(a) What are hanging wall and footwall of a fault? Describe the different classes of faults based on relative movement of walls…

(a) Hanging wall, footwall and classes of faults

A fault is a fracture along which rock blocks have moved relative to each other. For an inclined fault plane, the block above the plane is the hanging wall and the block below it is the footwall.

Classification by relative movement:

• Dip-slip faults, where movement is along the dip. In a normal fault the hanging wall moves down relative to the footwall, under tension; paired normal faults form a graben (down-dropped block) or a horst (raised block). In a reverse fault the hanging wall moves up, under compression; a thrust is a reverse fault with a dip below about 45°, and large thrusts form nappes.
• Strike-slip faults, where movement is horizontal along the strike. They are right-lateral (dextral) if the opposite block appears displaced to the right, and left-lateral (sinistral) if to the left, as in the San Andreas Fault (dextral). Transform faults are strike-slip faults at plate boundaries.
• Oblique-slip faults, with both dip-slip and strike-slip components.
• Rotational (hinge or scissor) faults, where one block rotates about an axis, so displacement increases along the fault.

Net slip is the true relative displacement, and the rake of the slip line gives the direction on the plane.

(b)(i) Atmospheric windows in remote sensing

Atmospheric windows are wavelength ranges in which the atmosphere transmits electromagnetic radiation with little absorption by water vapour, carbon dioxide and ozone. Sensors are designed to work in them. The main windows are the visible and near-infrared (about 0.3–1.3 µm), parts of the short-wave infrared (1.5–1.8 and 2.0–2.6 µm), mid-infrared windows (3–5 µm), the thermal infrared (8–14 µm), and the microwave region (beyond about 1 mm), which passes through cloud.

(b)(ii) Spectral reflectance curves

A spectral reflectance curve plots the percentage of incident energy that a material reflects against wavelength. It is the spectral signature of the material.

Clear water: low reflectance, highest in blue-green (about 0.4–0.6 µm), falling to almost zero in the near-infrared because water absorbs it.

Dry soil: reflectance rises smoothly and gradually from visible to near- and short-wave infrared, higher than water, with no sharp features, except moisture dips near 1.4 and 1.9 µm.

Healthy vegetation: low reflectance in blue and red (chlorophyll absorption), a small green peak near 0.55 µm, a sharp rise (red edge) near 0.7 µm and high reflectance in near-infrared (about 0.75–1.3 µm) because of leaf cell structure, with water dips at 1.4 and 1.9 µm.

Demarcation: in the red band, soil is brighter than vegetation; in the near-infrared, vegetation is brightest, soil intermediate and water darkest. A near-infrared band therefore separates water from land, and the red and near-infrared bands (NDVI) separate vegetation from soil.

(c) Drainage pattern, texture and anomaly in geological interpretation

Drainage pattern is the arrangement of streams in plan view. Drainage texture is the relative spacing of channels: fine, medium or coarse. A drainage anomaly is a local deviation from the regional pattern, such as an abrupt bend, straight stretch or captured stream.

Classification: patterns are dendritic, trellis, rectangular, parallel, radial, centripetal, annular, pinnate, angulate, barbed, braided and deranged. Texture is fine, medium or coarse.

Significance: dendritic indicates uniform, flat-lying rock or homogeneous granite; trellis indicates folded alternating strong and weak strata; rectangular and angulate indicate joints and faults; radial indicates domes or volcanoes; annular indicates eroded domes; parallel indicates uniform slope or structural control. Fine texture indicates impermeable, soft rock (shale); coarse texture indicates permeable rock (sandstone, granite). Anomalies mark faults, folds, domes and lineaments, which are guides to oil, groundwater and mineral zones.