Basic Geophysics: Geomagnetics

Summary

This video introduces geomagnetics, a method for mapping subsurface structures without excavation, particularly focusing on the old airfield in Karlsruhe. It explains the Earth’s magnetic field, the impact of subsurface structures on magnetic anomalies, and the tools used for measurement, including torsion magnetometers, proton magnetometers, fluxgate magnetometers, and gradiometers. The video highlights how these instruments detect variations in the magnetic field, enabling archaeologists to identify historical remains and other anthropogenic structures while emphasizing the importance of combining geomagnetic data with other investigative methods for accurate interpretation.

Takeaways

• ✈️ The old airfield in Karlsruhe was utilized by the American military until 1993 and is now a protected biotope for various species.
• 🔍 Geophysical methods, specifically geomagnetics, are used to address archaeological concerns related to subsurface remains that may affect the biotope.
• 🧭 Geomagnetics is a passive technique that uses the Earth's natural magnetic field to identify variations caused by subsurface structures.
• 🌍 The Earth's magnetic field is generated by convective currents in the outer core, creating a self-exciting geodynamo process.
• 📈 The magnetic susceptibility of subsurface materials influences local magnetic fields, which can be mapped to identify anomalies.
• 🧪 Four types of measuring instruments are introduced: torsion magnetometer, proton magnetometer, fluxgate magnetometer, and gradiometer.
• ⚖️ The torsion magnetometer is useful for single measurements but requires recalibration before each use.
• 💧 The proton magnetometer measures the total intensity of the magnetic field through the precession of hydrogen nuclei in a fluid.
• 📏 The fluxgate magnetometer measures specific vector components of the magnetic field by evaluating displacements in magnetization curves.
• 🔗 Gradiometers, consisting of two fluxgate magnetometers, are sensitive to near-surface anomalies and are suitable for large-scale mapping.
• 📊 The results from geomagnetic measurements can help locate historical structures like aircraft hangars and assess environmental impacts.

Q & A

What historical significance does the old airfield in Karlsruhe hold?

The old airfield in Karlsruhe was used by the American military until 1993. After its transfer to the city, the buildings were demolished, and a biotope for various species was established.

How does geomagnetics help in archaeological investigations?

Geomagnetics uses geophysical methods to map subsurface structures without excavation. It helps identify potential harmful remains from earlier buildings that could affect the biotope.

What is the Earth's magnetic field and how is it generated?

The Earth's magnetic field is generated in the outer core through the convection of hot, electrically conductive rock, which induces currents that reinforce the magnetic field, a process known as the geodynamo.

What is magnetic susceptibility and why is it important in geomagnetics?

Magnetic susceptibility is a dimensionless measure that indicates how a material responds to a magnetic field. It is important in geomagnetics because it helps define how magnetised bodies affect the Earth's magnetic field.

What are the main types of magnetism mentioned in the script?

The script mentions remanent magnetism, which occurs in the absence of an external magnetic field, and induced magnetism, which occurs only when an external field is applied.

What are the four measuring instruments introduced for geomagnetics?

The four instruments are: the torsion magnetometer, the proton magnetometer, the fluxgate magnetometer, and the gradiometer.

How does a torsion magnetometer operate?

A torsion magnetometer operates by suspending a small twistable bar magnet between two metal threads. The magnetic field causes a torque that can be measured to determine the magnetic field component.

What principle does a proton magnetometer rely on for measurements?

A proton magnetometer relies on the precession of nuclear spins of protons in a strong magnetic field, which induces a measurable voltage in a coil, proportional to the strength of the magnetic field.

What advantage does a gradiometer provide in measuring magnetic fields?

A gradiometer can measure vertical gradients of the magnetic field and is particularly sensitive to near-surface features, allowing it to effectively detect near-surface anomalies.

What are some applications of geomagnetics beyond archaeological investigations?

Geomagnetics is used for locating anthropogenic structures like cables and pipes, assessing landfills, investigating contaminated sites, and studying natural anomalies such as magma intrusions.