Question 3 of 10: Converting Geophysical Data into an Image — Gridding, Colour/Shading, and Interpretation

Question 3: Converting Geophysical Data into an Image — Gridding, Colour/Shading, and Interpretation (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.

From points to an image. Geophysical measurements are almost always made at scattered, irregularly spaced stations or along flight/traverse lines, not on a regular grid, so the first step is gridding (interpolation onto a regular array of cells): common algorithms include minimum-curvature gridding (fits a smooth surface with minimum bending energy, good for potential-field data), kriging (a geostatistical interpolator that weights neighbouring points by a fitted spatial-correlation model and additionally gives an uncertainty estimate), and simple inverse-distance weighting. The grid is then rendered as a contour map (lines of equal value), a colour raster/image map, or both together.

Colouring and shading. A continuous colour scale (e.g. a "rainbow" or, preferably, a perceptually uniform scale) maps the data range to colour so that magnitude is read at a glance; the choice of scale (linear, logarithmic — important for the extreme dynamic range of resistivity/conductivity data — or histogram-equalized to spread colour evenly across a skewed data distribution) strongly affects which features stand out. Shaded relief (artificial sun-angle illumination applied to the gridded surface as if it were topography) enhances linear, edge-like features by throwing them into highlight/shadow, and is especially effective for magnetic and resistivity data where lineaments (faults, dykes, contacts) are the target. Derivative images — the first vertical derivative, horizontal gradient, analytic signal, or tilt derivative of a potential-field grid — sharpen and better localize the edges of causative bodies compared with the raw field, at the cost of amplifying high-frequency noise.

Examples and what is extracted. A total-field aeromagnetic map coloured on a histogram-equalized scale and sun-shaded from the northwest highlights faults and dyke swarms as narrow linear highs/lows, from which strike direction, offset (apparent fault throw) and intrusive geometry are mapped directly by eye; an analytic-signal image of the same data instead produces a symmetric peak directly over the edge of a magnetic body regardless of magnetization direction, making it useful for locating body outlines without needing to first reduce-to-pole. A resistivity/EM conductivity map on a logarithmic colour scale highlights a conductive plume or clay unit as a warm-coloured zone whose shape traces the lateral extent of contamination or a buried channel. A seismic reflection time-slice or amplitude map, coloured to emphasize amplitude anomalies (a "bright spot"), can indicate a hydrocarbon-bearing sand or a buried channel's meander pattern in plan view.