Geophysics - introduction Flashcards
Many engineering and construction projects require detailed understanding of subsurface conditions, including waste disposal sites. Weak layers or underground excavations can pose significant risks to buildings, roads, and other infrastructure. For large infrastructure projects such as tunnels and subways, specific geological knowledge of the subsoil and excavation site is critical to prevent surface damage or issues along the excavation axis. In waste disposal sites, it is essential to have a
What are solid earth geophysics / Global (pure) geophysics?
- the use of physics to study the interior of the Earth, from land surface to the inner core divided in global (pure) geophysics and applied geophysics
- study of the whole or substantial parts of the planet; methods may be applied to a wide range of investigations: entire earth to explorization of a localized region of the upper crust
What are applied geophysics / Exploration (Applied) Geophysics?
- concerned with investigating the Earth’s crust and near-surface to achieve a practical and economic aim or with economic objective (e.g. discovery of fuel or mineral deposities)
- exploration geophysics is the use of seismic, gravity, magnetic, electrical, elctromagnetic, etc., methods in the search for oil, gas, minerals, water, etc., with the objective of economic exploitation of primary resources
What are advantages of applied geophysics?
- environmentally friendly
- no time spent on procedures to understand geological problems compared to classical geo-mechanical surveys
- cheaper procedures compared to classical geo-mechanical surveys (7-100 eiro/m, but depending on the local context, targets and technology used)
What are the disadvantages of applied geophysics?
- difficulties for data interpretation (quite expensive software and data logger equipment are needed)
- difficulties to execute the survey depending on the work site
- a base geo-mechanical survey is always needed
What is the difference among Geophysics and Applied Geophysics methods?
Geophysics is more theoretical and aimed at understanding Earth’s natural processes, while Applied Geophysics focuses on the practical application of geophysical methods to solve specific real-world problems.
What does direct or indirect method mean?
- direct methods involve physically sampling or observing the subsurface (e.g., drilling), providing precise but localized information
- indirect methods measure physical phenomena (like seismic waves or magnetic fields) to infer subsurface properties without direct access. These methods cover larger areas but require interpretation and can be less precise.
(In practise, indirect methods are more commonly used for initial large-scale exploration, while direct methods may be used later for verification and detailed analysis.
Mention 4 Geophysics methods and the physical parameter(s) that directly and/or indirectly are measuring!
- Seismic methods
- direct: the velocity of seismic waves through the subsurface materials
- indirect: subsurface structure and composition inferred from how seismic waves are reflected, refracted, or absorbed - Magnetic survey
- direct: variations in Earth’s magnetic field due to the presence of magnetic minerals in the subsurface
- indirect: subsurface geological structures and mineral deposits based on magnetic anomalies - Gravity Survey
- direct: variations in the Earth’s gravitational field caused by difference in subsurface density
- indirect: subsurface density variations and geological structures, such as the presence of dense ore bodies or voids - Electrical Resistivity Method
- direct: electrical resistivity or conductivity
- indirect: subsurface features like water tables, mineral deposits, and contamination based on how resistivity varies with depth and location
Describe 3 different scale of applications of Geophysics applied methods
- local scale (site-specific) : environmental assessments, engineering surveys, archaeology
- regional scale (exploration and resource assessment) : mineral and hydrocarbon exploration, groundwater resource management, geohazards
- global scale (planetary studies and large-scale monitoring) : earth’s internal structure, climate change and glaciology, space missions
What is a geophysical survey?
A geophysical survey involves the use of geophysical methods to measure physical properties of the Earth’s subsurface. It aims to detect and map variations in these properties to infer geological structures, resource locations, or environmental conditions.
What are the differences respect a geomechanical survey together with pros and cons?
Pros:
- non-invasive
- large area coverage
- versatile
Cons:
- indirect data
- resolution limitations
- cost
Describe 4 types of environmental and/or engineering problems that can be supported (and justified) by the use of a geophysics method?
- groundwater contamination assessment
- site characterisation for construction projects
- detection of underground storage tanks or buries infrastructure
- assessment of soil liquefaction potential
Which types of limitations geophysical methods can have?
- resolution and depth limitations
- data interpretation challenges
- environmental and physical constraints
- cost and accessibility
- resolution vs. coverage trade-off
- complex geological conditions
- data processing and analysis
What is coherent noise?
A type of noise that has a consistent and predictable pattern. It often originates from a specific, identifiable source that repeats over time or space.
example: in seismic surveys it might come from regular traffic vibrations or machinery that creates consistent interference in the data
What is incoherent noise?
Random noise that does not follow a consistent pattern. It is often caused by unpredictable or irregular sources of intererence.
example: in magnetic surveys, incoherent noise might come from random variations in the Earth’s magnetic field or atmospheric disturbances.
How can position fixing matter in planning a survey noise aspect?
Knowing the position of any data point accurately within a survey and relative to prominent ground features is essential (use of GPS)
