3.0 Layer attributes

Attributes refer to the physical properties assigned to a given model layer(polygon) and are used to calculate the predicted gravity and magnetic anomalies. Each layer contains five attributes:

  • Layer bulk density

  • Layer reference bulk density

  • Layer susceptibility contrast

  • Magnetic field vector angle A (Equal to the inclination of Earth’s field if magnetisation is induced only)

  • Magnetic field vector angle B (Equal to the declination of Earth’s field if magnetisation is induced only)

  • Magnetic field vector angle C (Angle between model profile positive X-axis and geographic north)

Layer attributes are set using the Attributes sidebar within the modelling window or from the Attribute Table which can be accessed from the top menubar. GMG is designed such that attributes can be easily varied when modelling. This allows for rapid assessment of the effect of varying each attribute. A given attribute need only be set if required for the layer, if attributes are not set, the given layer will not contribute to the modelled anomaly, be that gravity or magnetic.

Note

For the case when a layer requires a remanent magnetism, angles A and B will differ from the Inclination and Declination.

3.1 Bulk density

Gravity modelling is achieved using bulk density contrasts relative to a reference density.

Each layer requires:

  1. A bulk density

  2. A reference density

GMG defaults to a reference density of 0 \(kg\) \(m^{-3}\). In this case, densities should be input as density contrasts e.g., 200 \(kg\) \(m^{-3}\).

If you would prefer to use absolute values, then you can set the reference density to your chosen value and gmg will determine the density contrast to use when calculating the gravity anomaly. For example, popular choices for a bulk crustal reference density are 2670, 2700 and 2800 \(kg\) \(m^{-3}\) depending on the particular geology in the region of interest. See Hinze et al., (2003) for more details regarding the choice of reference density.

You can als use a different reference density for different layers. This is useful if you wish to model crustal structure and relative to, for example, an upper crust of 2670 \(kg\) \(m^{-3}\), lower crust 2900 \(kg\) \(m^{-3}\) and upper mantle of 3330 \(kg\) \(m^{-3}\).

To assign density values use the Attribute Sidebar or the Attribute table.

Note

All densities must be input using SI units: \(kg\) \(m^{-3}\).

For each layer both the bulk absolute density and the reference density must be set.

Example

To model a sedimentary basin typical absolute density values may be 2400 \(kg\) \(m^{-3}\) for the basin and 2670 \(kg/m^3\) as the reference density for the upper continental crust.

To model crustal thickened from a reference crustal thickness of 32 km, thickened to 36 km, a lower crustal density of 2800 \(kg\) \(m^{-3}\) maybe be modelled against a upper most mantle reference density of 3330 \(kg\) \(m^{-3}\).

Tip

To set all layer reference densities as a single value (e.g. when only modelling upper crustal structure), in the menubar navigate to:

Gavity Field -> Set background density...

3.2 Magnetic Susceptibility

Magnetic susceptibility, \(k\), is related to the intensity of magnetisation, \(J_{i}\), and the strength of the magnetising force, \(H\) by:

\[k = \frac{J_{i}}{H}\]

As both \(J_{i}\) and \(H\) have SI units \(A m^{-1}\), magnetic susceptibility is dimensionless.

Note

GMG uses SI units, hence magnetic susceptibilities are \(4 \pi\) larger than c.g.s values

3.3 Magnetisation Vector

Angle A

For induced magnetisation, Angle A is equal to the inclination of Earth’s field. If magnetistion is remnant or a mix of remnant and induced, then the direction of magnetisation will likely differ from Earth’s inclination and Angle A must be calculated outside of GMG.

Angle B

For induced magnetisation, Angle B is equal to the declination of Earth’s field. If magnetisation is remnant or a mix of remnant and induced, then the direction of magnetisation will likely differ from Earth’s declination and Angle B must be calculated outside of GMG.

Angle C

Angle C (Azimuth) is the angle (in degrees) that the strike of the model profile with respect to geographic north (the angle is positive when measured clockwise). See A2 for further details. The strike of the polygon (geological feature/layer) is assumed to be orthogonal to the model profile. For example, if the model is orientated West-East Angle C will equal 90.

3.3 Remnant Magnetism

Currently, modelling of remnant magnetism is not explicitly included within GMG. THis could, however, be achieved using user defined combinations of Angles A, B and C.