Geophysics Introduction

Question 1

What are the primary differences between geophysics and applied geophysics?

Answer 1

• Applied geophysicsfocuses on using these methods for practical purposes, especially for economic objectives like discovering resources (oil, gas, minerals) or solving engineering/environmental issues like waste management and contamination monitoring.
• Geophysicsis the study of Earth’s physical properties through quantitative observation, such as seismic, electromagnetic, and gravity methods, often for scientific exploration of the Earth’s interior.

Question 2

Can you list some pros and cons of geophysical surveys compared to geomechanics surveys?

Answer 2

Cons:
- Interpretation of data can be complex and requires specialized software.
- Certain work sites can make it difficult to conduct surveys.
- Geomechanical data is sometimes still necessary for validation.
Pros:
- Environmentally friendly.
- Faster and cheaper compared to classical geomechanical surveys.
- Provides broader surface coverage without intrusive drilling.

Question 3

What are the distinctions between active and passive geophysical methods? Provide examples.

Answer 3

• Passive methods:
  measure natural phenomena like gravity, magnetic fields, and seismic waves from earthquakes. For example, passive seismic methods record vibrations naturally occurring in the ground.
  
  • Active methods: involve transmitting an artificial signal into the ground, such as seismic waves generated by controlled explosions, and then analyzing how the signal is altered by subsurface structures. Examples include seismic reflection and refraction methods

Question 4

What are some geophysical properties that can be measured to assess subsurface conditions?

Answer 4

• Gravity(sensitive to density variations).
• Magnetism(sensitive to magnetic susceptibility).
• Seismic wave velocity(related to elastic properties and density).
• Resistivity(sensitive to conductivity, useful in identifying fluids or materials underground).
• Electromagnetic properties(detecting electrical conductivity and dielectric constants)

Question 5

How is noise addressed in geophysical surveys, and what types of noise can affect survey results?

Answer 5

Noise can beman-made(cultural noise) such as electric cables, vehicles, and pipelines, ornaturalsuch as wind, rain, and seismic activity. Noise reduction involves careful survey design and the use of modern technology to enhance the signal-to-noise ratio. Coherent noise (systematic, like power lines) and incoherent noise (random, like waves) require different mitigation approaches.

Question 6

What role do geophysical surveys play in waste management, particularly in preventing contamination?

Answer 6

Geophysical surveys are used in waste management to assess subsurface conditions at landfills. They help in identifying geological barriers that prevent contamination of groundwater and in monitoring potential leakage from waste disposal sites. Techniques like electrical resistivity tomography (ERT) are useful for detecting pollution plumes in the subsurface

Question 7

Why is it important to clearly define the survey’s objective before starting a geophysical investigation?

Answer 7

Clearly defining the objective helps determine the appropriate geophysical method, equipment, and survey design. Without a clear objective, data collection may be inadequate or irrelevant, leading to inconclusive or incorrect interpretations

Question 8

What are some challenges associated with interpreting data from geophysical surveys?

Answer 8

• Non-uniqueness: The same data may be explained by different subsurface structures.
• Noise interference: External noise can distort signals, especially when detecting weak anomalies.
• Data complexity: Advanced software is often required for processing and interpreting data, which adds to the cost and expertise needed

Question 9

How do seismic reflection and refraction differ in geophysical studies?

Answer 9

• Seismic reflectionmeasures waves that bounce off subsurface layers, useful for mapping structures like oil reservoirs.
• Seismic refractionmeasures waves that bend as they pass through different layers, often used for mapping deeper geological layers. Reflection is more focused on shallow, detailed surveys, while refraction can penetrate deeper