Fixed – executional geological mapping techniques 32

Several geological mapping techniques involve collecting data along a set course and interpreting the data after the survey is completed. The survey route does not change while the data is being collected – it is fixed. In many cases, an untrained geologist can conduct the survey, although interpretation of the data requires geological expertise.

2.1.2.1

Float/soil mapping

Float/soil mapping is used in areas in which there is no access to and/or exposure of bedrock. The distribution of a particular rock type is determined based on “float” or distinctive soil characteristics developed through weathering of the parent unit. “Float” is a geologic term that refers to pieces of rock that have become detached from an outcrop and which may have also been moved by erosion to a new location (e.g., at the base of a steep slope).

Float/soil mapping is particularly useful in filling in the gaps when constructing a large- scale reconnaissance map. This style of mapping results in a suite of samples that are subsequently used to compare with samples from outcrops within the area of interest. Contacts on geological maps are inferred based on the relative abundance of float

samples from different rock units, suggestive second-order data such as topography, and on interpretations that might be inferred from any outcrops within the field area. It is rare to produce an entire geologic map strictly on the basis of float and it is generally

considered the least effective geological approach to developing a map.

2.1.2.2

Geostatistical sampling

Geostatistical sampling is used in areas in which there is little or no bedrock exposure. It involves the systematic collection of data, either through collection of surface soil

samples or by drilling boreholes. Geostatistical sampling is meant to be predictive, and is based on the assumption that the properties of the overburden (e.g., geochemistry of soil) are dependent on geological principals (e.g., pattern of deposition or emplacement of underlying bedrock).

Samples are typically collected along some form of defined grid, which is designed to be dense enough to allow for statistically meaningful predictions for a given area. This method is often used for mineral exploration. Samples collected along a grid are assayed and the element of interest can be plotted on a geographic base (e.g., topographic map) using statistical analysis. Contour lines can be plotted which indicate higher and lower zones of concentration. This is a standard procedure that can be performed using GIS software. Often, the primary map can define areas that deserve more detailed sampling and mapping.

Geostatistical sampling can also be used in an attempt to sample all the geological units of a particular area. At each site, samples of a given size are collected and categorized based on rock type. The abundance of different rock types collected at different sites is assumed to represent the relative abundance of surrounding rock units. The products include a statistically significant number of samples and a geographic map showing sample locations. Various derivatives of this map can be created using statistical analysis (e.g., creation of a contour map showing element concentrations).

2.1.2.3

Field geophysical techniques

Geophysical techniques gather information of the subsurface remotely. Typically, for investigations on land, aerial surveys are initially conducted to assist geological mapping and to help identify target areas for more detailed surveys. Ground techniques are

subsequently used to test targets discovered by the reconnaissance surveys.

Geophysical data are plotted on graphs and/or can be represented on a geographic base using GIS software. Geophysical methods do not typically give unique, unambiguous solutions. The confidence of the interpretation increases with better understanding of the surface geology and topography. Remotely gathered geophysical data can be very useful in helping to fill in blank spots of a geological map by providing a better sense of either regional structure and fabric, or regional variations in the overall distribution of rock units. All geophysical data require extensive, post-collection data reduction and modelling to generate a useful product.

Gravity survey

Gravity surveying for geological studies is a passive technique that measures the variations in the gravitational pull of the Earth. Subtle changes in gravity result from variations in the density of materials within the subsurface. These variations can be due to local changes in rock types; for example, clastic sedimentary rocks are less dense than granite, which is in turn less dense than basalt. An understanding of the geology of a particular area by way of an accurate geologic map is necessary to extrapolate surficial geology to presumed subsurface rocks

Gravimetric surveys involve taking repeated measurements of the magnitude of

gravitational acceleration throughout the area in question, with periodic reacquisition of base stations to re-calibrate the gravimeter. The effects of tidal and instrument drift that would otherwise mask any subtle anomalies are overcome by repeat readings at a fixed base station throughout the survey. Accurate topographic levelling is carried out at each station in order to correct for the effects of terrain.

Magnetics survey

Magnetic profiling is a passive technique that involves measurement of the total

amplitude of the Earth's magnetic field. Magnetometer surveys map local disturbances in the earth's magnetic field that are caused by magnetic minerals in the upper regions of the earth's crust.

If the object of the survey is to make a rapid reconnaissance of an area, a magnetic- intensity profile is made only over the target area. If the object of the survey is to delineate already discovered structures, the geophysicist sets up a grid over the area and makes measurements at each station on the grid. The corrected data that is recorded is then entered on a scale drawing of the grid, and contour lines are drawn between points of equal intensity to give a magnetic map of the target area that may clearly indicate the size and extent of the anomalous body to the trained eye of the interpreting geophysicist.

Electrical and electromagnetic surveys

Electrical and electromagnetic methods are both used to map variations in the electrical properties of the subsurface and essentially measures electrical conductivity (i.e., how easily an electrical current can pass through a material). Electrical surveys pass an electrical current directly into the ground using electrodes and measure the resulting potential difference within the subsurface using detectors which are place along a surveyed line. In comparison, electromagnetic methods induce currents in the ground by the passage of electromagnetic waves. The indirect nature of electromagnetic methods makes it possible to take measurements from the ground and also from specially adapted aircraft, whilst electrical methods are restricted to ground based measurements.

Subsurface materials exhibit a very large range of electrical conductivity values, which is primarily governed by the amount of water filling the gaps between the mineral grains and the amount of salt dissolved in this water. Areas of high interstitial water content have lower resistance to an electrical current than do dry areas. Rock units of differing composition will have variable natural electrical conductivity. Metallic minerals

containing are very good conductors in their native metal state and are easily detected using electrical and electromagnetic methods (e.g., iron, copper, nickel, silver, gold).

Seismic surveys

Seismic surveys essentially generate a ‘picture’ of the subsurface geology. A controlled seismic source of energy is provided by a source ('shot') located on the surface. Energy radiates out from the shot point and travels laterally and/or vertically before returning to the surface. Reflected and refracted signals are recorded by an array of receivers called geophones. Sound waves travel through the subsurface and change speed when passing through rocks of different densities, and reflect (bounce back) at the contact between these rock types. By noting the time it takes for a reflected or refracted wave to arrive at a receiver, the depth of the feature that generated the reflection is estimated. In order to estimate this depth, the subsurface geology is assumed based on previous (surface) field work.

The detail and depth investigated by seismic surveys is based on input energy magnitude, input energy frequency, and number and spacing of the geophone lines. The quality of the final seismic survey is only as good as the input parameters to the model. Complex geology can go undetected if the modelling does not bring it out of the seismic signals. The depth of investigation can range from near surface to, in extreme cases, the radius of a particular planet, making it very useful for conducting investigations at multiple scales.

2.2 Review of the University of Western Ontario’s lunar