Applied Geophysics Overview

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Questions and Answers

What is the main purpose of Applied Geophysics?

To predict volcanic eruptions and earthquakes

To investigate the Earth's crust and near surface for practical and economic purposes (correct)

To study the composition of the Earth's core

To map the boundaries between different geological formations

Which of the following is NOT a physical phenomenon measured by geophysical techniques?

Magnetism

Temperature (correct)

Gravity

Elastic waves

What is the sub-surface property that is measured by electrical resistivity meter?

Seismic wave velocity

Resistivity (correct)

Density

Magnetic susceptibility

Which of the following is NOT a method of conducting geophysical surveys?

Satellite survey (C) Signup and view all the answers

What is the primary function of a gravimeter?

Detect changes in gravity due to variations in density (B) Signup and view all the answers

What is the term used for a mineral deposit that contains enough minerals to be mined for profit?

Ore (D) Signup and view all the answers

Why is an accurate estimation of the volume of mineral deposits important in mineral exploration?

It helps to determine the feasibility of mining the deposit (B) Signup and view all the answers

What is the primary reason why mineral exploration is considered 'capital intensive'?

The significant investment needed for exploration and development (B) Signup and view all the answers

What is the typical resistivity of sedimentary rocks?

10- 10*8 (A) Signup and view all the answers

Which electrode configuration is preferred for Vertical Electrical Sounding (VES) applications due to its strong vertical resolution and ease of setup?

Schlumberger (B) Signup and view all the answers

Which of the following best describes the purpose of Vertical Electrical Sounding (VES)?

To measure the variation of resistivity with depth at a specific location on the ground surface. (D) Signup and view all the answers

In an expanding-spread Wenner survey, how are the electrodes moved between readings?

All electrodes are moved along a straight line with equal spacing preserved. (D) Signup and view all the answers

What is the main difference between the Wenner and dipole-dipole arrays in electrical resistivity surveying?

The Wenner array is more sensitive to shallow resistivity variations, while the dipole-dipole array is more sensitive to deeper variations. (A) Signup and view all the answers

What is the approximate relationship between the spread length (L) and the potential electrode spacing (l) in a valid Schlumberger array measurement?

L should be at least 5 times l (B) Signup and view all the answers

Which of the following is NOT a type of curve obtained from a three-layer geo-electric section in VES?

S-type (D) Signup and view all the answers

What is the purpose of inversion in 1D VES?

To estimate the true resistivities and thicknesses of the subsurface layers. (D) Signup and view all the answers

What is the typical resistivity of pure water?

10*3 (C) Signup and view all the answers

Which of the following is NOT a typical electrode pattern used in resistivity surveying?

Triple-dipole (A) Signup and view all the answers

Which of the following software is commonly used for forward modeling of 2D resistivity data in CST?

DIPPRO (C) Signup and view all the answers

What type of noise can be effectively reduced through stacking in ER data acquisition?

Incoherent noise (C) Signup and view all the answers

Which of the following materials has the highest typical resistivity?

Igneous & Metamorphic rocks (B) Signup and view all the answers

What is the primary method used to remove noise in ER data?

Filtering (D) Signup and view all the answers

Which electrode configuration involves placing all four electrodes collinearly with equal spacing between them?

Wenner (B) Signup and view all the answers

Which of the following is NOT a typical characteristic of incoherent noise in ER data?

Repetitive pattern (A) Signup and view all the answers

What occurs during membrane polarization when pore space narrows?

Charge accumulation leads to net charge dipole. (A) Signup and view all the answers

What primarily influences the magnitude of electrode polarization?

The type of mineral present. (C) Signup and view all the answers

Which factor does NOT affect induced polarization (IP)?

Rock color. (A) Signup and view all the answers

In time-domain measurements of IP, what is compared to determine effects?

Residual voltage and steady voltage. (B) Signup and view all the answers

How does the presence of clay particles influence induced polarization?

It enhances charge accumulation. (D) Signup and view all the answers

When is the IP response typically better?

With disseminated mineralization. (D) Signup and view all the answers

What is a characteristic of time-domain measurements of IP effects?

Current flow stops after a short period. (B) Signup and view all the answers

Which statement about induced polarization is accurate?

IP varies with both time and frequency. (C) Signup and view all the answers

What is the primary measurement collected during an Induced Polarization survey?

Chargeability in mv/v (D) Signup and view all the answers

How is induced polarization (IP) typically measured?

In conjunction with spontaneous potential and resistivity (C) Signup and view all the answers

What is the relationship between impedance, input, and output in IP measurements?

Impedance is the ratio of the output to the input (B) Signup and view all the answers

During which process does switching off the current lead to voltage decay?

Induced Polarization measurement (B) Signup and view all the answers

What occurs when the current is switched on in an IP survey?

Voltage builds up (A) Signup and view all the answers

What distinguishes the time domain from frequency domain measurements in IP?

How the current is turned on and off (D) Signup and view all the answers

What is the role of stacking in IP measurements?

To average multiple voltage readings (C) Signup and view all the answers

What is NOT a characteristic of impedance in IP methods?

It provides the amount of charge stored (B) Signup and view all the answers

What is a key theoretical observation regarding frequency and time measurements in IP measurements?

Their results should theoretically be the same. (C) Signup and view all the answers

Which system is used to reduce operational time and enhance data coverage in IP surveys?

Multi-electrode & roll-along system (A) Signup and view all the answers

What is one application of IP measurements in environmental studies?

Mapping electrochemical reactions for pollutants (C) Signup and view all the answers

What is a significant disadvantage of IP compared to resistivity surveys?

It is slower and more costly. (D) Signup and view all the answers

How are IP results typically displayed?

In profiles plotted against location (D) Signup and view all the answers

What type of plot is used to visualize results from a specific electrode array in IP?

Pseudodepth plots with a tent shape (A) Signup and view all the answers

Which statement about the understanding of electrochemical phenomena in IP is accurate?

They remain poorly understood. (B) Signup and view all the answers

What measurement parameter is commonly plotted in IP profiles?

Percent frequency effect (C) Signup and view all the answers

Study Notes

Applied Geophysics (GPS 314)

Applied geophysics investigates the Earth's crust and near-surface to achieve practical and economic goals.
It utilizes various physical theories and experimental techniques to determine Earth's properties.
Geophysical techniques measure physical phenomena such as gravity, magnetism, elastic waves, electricity, and electromagnetic waves.
These phenomena are sensitive to subsurface physical properties like density, magnetic susceptibility, seismic wave velocity, resistivity, conductance/inductance/permittivity.

Branches of Geophysics

Solid Earth Geophysics: Studies the Earth's interior, from the surface to the core, using physics.
Global Geophysics (Pure Geophysics): Examines the Earth as a whole or substantial parts of it.
Engineering Geophysics: Applies geophysical methods to investigate subsurface materials and structures with significant engineering implications.
Environmental Geophysics: Investigates near-surface physico-chemical phenomena with potential implications for local environmental management.

Applied Geophysics Applications

Studies the crust's thickness for hydrocarbon exploration
Investigates shallow structures for engineering site investigations
Locates groundwater, minerals, narrow mine shafts, buried cavities, archeological remains, buried pipes and cables.

Geophysical Survey Environments

Ground survey: On land
Airborne or Aero-survey: In the air
Marine survey: At sea
Satellite-derived data: From space

Glacio Geophysics

Utilizes radar and seismic reflection methods to study the ice surface and subglacial interface in polar regions.

Geophysical Methods

Various methods measure different physical properties of the subsurface.
Some methods are suited for shallow investigations, others for deep ones.
Choice of method is based on the target and desired depth of investigation or resolution.

Mineral Exploration and Geophysics

Mineral exploration is the process of finding economically viable ore deposits.
A mineral deposit that holds enough minerals for profitable mining is termed an ore.
Mineral prospecting is the physical search for valuable minerals.

Geophysical Methods and Physical Properties

Geophysical methods measure various physical properties of subsurface rocks
Examples include density, magnetic susceptibility, propagation velocity, resistivity, and electromagnetic wave reflectivity.
These measurements reveal variations caused by geological features.

Planning a Geophysical Survey

A clear prime objective is essential from the start.
There are no ideal methods for all situations, techniques and interpretation methods need to be carefully chosen.
Comprehensive survey planning, informed by geophysical methods and their principles, yields cost-effective and efficient surveys within constraints.
Steps include defining survey objectives, site selection, selecting the most suitable geophysical methods, and data acquisition protocols.

Survey Limitations

Cost is a key factor, influenced by the survey location, site access, survey scale and logistics, and required equipment.
Geophysical surveys should be viewed as an investment in reducing costs and improving efficiency.
The cost of a geophysical survey should be weighed against its potential to generate cost savings in a larger project.

Target Identification

Geophysical methods find boundaries with significant differences in physical properties to find anomalies in subsurface targets.
These anomalies point to targets like oil/gas structures, mineshafts, pipelines, etc.

Anomalies from Geophysical Surveys

Anomalies are deviations from typical geophysical readings/measurements
They represent features of interest such as geological structures, underground features, etc.

Choosing the Right Method

The choice of geophysical method depends on the target or geologic feature sought.
The method best suited for the geological features being prospected will maximize efficiency and minimize errors.

Background Values and Variations

Background variations in physical properties (like resistivity, density, etc.) can obscure target anomalies.
Understanding background variations is essential for accurate target detection and delineation.

Types of Noise in Geophysical Surveys

Coherent noise (e.g., power lines) has predictable patterns and can frequently be removed through filtering procedures.
Incoherent noise (e.g., random wind signals) is less predictable.
Effective survey design and noise reduction techniques help get usable data and interpretations.

Noise Reduction Techniques

Filtering: Removes or reduces noise based on frequency characteristics.
Stacking: Averaging multiple measurements to reduce incoherent noise.
Careful survey design: Selecting optimal survey locations and methods to minimize noise interference.

Qualitative Interpretation

Preliminary interpretation often involves analyzing contour, density, or anomaly plots.
Visual inspection of data helps find regional features.
Data plotting and contouring of geophysical maps provide a visual picture, identifying regions with unusual characteristics.

Quantitative Interpretation

Uses quantitative methods (inversion, numerical modeling, or neural network techniques).
This converts and explores data into 2D models or 3D models of the survey area.
The goal is to model or quantify geologic features, structures, or other subsurface parameters (e.g., depth, extent, etc.).

Electrical Method Applications

Shallow ore prospecting (massive ore deposits).
Geological mapping.
Groundwater and engineering investigations.
Environmental studies
Hydrocarbon exploration.

Induced Polarization (IP)

A current-driven phenomenon, offering an additional method in subsurface exploration.
Useful for locating disseminated minerals, where other methods might not be effective.
A technique based on the capacitive and electro-chemical nature of geological materials
Measurements yield properties such as chargeability.

Factors Affecting Induced Polarization

Mineral concentration
Physical characteristics of the host rock
Electrolyte types
Presence of water (water conductivity/porosity)
Current intensity
Current frequency

IP Measurements

Multiple measurement techniques are used, dependent on data needs, and may use more sophisticated multi-electrode and roll-along systems.
The resulting data can be interpreted, identifying and evaluating subsurface targets and geological parameters (e.g., electrical resistivity).

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Description

Test your knowledge on the principles and techniques of applied geophysics. This quiz covers various methods, concepts, and the importance of measurements in the field. Explore topics such as mineral exploration and electrical resistivity.