Gradient Calculator

The program provides a simulation of the magnetic response from a total field gradiometer over a target defined by preset values or by the user. The purpose of the program is to use graphical displays to highlight the recorded magnetic response from a buried target for a given set of gradiometer separations, survey speeds and sampling separations. The displays include graphs showing the magnetic response from two total field sensors and the transverse or vertical gradient, a side and plan view of the survey geometry with vector arrows showing the direction of field or with an amplitude colour chart. A table of model values is presented and gives useful values calculated by the program.

This program has two wizards to explain the different aspects and features of the program. The simple wizard uses the program’s predefined targets and basic survey options to give quick results for standard surveys, while the advanced wizard uses more complex options and gives more detail about the individual input fields. Also, it leads you through the process of defining your own targets. On the completion of these wizards, the results will be shown in the main program and gives access to all the adjustable parameters. There is an option to skip the wizards and go straight to the main program if you wish.

Select A Program

Simple : Step 1 - Target Parameters

Firstly, select a Target from the drop down list that are predefined in the list.

Use the slider to change the depth below the Seabed/Ground Surface and the Distance the Target is from the centre of the survey line.
Depth of Target:
m

Distance (+ East/- West):
m

Simple : Step 2 - Positional Parameters

Secondly, insert your Latitude and Longitude in decimal degrees, and Height above WGS Spheroid in km for your survey. Also enter the Date for the survey.

Latitude (°):

Longitude (°):

Height above WGS Spheroid (km):

Date:

Map Diagram

Elevation Diagram

Simple : Step 3 - Survey Parameters

Finally, select the Transverse or Vertical Gradiometer setup and use the sliders to adjust the Sensor Separation and Sensor Height. When all the parameters are correct, click the Calculate button.

Note: For Vertical Gradiometer the Sensor Height is the height of the Lowest Sensor.

Gradiometer:

Sensor Separation:
m

Sensor Height above Seabed/Ground Surface:
m

Advanced : Step 1 - Defining your Target

Firstly, define the target's position in the survey area relative to the mid-point of your survey line and the depth beneath the seabed/ground surface.

Distance (+ North/- South):
m

Distance (+ East/- West):
m

Depth of Target (m):
m

Target Dimensions

Next, define the properties of the target, such as size, magnetic susceptibility and shell thickness if the target is hollow.

Breadth (m):

Width (m):

Height (m):

Hollow

Shell Thickness (m):

Magnetic Susceptibility

Magnetic Susceptibility is a dimensionless value that determines how easily a magnetic field can be induced in target. Typical Susceptibility values are shown in the table.

Note: The Calculator assumes a null background susceptibility therefore the target's Magnetic susceptibility value is the contrast value.

Magnetic Susceptibility:

Material	Magnetic Susceptibility Value
Iron	0.138
Steel	0.051
Hematite	0.001 to 0.007
Magnetite	0 to 3
Pyrite	-0.000001 to 0.0001
Quartz	-0.00001
Orthoclase	-0.00001
Biotite	0.001 to 0.003
Ilmenite	0 to 3

Advanced : Step 2 - Positional Parameters

Secondly, insert your Latitude and Longitude in decimal degrees, and Height above WGS Spheroid in km for your survey. Also enter the Date for the survey. These values are used in an IGRF model to calculate Inclination, Declination and Regional Field Intensity.

Latitude (°):

Longitude (°):

Height above WGS Spheroid (km):

The Heading indicates the angle with North that the survey is travelling. At 0° the traverse is progressing in the North direction

Heading (°):

Date:

Map Diagram

Elevation Diagram

Advanced : Step 3 - Survey Parameters

Finally, define the type of Gradiometer you are going to use between a Transverse or Vertical Gradiometer, the Sensor Separation and Sensor Height. You can also define the Speed at which you take the survey in m/s or knots and the sampling rate of your instrument.

Note: For Vertical Gradiometer the Sensor Height is the height of the Lowest Sensor.

Gradiometer:

Sensor Separation:
m

Sensor Height:
m

Survey Speed:

Sampling Rate:
readings per second

Anomaly Signature

Show Total Field

Show Gradient

Anomaly Amplitude (nT)

Traverse Length (m)

Sensor 1 Sensor 2

Gradient

Survey Geometry

Change View

Show Side View Show Plan View

Model Display

Show Vector Field Show Amplitude

Smoothing Contours may take a minute to calculate.

Model Values

Gradient Maximum Peak
Gradient Maximum Trough
Sensor 1 Maximum Peak
Sensor 1 Maximum Trough
Sensor 2 Maximum Peak
Sensor 2 Maximum Trough
Instrument Noise
Survey Noise
Regional Field
Inclination of Field
Declination of Field

Positional Parameters

Latitude (°): Geodetic Latitude from GPS to 2 decimal places.

Longitude (°): Longitude from GPS to 2 decimal places.

Height above WGS Spheroid (km): Elevation taken from GPS, or use Height above Mean Sea Level in km.

Date:

Heading (°): Direction of travel where 0° is travelling in the North Direction.

Calculations can take between 10 and 20 seconds

Survey Parameters

Gradiometer: Transverse: Sensor 1 and 2 are positioned at same height and are separated either side of survey line.
Vertical: Sensor 1 positioned above survey line at the sensor height, with Sensor 2 vertically separated above Sensor 1.

Sensor Separation: Distance between Sensors in metres.
m

Sensor Height: Height of sensors above seabed/ground surface in metres. For a vertical gradiometer, this is the height of lowest sensor.
m

Survey Speed: How fast the sensors are travelling along the survey line in either m/s or knots.

Sampling Rate: Number of readings per second. This value depends on the type of sensor being used.
readings per second

Add Noise This adds a noise function to the calculated Total Field Values for each sensor. The function models instrument noise and survey noise. Survey Noise is a randomly generated trend through the data with amplitudes between 0.1 and 0.2 nT.

Instrument Sensitivity: Presented in the drop down list are a range of typical sensitivity values for various magnetometers which is quoted in nT/√Hz. The instrument noise is determined using the smapling rate and sensitivity values. Geomatrix recommend customers calculate the sensitivity of their instrument and then enter the value using the sliding bar.

nT/√Hz

Calculations can take between 10 and 20 seconds

Target Parameters

Advanced Options Advanced Options allow you to define up to 3 targets and adjust a larger range of properties for the target, including Magnetic Susceptibility. Unchecked uses the predefined targets shown in the list.

Distance (+ North/- South): Distance from centre of traverse line in the North/South direction in metres.
m

Distance (+ East/- West): Distance from centre of traverse line in the East/West direction in metres.
m

Depth of Target: Depth below seabed/ground Surface in metres.
m

Number of Targets:

Target 1

Distance (+ North/- South): Distance from centre of traverse line in the North/South direction in metres.
m

Distance (+ East/- West): Distance from centre of traverse line in the East/West direction in metres.
m

Depth of Target: Depth below seabed/ground surface in metres.
m

Magnetic Susceptibility: How easily the Target is magnetised by the Earth's Magnetic field.

Breadth (m): Extent of the Target in the North/South Direction.

Width (m): Extent of the Target in the East/West Direction.

Height (m): Extent of the Target in the up/down direction.

Hollow

Shell Thickness (m): When the hollow checkboxed is selected this value will define the thickness of the hollow target. Note that this value will round the target dimensions to best fit the shell thickness.

Target 2

Distance (+ North/- South): Distance from centre of traverse line in the North/South direction in metres.
m

Distance (+ East/- West): Distance from centre of traverse line in the East/West direction in metres.
m

Depth of Target: Depth below seabed/ground surface in metres.
m

Magnetic Susceptibility: How easily the Target is magnetised by the Earth's Magnetic field.

Breadth (m): Extent of the Target in the North/South Direction.

Width (m): Extent of the Target in the East/West Direction.

Height (m): Extent of the Target in the up/down direction.

Hollow

Shell Thickness: When the hollow checkboxed is selected this value will define the thickness of the hollow target. Note that this value will round the target dimensions to best fit the shell thickness.

Target 3

Distance (+ North/- South): Distance from centre of traverse line in the North/South direction in metres.
m

Distance (+ East/- West): Distance from centre of traverse line in the East/West direction in metres.
m

Depth of Target: Depth below seabed/ground surface in metres.
m

Magnetic Susceptibility: How easily the Target is magnetised by the Earth's Magnetic field. Note that the background susceptibility is assumed zero.

Breadth (m): Extent of the Target in the North/South Direction.

Width (m): Extent of the Target in the East/West Direction.

Height (m): Extent of the Target in the up/down direction.

Hollow

Calculations can take between 10 and 20 seconds