Geophysics Images

Following are detailed descriptions and screenshots of the NSW geophysical images available for phones and tablets.

Radioelements

Radioelements

Ternary radioelement potassium(K)-thorium(Th)-uranium(U) channel data. The image is a red-green-blue (RGB) composite using a histogram-equalised colour-stretch for each of the three channels. The red, green and blue channels represent K, Th and U respectively. Mixed compositions are indicated by the proportional blend of the corresponding additive primary colours (e.g. yellow indicates the presence of both K and Th, magenta the presence of K and U while aqua indicates the presence of Th and U). Black indicates low concentrations and white represents high concentrations for all three radioelements. The distribution of radioelements reflects the geochemistry and mineralogy of the near-surface, which may constitute either bedrock or regolith materials. See the NSW statewide grid DVD for K values in percent (%), Th values in parts per million (ppm) and U values in parts per million (ppm).

Total magnetic intensity (TMI) - reduced to pole

Total magnetic intensity

This image shows TMI RTP data in pseudocolour. A histogram-equalised colour-stretch has been applied to the dataset to enhance image. Cooler colours indicate lower values and warmer colours represent increasingly higher magnetic intensity values. Two sun filters were applied to the image, a 3x3 west-east and a 3x3 south-north. 

Variations in the Earth’s magnetic field respond to the intensity of magnetisation, which is controlled by lithological factors, principally magnetite (and or pyrrhotite) content in the ground. Reduction to the pole simulates the form that the anomalous TMI would take if the Earth's magnetic field were locally vertical, as it is at the magnetic pole, assuming that all magnetic sources are inductively magnetised. See the NSW statewide grids DVD set for TMI RTP values in nanoTeslas (nT).

TMI - first vertical derivative - reduced to pole

TMI - first vertical derivative

This greyscale image of 1VD TMI RTP data has been enhanced using a histogram-equalised stretch with input limits of -0.1 to 0.1 nanoTeslas per metre (nT/m). Darker tones indicate lower values and lighter tones represent increasingly higher values. This image shows the vertical rate of change in the Earth's total magnetic field. A 1VD filter enhances geophysical boundaries and structural detail of shallow sources. See the NSW statewide grids DVD set for 1VD TMI values in nanoTeslas per metre (nT/m).

TMI RTP - TMI RTP tilt filter

TMI RTP - TMI RTP tilt filter

This image is a composite of two filtered TMI datasets. No sun angle illumination has been applied. The top layer is partially-transparent pseudocolour TMI RTP data, with a histogram-equalised colour-stretch. Cooler colours indicate lower values and warmer colours represent increasingly higher magnetic intensity values. The layer beneath is a greyscale image of tilt-angle filtered TMI RTP data, presented with a histogram-equalised colour-stretch. Dark tones indicate lower values and lighter tones represent increasingly higher values of the phase angle of the magnetic values. 

Combining the TMI RTP data over TMI RTP tilt-filtered data provides information on the amplitude of the magnetic field, whilst sharpening boundaries of geological features.

The tilt-angle filter is less sensitive to source depth or anomaly amplitude than are simple derivative filters (such as 1VD filter) making tilt-filtered TMI a useful tool for tracing geological structure below variable depths of sedimentary cover. The output of the tilt-filter is an angle, varying between -π/2 and +π/2 radians (i.e. -90° to +90°). This is not related to the strength of the magnetic anomaly, but the spacing between contours of equal tilt angle is related to the depth of the source. The tilt-angle is a local positive maximum over a magnetic source and is zero near the edge.

Isostatic gravity

Isostatic gravity

This is a pseudocolour image of spherical-cap Bouguer gravity data that has been isostatically corrected. A histogram-equalised colour stretch and a 3x3 sun filter has been applied to the data. Sun illumination was applied at 65 degrees elevation and 80 degrees azimuth. Cooler colours indicate lower values and warmer colours represent increasingly higher Bouguer gravity values. Gravity data are presented after removal of latitude and 'free-air' corrections, Bouguer corrections (assuming a crustal density of 2.67 T/m3) and subtraction of gravity due to a model of the isostatic response to changes in elevation. Isostatic correction of the gravity data removes the contribution due to Airy isostasy, the effect in which the depth of the base of the crust is greater in regions of higher topography. Variations in isostatically-corrected gravity should directly reflect differences in the geology of the crust, rather than its thickness. Isostatic gravity shares the same units as Bouguer gravity (µms -2).

Isostatic gravity - TMI RTP tilt filter

Isostatic gravity - TMI RTP tilt filter

This is a composite image made from two datasets. No sun angle illumination has been applied. The top pseudocolour layer is isostastic gravity with an equalised-histogram colour stretch. Cooler colours indicate lower values and warmer colours represent increasingly higher Bouguer gravity values.

The layer beneath is TMI RTP tilt-filtered data in greyscale. Dark tones indicate lower values and lighter tones represent increasingly higher values of the phase angle of the magnetic values.

Combining gravity and magnetic data helps visual interpretation of geology. For example a blue gravity low with a ring of alternating black and lighter tones is often the result of a granite. The granite has a lower density than the surrounding rocks, appearing as a low in the gravity data. The ring was caused by magnetic minerals in the surrounding rock that formed due to heat from the intruding granite.

Elevation

Elevation

Elevation data presented as a pseudocolour image with an histogram-equalised colour-stretch. Cooler colours indicate lower values and warmer values represent increasingly higher elevation. The elevation is derived from airborne magnetic line data, by subtracting the ground clearance given by the radar altimeter from the Global Positioning System (GPS) altitude. Where there are gaps in the airborne survey coverage Geoscience Australia data has been used (DEM-9s). Two sun filters were applied to this image, a 3x3 west-east and 3x3 south-north.

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