Magnetotellurik Ch. Dasar Teori - Pembimbing Akademik

Summary

This video explains the foundational theory behind the Magnetotelluric (MT) method, an electromagnetic technique that uses natural geomagnetic variations as its signal source. It covers how electromagnetic waves interact with the Earth’s subsurface, providing insight into material conductivity and depth. The video details the different frequencies involved—high, medium, and low—along with their respective sources such as lightning, ionospheric currents, and solar storms. Key equations from Maxwell's laws are used to describe the wave propagation and its relation to resistivity. The MT method is presented as a tool for exploring subsurface electrical properties, offering insights into geological features like magma movement.

Takeaways

• 😀 Magnetotellurics (MT) is a passive electromagnetic method that uses natural geomagnetic variations as the source of electromagnetic signals.
• 😀 Geomagnetic variations are caused by phenomena such as lightning activity, ionospheric magnetic field resonance, and solar storms, including the solar wind.
• 😀 The electromagnetic signals induced by these variations interact with the Earth's subsurface materials, providing data on their electrical properties.
• 😀 MT signals can be classified into high, medium, and low frequencies, each generated by different sources like lightning, ionospheric currents, and solar wind interactions.
• 😀 High-frequency signals are caused by distant lightning and penetrate shallow depths, while low-frequency signals, like those from solar wind storms, penetrate much deeper into the Earth.
• 😀 The depth of signal penetration depends on both the frequency of the signal and the electrical conductivity of the Earth's layers.
• 😀 The propagation of electromagnetic waves in the Earth follows Maxwell's equations, particularly the laws of Gauss and Faraday, which explain the relationship between electric and magnetic fields.
• 😀 The MT method provides information about subsurface conductivity and resistivity through impedance, which is the ratio of electric field to magnetic field.
• 😀 Electromagnetic waves are attenuated faster at higher frequencies and penetrate less deeply, while lower frequencies can penetrate hundreds of kilometers into the Earth.
• 😀 By analyzing the complex impedance from the MT data, resistivity values can be derived, which reveal details about subsurface electrical properties and the distribution of different geological materials.
• 😀 The MT method is widely used in geophysical exploration for understanding the Earth's subsurface, including magma systems, through its ability to detect and measure electrical conductivity variations.

Q & A

What is the principle behind the magnetotelluric method?

The magnetotelluric method (MT) is an electromagnetic technique that uses natural geomagnetic variations as its signal source. These variations are caused by phenomena like lightning, ionospheric magnetic field resonance, or solar wind storms, which induce fluctuating electric and magnetic fields in the Earth.

What is the role of geomagnetic variation in magnetotellurics?

Geomagnetic variation acts as the electromagnetic signal source for MT methods. These variations occur due to natural processes like thunderstorms, ionospheric activity, and solar wind, which influence the Earth's magnetic and electric fields, allowing for the study of subsurface materials.

How do different frequencies of electromagnetic signals affect penetration depth in the Earth?

The penetration depth of electromagnetic signals into the Earth depends on their frequency. Higher-frequency signals have shallow penetration, while lower-frequency signals can penetrate deeper, reaching several hundred kilometers into the Earth's crust.

What are the sources of electromagnetic signals in magnetotellurics?

The primary sources of electromagnetic signals in MT include lightning (high-frequency signals), ionospheric currents (medium-frequency signals), and solar wind interactions with the Earth's magnetic field (low-frequency signals).

What is the significance of Maxwell's equations in the magnetotelluric method?

Maxwell's equations are crucial in describing the behavior of electromagnetic fields in MT. They provide a theoretical foundation for understanding how electric and magnetic fields interact with subsurface materials, helping to interpret the resulting data.

What is the concept of impedancy in magnetotellurics?

Impedance in magnetotellurics is the ratio of the electric field to the magnetic field at the Earth's surface. It is a key parameter for interpreting the electrical conductivity of subsurface layers. Impedance is a complex quantity with both real and imaginary components.

How does the conductivity of subsurface layers affect electromagnetic signal behavior?

The conductivity of subsurface layers directly influences how electromagnetic signals attenuate as they travel through the Earth. Higher conductivity leads to greater attenuation, meaning the signal weakens more quickly as it penetrates deeper.

What are the two primary properties derived from the impedance in MT?

From the impedance in MT, two primary properties can be derived: resistivity and phase. Resistivity relates to the electrical conductivity of subsurface materials, while phase provides information about the temporal relationship between electric and magnetic fields.

Why is it important to measure both electric and magnetic fields in the MT method?

Measuring both electric and magnetic fields is essential in MT because these two fields are interrelated, and their relationship helps in determining the subsurface resistivity and conductivity. The variation in these fields allows for a more accurate interpretation of the Earth's internal properties.

How is the magnetotelluric data processed to determine subsurface resistivity?

Magnetotelluric data is first collected by measuring electric and magnetic fields in the time domain. This data is then transformed into the frequency domain, from which impedance is calculated. The impedance values allow for the estimation of subsurface resistivity, providing insights into geological structures.