Question 7 of 10: Displaying and Enhancing Geophysical Data

Question 7: Displaying and Enhancing Geophysical Data (16.66% of paper)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Raw geophysical data (profile stacks, gridded maps) rarely reveal every geologically useful feature directly; display and enhancement choices trade off suppressing noise/regional trends against preserving true signal, and different enhancements make different features visible.

Profile (stacked line) plots. Individual survey-line data plotted as a wiggle trace or line graph against distance. Assists: preserves exact amplitude and true resolution along a line, ideal for picking crossover points, inflection points, or comparing observed vs. modelled response directly. Hinders: gives no sense of the 2-D/3-D spatial pattern between lines, so a feature's true strike, extent or geometry is easily missed.

Contour/colour-shaded grid maps. Gridded (interpolated) data displayed as contour lines or a continuous colour-graded raster. Assists: immediately reveals the spatial pattern, strike, and extent of anomalies across the whole survey area, essential for target selection and geological correlation. Hinders: gridding/interpolation between lines can create or smear artifacts, especially with wide line spacing relative to target size (aliasing); colour-scale choice itself can visually exaggerate or suppress features (a poorly chosen non-linear or non-perceptual colour scale can create false "bullseye" anomalies).

Shaded-relief (sun-illumination) maps. A grid rendered with an artificial low-angle illumination from a chosen azimuth, producing shadow/highlight relief that emphasizes gradients. Assists: dramatically enhances subtle linear features (faults, dykes, lineaments) that a flat colour map would hide, especially those aligned perpendicular to the illumination azimuth. Hinders: features parallel to the illumination azimuth are suppressed rather than enhanced, so a single illumination direction can miss real structure — multiple illumination azimuths are needed for a complete picture, and the technique can introduce apparent linear artifacts unrelated to real geology (grid-line or flight-line noise, similarly enhanced).

Vertical/horizontal derivatives and analytic signal. First or second vertical derivative (or horizontal gradient/analytic signal) transforms sharpen edges and boost short-wavelength (near-surface, shallow) features relative to long-wavelength (deep, regional) ones. Assists: resolves closely-spaced or overlapping sources, sharpens body edges for contact-location mapping, useful for structural/lineament mapping. Hinders: strongly amplifies high-frequency noise, so derivative maps require clean, well-processed input data or the noise itself becomes the dominant "signal" displayed.

Upward continuation / low-pass filtering. Mathematically (or physically) continues the field to a greater observation height, or otherwise removes short wavelengths, isolating the deep/regional component. Assists: separates regional geological trends (basement structure, large intrusions) from shallow noise/near-surface clutter, useful as a first step in regional-residual separation. Hinders: inevitably also attenuates real shallow signal along with the noise, and the choice of continuation height/cutoff wavelength is somewhat subjective, so an overly aggressive filter can remove genuine near-surface targets.

Pseudosections/cross-sections. 2-D vertical-slice displays (resistivity, IP, seismic) along a survey line. Assists: the natural display for depth-resolving methods, directly shows depth extent and dip of a target. Hinders: a raw (uninverted) pseudosection is not true depth/resistivity and can visually mislead if presented as if it were a true cross-section.