochemical data (e.g., contaminant concentrations, redox conditions) help characterize the nature and extent of contamination and guide the interpretation of geophysical anomalies
- Integration of geophysical, geotechnical, and geochemical data enables the development of a comprehensive site conceptual model, identifying potential contaminant sources (e.g., leaking tanks, spills), pathways (e.g., fractures, permeable layers), and receptors (e.g., wells, surface water bodies)
- Data integration techniques include co-located data analysis (e.g., comparing resistivity and borehole logs), geostatistical methods (e.g., kriging), and coupled inversion of multiple geophysical datasets (e.g., joint inversion of resistivity and seismic data)
Applications in Site Assessment and Remediation Planning
- The integrated site model supports the design of targeted sampling and monitoring programs, optimizing the placement of wells and remediation infrastructure (e.g., extraction wells, injection points, barriers)
Geophysics in Subsurface Monitoring and Remediation
Monitoring Subsurface Processes
- can monitor changes in subsurface properties and processes, such as the migration of contamination plumes or the progress of remediation efforts
- Electrical resistivity and IP monitoring can track the movement of conductive or polarizable contaminants, as well as changes in subsurface geochemistry induced by remediation activities (e.g., bioremediation, chemical oxidation)
- GPR surveys can detect changes in the saturation and distribution of non-aqueous phase liquids (NAPLs) during remediation, as well as monitor the integrity of containment systems (e.g., slurry walls, caps)
- Seismic methods can monitor the evolution of subsurface mechanical properties, such as the development of fractures or the consolidation of treated soils
Assessing Remediation Effectiveness
- Geophysical monitoring data can be used to calibrate and validate numerical models of subsurface flow, transport, and remediation processes, improving the predictive capabilities and supporting the optimization of remediation strategies
- The effectiveness of remediation can be assessed by comparing geophysical monitoring results with baseline surveys and performance metrics, such as the reduction in contaminant mass or the achievement of target geophysical properties (e.g., increased resistivity, reduced GPR attenuation)
Key Terms to Review (28)
Attenuation of gpr signals: The attenuation of GPR signals refers to the gradual loss of signal strength as it travels through different materials in the subsurface. This phenomenon is crucial in understanding how radar waves interact with various geological and environmental conditions, impacting the clarity and quality of data collected during ground-penetrating radar surveys.
Comprehensive site model: A comprehensive site model is a detailed representation of a specific geographical area that integrates various environmental and geological factors, including subsurface conditions, contamination sources, and hydrology. This model aids in understanding the potential impacts of human activities and natural processes on the environment, facilitating better decision-making for site characterization and remediation strategies.
Contaminant plumes: Contaminant plumes are distinct, elongated bodies of contaminated groundwater or soil that spread from a source of pollution, often due to leaching from landfills, industrial sites, or spills. Understanding these plumes is crucial for assessing environmental impacts, guiding remediation efforts, and protecting water resources and public health.
Contamination assessment: Contamination assessment is the process of evaluating and characterizing the presence and extent of harmful substances in the environment, particularly in soil, groundwater, and sediments. This assessment helps identify potential risks to human health and the ecosystem, guiding decisions for remediation and site management.
Data acquisition systems: Data acquisition systems are integrated setups used to collect, measure, and analyze physical data from various sources in real-time. These systems play a crucial role in environmental geophysics and site characterization by enabling researchers to gather information about the physical properties of the subsurface, such as soil composition, groundwater levels, and contamination. By using various sensors and instruments, data acquisition systems convert analog signals into digital data, which can be processed and interpreted for environmental assessments and decision-making.
Direct current resistivity method: The direct current resistivity method is a geophysical technique used to investigate subsurface properties by measuring the electrical resistivity of the ground. This method involves injecting a direct current into the soil and measuring the resulting voltage drop, allowing for the mapping of different materials based on their resistivity characteristics. It is widely used in environmental studies, site characterization, and resource exploration.
Electrical resistivity: Electrical resistivity is a measure of how strongly a material opposes the flow of electric current, expressed in ohm-meters (Ω·m). It is crucial in various applications, as it helps in understanding subsurface materials and their properties. By analyzing resistivity, scientists can infer information about geological formations and contaminants, making it an essential tool in resource exploration and environmental studies.
Electromagnetic data interpretation: Electromagnetic data interpretation is the process of analyzing and extracting meaningful information from electromagnetic survey data to understand subsurface conditions. This technique is vital in environmental studies as it helps in site characterization by revealing information about soil, groundwater, and contamination levels without the need for invasive methods. By utilizing electromagnetic methods, geophysicists can map the distribution of materials and identify areas of concern, aiding in environmental assessments and decision-making.
Electromagnetic induction: Electromagnetic induction is the process by which a changing magnetic field generates an electric current in a conductor. This principle forms the basis for various technologies, including generators and transformers, and is essential in the exploration of subsurface properties using electromagnetic methods. The understanding of electromagnetic induction is crucial for assessing environmental conditions and characterizing sites for potential geophysical investigations.
Geophysical Sensors: Geophysical sensors are devices used to detect and measure various physical properties of the Earth, such as its magnetic, electrical, gravitational, and seismic characteristics. These sensors play a critical role in environmental assessments, helping scientists and engineers gather data that informs site characterization, resource exploration, and hazard assessment.
Geostatistical Analysis: Geostatistical analysis is a set of statistical techniques used to analyze spatially correlated data and make predictions about unknown values based on known observations. This method is crucial in assessing environmental conditions, as it allows researchers to create models that represent the distribution of variables such as soil composition, contamination levels, and groundwater quality across a given area.
Gpr reflections: GPR reflections refer to the echoes received from subsurface structures when ground-penetrating radar (GPR) waves encounter materials with different electrical properties. These reflections provide critical information about the geological and environmental conditions below the surface, making GPR a valuable tool in assessing site characteristics and environmental impacts.
Ground-penetrating radar: Ground-penetrating radar (GPR) is a non-invasive geophysical method that uses radar pulses to image the subsurface, revealing structures, features, and changes in material properties beneath the ground. It operates by sending electromagnetic waves into the ground and measuring the reflected signals, providing detailed information about the subsurface, which is essential for various applications such as groundwater studies, environmental assessments, and site investigations.
Groundwater contamination: Groundwater contamination refers to the presence of harmful substances in the water that is stored underground in aquifers. This can occur from various sources such as industrial discharges, agricultural runoff, and leaking underground storage tanks, leading to significant environmental and health risks. Understanding groundwater contamination is crucial for assessing site conditions and developing remediation strategies in environmental studies.
Hazardous waste site investigation: A hazardous waste site investigation is a systematic process used to assess and characterize sites contaminated with hazardous materials, aiming to identify the extent of pollution and the associated risks to human health and the environment. This investigation includes collecting data through various methods, such as sampling soil and groundwater, and employing geophysical techniques to locate buried waste and understand subsurface conditions. These investigations are crucial for effective remediation and ensuring public safety.
Induced polarization method: The induced polarization method is a geophysical technique used to investigate subsurface materials by measuring their electrical properties, specifically the ability of the materials to retain electrical charge after an external electric field is removed. This method is particularly valuable in environmental studies, helping to differentiate between various types of materials, such as metal ores, clays, and contaminated soils, based on their polarizability characteristics.
Inversion modeling: Inversion modeling is a mathematical and computational technique used to interpret geophysical data by inferring the subsurface properties of the Earth from surface measurements. This process involves creating a model that explains the observed data and adjusting it iteratively until a satisfactory fit is achieved, allowing for the characterization of environmental conditions and site-specific characteristics.
Magnetic surveys: Magnetic surveys involve measuring variations in the Earth's magnetic field to identify subsurface geological structures and features. This technique helps in locating mineral deposits, mapping archaeological sites, and assessing environmental conditions by detecting anomalies caused by different materials in the ground.
Metallic objects detection: Metallic objects detection refers to the methods and technologies used to identify and locate metal items buried in the ground or submerged in water. This process is crucial for various applications, including environmental assessments and site characterizations, as it helps to reveal hidden contamination or anthropogenic artifacts that can impact land use and safety.
Monitoring subsurface properties: Monitoring subsurface properties refers to the process of using various geophysical techniques to analyze and assess the characteristics of materials and fluids located beneath the Earth's surface. This practice is crucial for understanding environmental conditions, identifying potential hazards, and making informed decisions regarding land use, construction, and resource management.
National Environmental Policy Act: The National Environmental Policy Act (NEPA) is a United States environmental law enacted in 1969 that requires federal agencies to assess the environmental effects of their proposed actions before making decisions. NEPA's primary aim is to promote informed decision-making and ensure that the environment is considered during the planning stages of projects, facilitating transparency and public participation in the evaluation process.
Permeability: Permeability is a measure of how easily fluids can flow through a material, particularly in geological formations. It plays a critical role in various processes, including the movement of groundwater, hydrocarbon migration, and heat transfer in geothermal systems. Understanding permeability helps in evaluating resource potential and environmental impacts associated with subsurface fluids.
Porosity: Porosity is the measure of the void spaces in a material, specifically in rocks and sediments, expressed as a fraction or percentage of the total volume. It is a crucial property that influences various geological processes, including fluid flow, storage capacity, and the overall behavior of subsurface materials in relation to their electrical and thermal properties.
Resource Conservation and Recovery Act: The Resource Conservation and Recovery Act (RCRA) is a federal law enacted in 1976 that governs the disposal of solid and hazardous waste in the United States. The act establishes a framework for the management of hazardous waste from its creation to its ultimate disposal, promoting conservation and encouraging recycling, treatment, and safe disposal methods. RCRA plays a crucial role in environmental protection by ensuring that waste management practices do not pose risks to human health or the environment.
Seismic Reflection: Seismic reflection is a geophysical technique used to analyze subsurface structures by sending seismic waves into the ground and recording the waves that bounce back from different geological layers. This method helps in understanding the composition, properties, and depth of subsurface materials, making it crucial for applications like resource exploration, environmental assessments, and geotechnical investigations.
Seismic refraction: Seismic refraction is a geophysical method that measures the time it takes for seismic waves to travel through different layers of the Earth's subsurface, allowing for the analysis of material properties and layer boundaries. This technique utilizes the bending of seismic waves as they pass through materials with varying densities, helping to identify subsurface structures and conditions. By interpreting the travel times of these waves, geophysicists can infer the geological characteristics of an area, which is crucial for understanding site conditions and potential risks.
Subsurface Mapping: Subsurface mapping is the process of creating detailed representations of the underground features and structures of the Earth, often utilizing geophysical methods and data to visualize subsurface conditions. This technique is crucial for understanding geological formations, locating resources like water or minerals, and assessing environmental impacts. It relies on measurements from various geophysical surveys to produce accurate and informative maps that guide exploration and decision-making.
Time-lapse geophysical surveys: Time-lapse geophysical surveys are a technique used to monitor changes in subsurface conditions over time by repeating geophysical measurements at the same location. This method is particularly valuable in environmental studies, allowing for the assessment of changes due to natural processes or human activities, and aids in site characterization by revealing information about subsurface fluid movement, contaminant transport, and geological alterations.