Geologic map

Summary

A geologic map or geological map is a special-purpose map made to show various geological features. Rock units or geologic strata are shown by color or symbols. Bedding planes and structural features such as faults, folds, are shown with strike and dip or trend and plunge symbols which give three-dimensional orientations features. Stratigraphic contour lines may be used to illustrate the surface of a selected stratum illustrating the subsurface topographic trends of the strata. Isopach maps detail the variations in thickness of stratigraphic units. It is not always possible to properly show this when the strata are extremely fractured, mixed, in some discontinuities, or where they are otherwise disturbed. Symbols Lithologies Rock units are typically represented by colors. Instead of (or in addition to) colors, certain symbols can be used. Different geologic mapping agencies and authorities have different standards for the colors and symbols to be used for rocks

Official source

https://en.wikipedia.org/wiki/Geologic_map

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Calculation of geological uncertainties associated with 3-D geological models

Ian Pomian-Srzednicki

3-D geological models are built with data collected in the field such as boreholes, geophysical measurements, pilot shafts or geological mapping. Unfortunately, these data are always limited in number. It implies that geological information is sparse and subsurface models are thus always built of both subjective interpretation and mathematical interpolation/extrapolation techniques. These models are therefore uncertain and this uncertainty is rarely pointed out in a geological prognosis. Our study proposes to bring a new methodology for the evaluation of geological uncertainties related to 3-D subsurface models and to test its potential use. The methodology we propose is based on the 3-D subsurface model, which is here considered as the most probable prediction (notion of best guess). The various geological interfaces that compose the subsurface model are handled individually as Gaussian random fields. At each location of an interface, the random function Z(u) describing the position of this interface is composed of a deterministic part m(u) which represents the expected position, and a random part σ(u)ε(u) which describes fluctuations around the predicted position. Then, a model of spatial variability (a variogram function γ(h)) is proposed in order to condition the random field according to available observations. Several structural constraints, such as the shape of folds and the thickness of layers can also be accounted for in this model. At this point, we are able to estimate the local variance all over the study area by the application of the kriging technique. Finally, the variability is converted into three-dimensional information by calculating probabilities, this describes the occurrence of the various rock masses that are present in the study area. The probabilities are calculated according to intersection rules that govern the stratigraphic sequence of the subsurface model, and they allow us to build a probabilistic model of subsurface structures in the form of a three-dimensional probability field. All of this has been incorporated in a computer program.

EPFL2002

When we have to valorise the geological information for land planning or engineering applications, it is difficult to integrate the large complexity of the maps of the Swiss Geological Atlas at the scale of 1:25'000. The heterogeneity of the legends between neighbouring maps is often another difficulty. That is the reason why we proposed the notion of «geo type», a concept that describes in a simplified and homogeneous manner the principal geological formations at the scale of a canton. The experience in the Canton de Vaud demonstrates that the use of geotypes allows a translation of the geological information of maps and boreholes in terms of properties that are useful for applied geology. Each geotype is characterized by a table of properties like, for example, an order of magnitude of thermal conductivity, permeability, grain size distribution, plasticity etc. This characterization proved its robustness in different domains like seismic microzoning, groundwater resources, geothermy and slope stability. A recent application is the DEEP CITY project which characterizes the underground resources below the cities where 3D models in geotypes are established. The numerical modelling operations are easier to be performed. The geo types do not replace the academic geological information, which remains the base, particularly the geological maps of the Swiss Atlas.

2015

Calculation of geological uncertainties associated with 3-D geological models

Ian Pomian-Srzednicki

EPFL2002