EarthSciences for Beginners

Earth science is the study of the Earth and its processes. It encompasses various disciplines such as geology, geophysics, oceanography, atmospheric and environmental sciences.

It utilises the principles of geology and physics to understand the structure, composition, processes, and history if the Earth.

Basic Terminology in Geology

Rocks: are naturally occurring solid aggregates of minerals, mineraloids, or organic materials.

Types of Rocks

They are classified into three main types based on their origin and formation processes.

Igneous Rocks: Formed from the cooling and solidification of molten rock, either magma (below the surface) or lava (at the surface).

Examples of igneous rocks granite, and basalt

Sedimentary Rocks: Formed from the accumulation and lithification of sediment, which can be fragments of other rocks, minerals, or organic material.
Examples of sedimentary rocks are sandstone, limestone, coal, and chalk

Metamorphic Rocks: Formed from the alteration of existing rocks (igneous, sedimentary, or other metamorphic rocks) through heat, pressure, and chemically active fluids.
Examples are marble, slate, schist, gneiss, and quartzite.

Fold: A bend in rock layers resulting from compressive forces, creating wave-like structures.

Anticline: An arch-shaped fold in rock layers with the oldest rocks at its core, where layers dip away from the center.

Syncline: A trough-shaped fold in rock layers with the youngest rocks at its core, where layers dip towards the center.

Fault: A fracture in Earth’s crust along which movement has occurred, causing displacement of rock masses.

Normal Fault: A type of fault where the hanging wall moves downward relative to the footwall, typically caused by extensional forces.

Reverse Fault: A type of fault where the hanging wall moves upward relative to the footwall, typically caused by compressional forces.

Cleavage: The tendency of certain rocks to split along specific, parallel planes of weakness.

Dip: The angle at which a rock layer or fault surface inclines from the horizontal plane.

Strike: The compass direction of the line formed by the intersection of a rock layer or fault surface with a horizontal plane.

Branches of Geology

Petrology: Study of rocks, their origins, composition, and structure.
Mineralogy: Study of minerals, their structure, properties, classification, and the processes of their formation.
Sedimentology: Study of sedimentary rocks and the processes of sediment deposition.
Paleontology: Study of fossils and ancient life forms.
Stratigraphy: Study of rock layers (strata) and layering (stratification).
Geochemistry: Study of the chemical composition of the Earth and its rocks and minerals.
Volcanology: Study of volcanoes, lava, magma, and related geological phenomena.
Geomorphology: Study of landforms and the processes that shape them.
Hydrogeology: Study of the distribution and movement of groundwater.
Environmental Geology: Study of the interactions between humans and the geological environment.

Geophysics

Geophysics (Geo + Physics i.e. Physics of Earth). The geophysics is the branch of Earth sciences that applies principles of physics to study the Earth’s structure, composition, and processes, including its magnetic and gravitational fields, seismic activity, and internal heat flow.

Branches of Geophysics

The study of Earth’s physical properties and the processes acting upon it using quantitative methods. Learn More

Exploration Geophysics: It involves using geophysical methods to locate mineral and energy resources. These methods help geophysicists infer the composition, structure, and physical properties of subsurface formations, aiding in resource discovery and evaluation. The aim is to identify and map minerals, hydrocarbons, groundwater, geothermal energy, and environmental concerns, such as subsurface contamination and waste management. It is also applied to societal needs like natural hazard mitigation and geological resource management.
Seismology: Study of earthquakes and the propagation of elastic waves through the Earth.
- Seismic: In seismic we use artificially generated controlled source, instead of natural source like earthquakes (uncontrolled source) to study the propagation of seismic waves in subsurface. Learn More
Magnetotellurics: Study of the Earth’s electromagnetic fields to determine the Earth’s subsurface structure.
Gravimetry: Measurement of the gravitational field of the Earth.
Geoelectricity: Study of electrical properties of the Earth’s subsurface.
Geodesy: Measurement and representation of the Earth, including its gravitational field.
Tectonophysics: Study of the physical processes that control the deformation of the Earth’s crust.
Volcanic Geophysics: Study of geophysical aspects of volcanic activity.
Remote Sensing and Geophysical Imaging: Use of satellite and airborne instruments to gather geophysical data. The technique of obtaining information about objects from a distance, typically using satellites or aircraft. This method involves detecting and measuring radiation reflected or emitted from the Earth’s surface and atmosphere. Remote sensing data can be used for various applications, including environmental monitoring, natural resource management, weather forecasting, and mapping land use changes. It utilize different types of sensors, such as optical, radar, and thermal, each suited for specific purposes and conditions.
Oceanography: The study of the Earth’s oceans, including marine organisms, ecosystem dynamics, ocean currents, and the physical and chemical properties of the sea.
Marine Geophysics: Marine geophysics is a specialized branch of oceanography where, geophysics, dealing with the physical properties and processes of the ocean floor and sub-seafloor structures. Key areas of marine geophysics include:
- Seafloor Mapping: Using sonar and other remote sensing technologies to map the topography and features of the ocean floor.
- Plate Tectonics: Studying the movements and interactions of tectonic plates under the ocean, including mid-ocean ridges, subduction zones, and transform faults.
- Submarine Volcanism and Earthquakes: Investigating underwater volcanic activity and seismic events to understand geophysical processes.
- Marine Magnetics and Gravity: Measuring magnetic and gravitational fields to infer the structure and composition of the oceanic crust and mantle.
Meteorology: The study of the atmosphere and weather patterns, including the causes of weather systems and climate change.
Geomagnetism: The study of the Earth’s magnetic field.

Geophysical problems

Forward Problem
Inverse Problem

Forward Problem

In geophysics, the forward problem involves predicting the measurements or observations that would result from a given model of the Earth’s subsurface properties. This process requires a theoretical or computational approach to simulate the physical phenomena, such as seismic waves, electromagnetic fields, gravity, or magnetic fields, based on a specified distribution of geological materials and their properties (e.g., density, seismic velocity, conductivity etc.).

The steps involved in solving the forward problem typically include:

Defining a Geological Model: Establishing a model that includes the spatial distribution of physical properties (e.g., rock types, densities, elastic moduli).
Applying Physical Laws: Using the relevant physical laws (such as Maxwell’s equations for electromagnetic or the wave equation for seismic) to calculate how these properties influence the geophysical fields.
Computing Observables: Generating synthetic data, such as seismic waveforms, gravitational anomalies, or magnetic fields, that would be measured by instruments on the surface or at specific locations.

Solving the forward problem is crucial for understanding how theoretical models relate to actual geophysical measurements and for validating the models by comparing synthetic data with real observations.

Inverse Problems

An inverse problem is a type of problem where you start with the results and work backwards to find the cause of the given result or effects.

For example, imagine you find a set of footprints on a sandy beach (the results). An inverse problem would be figuring out who made those footprints and what path they took (the cause).

Examples of an Inverse Problem

The inverse problem is common in many fields like medical imaging (figuring out the internal structure of the body from X-ray images), archaeology (reconstructing ancient civilizations from ruins), and seismology (determining the structure of the Earth’s interior from earthquake data).

In geophysics, this often means using data gathered from methods like seismic waves, gravitational fields, or magnetic fields to infer the distribution of materials, structures, or processes within the Earth. For example, in seismic tomography, the inverse problem is to determine the Earth’s internal structure from the travel times of seismic waves recorded at the surface.

How to Solve an Inverse Problem?

Identify Possible Models: List various potential models that could explain the geophysical phenomena.

Collect Data: Collect all available data and clues related to the problem.

Reconstruct the Story: Use all the collected information to infer the underlying cause.

Match Story with Models: Try to match re-created story from collected data and clues with the potential models.

Minimize Misfit (Cost Function): Adjust the model parameters to reduce the discrepancy (misfit) between observed data and model predictions. A cost function, in the context of inverse problems, is a mathematical function that measures how well a particular solution matches the observed data. It quantifies the difference between the predicted values (from a model) and the actual observed values (from data). The main objective in an inverse problems is to minimize this cost function to find the best-fitting solution. The term “misfit” is often used interchangeably with the cost function. It refers specifically to the measure of discrepancy or error between the observed data and the data predicted by a model. The “cost function” is a broader term used in various optimization problems, while the “misfit” is a term commonly used in geophysics.

Select the Best Fit: The model that best fits the data with the least misfit is considered the most plausible solution to the inverse problem.

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