Interpretation of Geophysical Data Questions and Answers

Answer: B

The principle describes Seismic Tomography.

Tomography uses many seismic rays crossing the same region from different directions. Each ray's travel time is compared with that predicted by a reference model, and combining the anomalies along all the crossing paths locates where waves travel faster or slower, building a three-dimensional image of the interior, much as a medical CT scan does with X-rays.

Seismic inversion is the general mathematical step of deriving a model from data, not this crossing-ray method. Reflection and refraction surveys use waves returned from or bent along layer boundaries, usually from one shot direction at a time.

Answer: D

All four statements are correct, so the answer is (i), (ii), (iii) and (iv) are correct.

• Seismic reflections arise where acoustic impedance (density times velocity) changes between layers (i).
• Gravity surveys detect lateral changes in density, which produce anomalies in the gravity field (ii).
• Magnetic surveys respond to contrasts in magnetic susceptibility (and remanent magnetisation) of rocks (iii).
• Electrical methods map variations in electrical resistivity (iv).

Each statement gives the physical property its method measures, so any option leaving one out is incomplete.

Answer: C

Aliasing occurs when the signal is sampled at both 8 kHz and 5 kHz.

The Nyquist criterion requires a sampling rate of at least twice the highest signal frequency:

\[ f_{s} \ge 2 f_{m} = 2 \times 5 = 10\ \text{kHz} \]

At 12 kHz the condition is met and the 5 kHz tone is recovered. At 8 kHz it appears as a false 3 kHz tone (8 - 5), and at 5 kHz it aliases to zero frequency, so both of these produce aliasing.

That rules out the options naming 12 kHz and the option saying aliasing never occurs.

Answer: A

The Earth acts as a low pass filter, which attenuates the high frequency signals.

As seismic waves travel through rock, internal friction (anelastic absorption) removes a fixed fraction of energy per cycle. High-frequency waves complete more cycles over a given distance, so they lose energy faster, while low frequencies pass through. Distant earthquakes therefore record as low-frequency signals, and seismic resolution decreases with depth.

A high pass filter would let high frequencies through, the opposite of what the Earth does, and option D's low pass filter attenuating low frequencies is self-contradictory.