Principles of Geophysics Quiz

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Questions and Answers

What is a significant advantage of electric resistivity methods compared to drilling?

They are faster than all other methods.

They can detect deeper geological formations.

They provide higher resolution images.

They are less expensive. (correct)

Which of the following parameters is NOT influenced by wave velocities in seismic analysis?

Depths to beds

Layering of anomalous regions (correct)

Temperature

Elastic moduli

How does seismic velocity typically change with increase in pressure and depth of burial?

It varies randomly.

It decreases.

It increases. (correct)

It remains constant.

Which statement about reflection seismic methods is correct?

They detect geometrical types of subsurface structures. Signup and view all the answers

What does the reflection coefficient (R) depend on at oblique incidence?

Incidence angle (Φ). Signup and view all the answers

What does analytic signal enhance in relation to magnetized bodies?

Edges of magnetized bodies Signup and view all the answers

In the context of aquifer transmissivity, what does the variable 'T' represent?

Transmissivity in m²/d Signup and view all the answers

What causes asymmetric magnetic signatures in relation to unexploded ordnance (UXO)?

Preferred alignment of the long axes of UXO Signup and view all the answers

Which method is used to locate buried tanks, pipes, and debris?

Frequency Domain Electromagnetic Methods Signup and view all the answers

In Very Low Frequency Method, what is the measured EM field range?

10-30 kHz Signup and view all the answers

What is measured in the Magnetic Gradiometry method to confirm effects?

Diurnal magnetic field effects Signup and view all the answers

Which feature is analyzed using Natural gamma-ray intensity?

Lithology and shale volume Signup and view all the answers

How does the duration of pulses affect apparent chargeability (Ma) in IP methods?

Increases with pulse duration Signup and view all the answers

What is a necessary step to confirm ambiguous geophysical interpretation?

Drilling Signup and view all the answers

Which geophysical method is described as having a grid length starting from 2 to 10 m?

High-resolution microgravity surveys Signup and view all the answers

What does normal correction remove from gravity data interpretation?

Regional gravity effect Signup and view all the answers

What is the main cause of local magnetic anomalies?

Different subsurface susceptibilities Signup and view all the answers

What is the relationship depicted in the Magnetic Hysteresis loop?

Between magnetic field strength (B) and magnetizing force (H) Signup and view all the answers

Which statement about gravity survey grid lengths is accurate?

Grid length must be less than the depth of the geologic feature. Signup and view all the answers

What is the sensitivity of gravity field derivatives affected by?

Near-surface topographic irregularities Signup and view all the answers

What absolute accuracy is reported for the Proton precession magnetometer?

0.1 nT Signup and view all the answers

Study Notes

Principles of Geophysics

Structural elements like layer thickness, depth, and dip are geometrical parameters derived from geophysical measurements.
Cracking, weathering, contamination, and radiological parameters are spatial distributions of petrophysical/geophysical parameters.
Combining various geophysical methods enhances interpretation reliability.
Drilling is often necessary to confirm interpretations due to ambiguities.
Physical properties of rocks in dams, tunnels, and shafts are important civil engineering concerns.
Centrifugal force is one source of Earth's gravitational field.
Grid length should be smaller than the depth of the geological feature.

Gravity Surveys

Large-scale gravity surveys (1-10 km) map regional geological structures.
Small-scale gravity surveys (10-100 m) detail local features.
Microgravity surveys use a grid length of 15-30 m for reconnaissance.
High-resolution microgravity surveys use a grid length of 2-10 m.
Normal correction removes the regional gravity effect using regression analysis.
Gravity field derivatives are sensitive to near-surface topography noise, enhancing small/shallow features.
Gravity anomalies of 10 µGals have been detected; subsurface features like caves and voids were estimated by inversion.

Other Geophysical Methods

Magnetic field direction is compared to ambient field direction.
Magnetic hysteresis loops graphically show the relationship between B and H in ferromagnetic cores.
Proton precession magnetometers measure precession frequency to derive magnetic field.
Absolute accuracy of Proton precession magnetometers is ~0.1 nT.
Analytic signals enhance features like faults, edges of magnetic bodies.
Magnetic gradiometry eliminates diurnal effects (daily changes) automatically.
Mud brick walls, using magnetic methods, can measure 10-50 nT.
Asymmetric signatures can be caused by preferred alignment of UXO (unexploded ordnance) targets.
Aquifer transmissivity is calculated as T=kh (m²/d) where h is thickness.
Induced polarization methods measure apparent polarizability.
Frequency-domain electromagnetic (FDEM) methods locate buried tanks, pipes, etc.
Very Low Frequency (VLF) methods investigate objects up to 100 m distance, using 10-30 kHz transmitters.
Objects with high resistance have a smaller phase shift in VLF.
Magnetotelluric (MT) method depth of investigation changes with frequency.
Time-domain electromagnetic (TDEM) methods are relatively insensitive to ground structures.
Ground-penetrating radar (GPR) emits high frequencies (25 MHz-2.6 GHz) to investigate ground structures and record reflected signals.
Natural gamma-ray intensity used to determine lithology, clay mineral classifications, and effective layer thicknesses.
Temperature logs measure fluid inflow/outflow, abnormal radioactivity, and oxidation regions.

Additional Geophysical Methods

NMR (Nuclear Magnetic Resonance) is applied for free-water porosity, irreducible water saturation, pore size distribution, and hydraulic conductivity.
Traditional variable-frequency IP (using two or more frequencies of <10 Hz) is a technique used for IP measurements.
IP is sensitive to dielectric rather than conductivity characteristics.
Apparent chargeability increases with pulse duration (3-5 seconds).
Spectral (complex resistivity) IP uses AC current at various frequencies (30-4000 Hz).
Electrical resistivity methods are cost-effective compared to drilling but have lower resolution.
Apparent resistivity depends on the shape/size of anomalies, layering, and the relative resistivities.
Wave velocity values relate to the medium's elastic moduli and density.
Seismic methods can monitor micro-earthquakes during mining or oil/gas production (fracking).
Refraction seismics detect high-velocity formations (depths, velocities, gradients).
Reflection seismics identify subsurface structures and sediment velocity.
Seismic velocities generally increase with pressure/depth and decrease with temperature.
Surface wave velocities are sensitive to K, μ, and p (parameters related to the medium).
Reflection depends on wave velocity and is insensitive to density.
Oblique incidence reflections (R) are used for Amplitude Variation with Offset (AVO) analysis.
Higher frequency surface waves are sensitive to shallower depths; plots of this dependence V(f) are called velocity dispersion.

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Description

Test your understanding of the fundamental principles of geophysics and gravity surveys. This quiz covers topics like layer thickness, geophysical measurements, and the importance of physical properties in civil engineering. Dive into the specifics of various surveying techniques and their applications.