Exercises: Sources of geological information

A hiking group wants to know the altitude of the peaks and the gradient of a route through the Sierra de Gredos. Which type of map should they consult and which graphical feature will provide this information? Explain how to read this feature.

Explain in your own words the difference between an aerial photograph and a satellite image, and give a specific example of a geological application for each.

Place the following tasks of a geological study of a region in order and justify the chosen sequence:

1. a) draw the final geological map;
2. b) visit outcrops and collect data in the field;
3. c) consult existing maps and aerial photographs.

On a topographic map the contour lines pass through elevations of $400\ \text{m}$, $420\ \text{m}$ and $440\ \text{m}$. What is the contour interval of the map? Explain how you would estimate the elevation of a point located between two contours.

What information will you find in the legend of a geological map? Name at least four elements, such as the colours and ages of the units, the symbols for structures, the contacts or the scale, and explain what each one provides.

What is an outcrop and why is it considered a direct source of geological information, as opposed to aerial photographs or maps, which are indirect sources?

Aerial photographs are taken in overlapping pairs so that they can be viewed through a stereoscope. What advantage does this three-dimensional view of the relief bring to geological study?

A map only shows the numerical scale $1:10\,000$. Explain what this figure means and calculate the real length of a path that measures $4,5\ \text{cm}$ on the map.

The diagram shows the contour lines of a hill with a contour interval of $100\ \text{m}$. Compare the gradient of the slope at point A with that at point B and explain, based on the spacing of the contour lines, which of the two you would climb with less effort.

On a topographic map at a scale of $1:50\,000$, the distance between two towns is $6,4\ \text{cm}$. Calculate the actual distance between them, expressed in kilometres.

In a satellite image of the mouth of the Ebro, a triangular plain with channels branching towards the sea can be seen. What sedimentary landform are you observing and which external geological process formed it?

On a map at a scale of $1:25\,000$ the distance between two outcrops is $7,6\ \text{cm}$. Calculate the real distance in metres and, if the difference in elevation between them is $190\ \text{m}$, estimate the average gradient of the route as a percentage.

Explain, step by step, how a topographic profile is constructed from the contour lines crossed by a straight line on the map, and what information the profile provides that cannot be seen directly in plan view.

In aerial photographs taken early in the morning, scarps and faults are easier to recognise than at midday. Explain what role shadows play in the photogeological interpretation of the relief.

In a false-colour satellite image, healthy vegetation appears in reddish tones and bare soil in greys. Explain how this contrast can be used to map rocky outcrops or areas burnt by a fire.

A topographic map shows contour lines very close together at the head of a basin and very far apart on the valley floor. Interpret this pattern in terms of gradient and explain in which zone you would expect more intense fluvial erosion.

When correlating two stratigraphic boreholes located $15\ \text{km}$ apart in the same basin, a Jurassic bioclastic limestone marker bed is identified. In borehole A, the bed is located at an elevation of $+420\ \text{m}$ and is $12\ \text{m}$ thick; in borehole B, it is found at $+180\ \text{m}$ and is only $2\ \text{m}$ thick.

1. a) A preliminary report claims that borehole B subsided due to a normal fault and that the reduction in thickness is solely due to recent superficial erosion. Refute this statement by proposing an alternative tectonic explanation and an alternative sedimentological explanation that coherently account for both discrepancies.
2. b) What field or microfacies criteria in borehole B would make it possible to discern whether the thickness reduction was due to pinch-out caused by a low sedimentation rate (paleogeographic control) or to a synsedimentary erosional unconformity?

In a preliminary geological map, the trace of two parallel normal faults appears separated by $9\ \text{cm}$ on the map. The actual horizontal distance measured on the ground between them is $2,7\ \text{km}$, but the terrain features a hillside with an average slope of $25\,\%$.

1. a) Determine the numerical scale of the map and state with reasoning whether it would allow representing a fault breccia zone $12\ \text{m}$ wide if the cartographic visual perception limit is $0,2\ \text{mm}$.
2. b) A technician claims that the actual distance they will walk on foot along the slope between both faults coincides with the projected horizontal distance deduced from the map ($2,7\ \text{km}$). Demonstrate quantitatively whether their statement is correct by calculating the actual distance along the terrain and the absolute error made.

When comparing a frame from the 1956 photogrammetric flight with a current orthophoto of a sandy stretch of coast, a technician observes that the coastline has retreated $60\ \text{m}$ and hastily concludes that the mean sea level has risen at a linear rate of $1\ \text{m}/\text{year}$ due to climatic causes.

1. a) Refute their conclusion by arguing two sources of methodological error linked to the image capture conditions (such as tidal range or seasonal beach profile) that must be corrected before quantifying real retreat.
2. b) Assuming a real net retreat, formulate two hypotheses linking this sedimentary deficit to specific anthropogenic actions (in the river basin or along the coastal strip itself) and describe what evidence you would look for in the images to verify the alteration of longshore drift transport.

To evaluate the flood attenuation capacity, the catchment basin of a reservoir is delimited on a $1:50\,000$ scale topographic map using a millimetre grid, yielding a cartographic area of $32\ \text{cm}^2$.

1. a) Calculate the real area of the basin in hectares and in $\text{km}^2$. If a torrential event generates a net precipitation of $75\ \text{L}/\text{m}^2$ in $24\ \text{hours}$ with an average runoff coefficient of $0,40$, calculate the runoff volume (in $\text{hm}^3$) that will enter the reservoir.
2. b) An operator digitises the basin on a paper copy of the map that was enlarged to $110\,\%$ of its original size (a $10\,\%$ linear increase) along each axis when reprographed, but uses the nominal scale of $1:50\,000$. Quantitatively demonstrate the percentage of overestimation in the calculated surface area and justify why the presence of a printed graphic scale would have avoided this error.

When comparing the 1956 topographic map (scale $1:33\,000$) and a current high-resolution orthophoto of a Pyrenean glacial cirque, the fragmentation of the glacier into relict ice patches, the exposure of a moraine ridge, and the formation of a tarn dammed by a rock threshold are observed.

1. a) Justify, taking into account the dynamics of glacial mass balance and the concept of response time, why the position of the front does not necessarily reflect the meteorological conditions of the previous year, but rather an accumulated climatic trend.
2. b) Design a methodological protocol to accurately quantify the average annual rate of linear retreat of the front (in $\text{m}/\text{year}$) and the loss of area (in $\text{ha}/\text{year}$), indicating which fixed ground references you should use to correct potential georeferencing discrepancies between both documents.

In two sectors of a basin separated by $8\ \text{km}$, two stratigraphic columns present at their top a bioclast-rich limestone layer with ammonites from the same biozone. In the western column, the limestone rests conformably on $40\ \text{m}$ of fluvial sandstones; in the eastern one, the same limestone rests directly on Cambrian metamorphic slates.

1. a) Justify on the basis of which stratigraphic principles the temporal equivalence of the limestones is established and deduce the relative nature of their deposition with respect to the basement.
2. b) Propose two alternative geological models (one tectonic-structural and one paleogeographic involving transgression over a paleorelief) that explain the absence of the sandstones in the eastern column.
3. c) Describe what observable field criterion at the basal contact of the limestone in the eastern column would make it possible to validate the marine nonconformity model versus an omission caused by a low-angle normal fault.

On a topographic map with a contour interval of $20\ \text{m}$ and a scale of $1:25\,000$, a north-south mountain range presents asymmetrical slopes: on the west slope, adjacent contour lines are $1\ \text{cm}$ apart on the map, whereas on the east slope they are $4\ \text{mm}$ apart. The geological series consists of alternating limestones and marls with a uniform dip of $18^\circ$ towards the east.

1. a) Calculate the topographic gradient (in percentage and in approximate degrees) of both slopes.
2. b) To build a road, an excavation is planned that will increase the cut slope angle to $30^\circ$ on the east slope. Evaluate the geotechnical stability of both slopes by classifying them as anaclinal or cataclinal and predict, with reasoning, what type of failure or instability this excavation will trigger on the east slope.

To map the reactivation of a slope landslide with a maximum width of $50\ \text{m}$ that threatens a transport route, multispectral satellite imagery is available with a spatial resolution of $30\ \text{m}$ per pixel and a revisit time of $16\ \text{days}$.

1. a) Technically justify why this resolution is unfeasible for delimiting the crown scarp and lateral margins of the landslide, explaining the concept of mixed pixels and the minimum required ratio between object size and spatial resolution.
2. b) Design a methodological remote sensing alternative that combines a very high spatial resolution source (such as sub-metre aerial or satellite orthophotography) for geometric delineation with a radar technique (InSAR interferometry) to measure millimetre-scale deformation velocity, explaining what each source contributes.