A geological survey team traverses a east–west line across rugged terrain suspected…
PHYSICS · Multi-part · CBSE · Physics · Magnetism and Matter · Magnetism and Matter
Problem
A geological survey team traverses a 10km east–west line across rugged terrain suspected to contain a large subsurface iron ore deposit. At 500m intervals they record the horizontal component BH of Earth’s magnetic field using a fluxgate magnetometer. Far from the deposit the regional value is BH,reg=21μT pointing 3∘ east of geographic north. The table lists their measurements:
Distance east of western end (km) | BH (μT)
0.0 | 21.0
0.5 | 20.8
1.0 | 20.4
1.5 | 19.7
2.0 | 18.9
2.5 | 18.1
3.0 | 17.5
3.5 | 17.3
4.0 | 17.4
4.5 | 17.8
5.0 | 18.5
5.5 | 19.3
6.0 | 20.1
6.5 | 20.7
7.0 | 21.0
Assume all other geomagnetic influences are uniform over the survey region.
(a) Plot BH versus distance and describe the key features of the profile, identifying the approximate center and lateral extent of the magnetic anomaly.
(b) Estimate the percentage decrease in BH at the anomaly’s center relative to the regional field. Explain what this indicates about the induced magnetization of the underlying ore body.
(c) The team must correct their compasses while working near the anomaly. Quantitatively determine the maximum compass error (in degrees) a navigator could experience along the survey line, assuming the vertical component of the regional field is unchanged.
(d) Design an experimental procedure (equipment, measurement spacing, and data analysis steps) that would allow the team to map the two-dimensional shape of the ore body’s magnetic signature with higher resolution. Justify how each element of your procedure improves upon the initial traverse.
(e) Discuss one limitation of interpreting magnetic anomalies solely with a dipole model and propose an additional geophysical technique that could independently confirm the presence of iron ore at depth.