Work with Gravity Data in GM-SYS Profile

Transcription

1 Work with Gravity Data in GM-SYS Profile In GM-SYS Profile, a "Station" is a location at which an anomaly component is calculated and, optionally, was measured. In order for GM-SYS Profile to calculate a response at a location, the X, Y, and Z coordinates of the station must be specified. However, observed values are not required. If Gravity Gradient calculations are enabled, you may also import observed gravity gradient components measured at the same locations. Adding or Importing any new gravity or gravity gradient data replaces all existing gravity and gravity gradient data. Likewise, importing any new magnetic or magnetic gradient data replaces all existing magnetic and magnetic gradient data. Each time new gravity stations are created or imported, you must set the appropriate Background Density for your survey. You my select and configure one of the 1D FFT filters to apply to the Gravity anomaly profiles. The available filters are: Bandpass Butterworth Cosine Roll-off Gaussian High Pass Low Pass The Filtering options are only available with a Geophysics license. Choosing the Gravity Anomaly to Model: Free-Air, Residual, or Bouguer If you are attempting to fit calculated values to observed values of gravity, you may use free-air, residual, Bouguer gravity and terrain-corrected Bouguer gravity as the observed values. The model calculations will include the contributions of the blocks extending above sea level using the selected densities. Therefore, if you use observed Bouguer or terrain-corrected Bouguer anomaly values, you must either change the density of the "Background block" to the Bouguer reduction density (recommended) or convert the densities of all blocks above sea level to density contrasts relative to the Bouguer reduction density. Typical densities for various rock types may be found in Clark (1966), Dobrin and Savit (1988), or many other references. 1

2 Gravity Stations Use the Specify Gravity Anomaly Profile dialog to define the locations of gravity calculations and observations, as well as the background (reduction) density appropriate to your observed data. This must be correctly defined in order to calculate the model response properly. There are three methods to add Gravity Data to your model: from a database line, from a text file, or by creating equallyspaced calculation stations with dummy observations. Each method is detailed below and in the Application Help. Importing From a Database Line Add a new profile of gravity observations and/or gravity gradient observations to your model by extracting from a specified line in a Geosoft Database. Specify the existing database file containing your observed data profile. The current X and Y channels and the Coordinate System associated with those channels will be displayed. Gravity gradient components are calculated at the same station locations as the gravity calculations. All fields must be filled in this dialog, in order for GM-SYS Profile to be able to calculate the gravity response for the model. The X,Y channels must be defined for the database and a valid coordinate system defined. To Import Gravity Data From a Geosoft Database 1. On the Profile menu, select Manage Anomaly Data. The Manage Anomaly Data spreadsheet appears. 2. On the Gravity Survey menu, select Add New/Replace Existing. The Specify Gravity Anomaly Profile dialog appears. 3. Select Import from database line. The Add Gravity Data from Database dialog appears. 4. On Selected database, browse to the file which contains your gravity data. The X and Y channels are detected and displayed as read-only fields. 5. The Coordinate system and distance units, if specified, are displayed as read-only fields. 6. From the Selected line drop-down list, select the line in the database to import from the list. 2

3 7. From the Elevation channel drop-down list, select the channel containing the station Z-coordinate. Elevations are expected to be positive upward. 8. From the Gravity channel drop-down list, select the channel containing the observed gravity (Gz) values. 9. Click OK. You are returned to the Specify Gravity Anomaly Profile dialog. 10. Specify the Background density. For Bouguer gravity, this would be the reduction density. 11. Click OK to close the Specify Gravity Anomaly Profile dialog. Your new stations will populate the Gravity tab of the spreadsheet. 12. Click OK to close the spreadsheet. You should see the gravity station locations in the Plan View and Depth Panes as black symbols. For additional information about More options, refer to the Add Gravity Profile from Database dialog in the Application Help. Importing From a Text File Use this dialog to import anomaly data from space-delimited ASCII files into GM-SYS Profile. Specify the fields containing each type of data. Leave the field blank or enter zero if the column is not present in the data file. If Gravity Gradients are not enabled, the dialog will not contain fields for the gradient components. To Import a Gravity Profile From a Text File 1. On the Profile menu, select Manage Anomaly Data. The Manage Anomaly Data spreadsheet appears. 2. On the Gravity Survey menu, select Add New/Replace Existing. The Specify Gravity Anomaly Profile dialog appears. 3. Select Import profile from text file. The Import Grav. From File dialog appears. 3

4 4. On the File field, browse to the location of the text file containing your gravity data profile. 5. Select the file. Click Open.The file appears in a new window. Review the file for the following information: Number of comment lines Fields which contain the X,Y, Z and data field(s) and the units they represent Do the coordinates represent model coordinates or external coordinates? Do the Z values represent Depth (positive down) or Elevation (positive up)? 6. Click OK. You return to the Import Grav. From File dialog. 7. Select the Direction. Specify whether the Z values in the file are positive Up (elevation) or Down (depth). 8. Select the Coordinates. Specify whether the X,Y coordinates in the file represent Model coordinates (distance along line, offset from line), or Real World coordinates. The Real World coordinates in the input file must share the same Coordinate System as the model. 9. Enter the Skip lines value if the data does not begin on line 1. Header lines not skipped will be imported as zeroes in all fields. 10. Set the Field Index for X, Y, and Z as determined by the column location of the data. Specify the units used for each field. 11. In the Obs field specify the column containing the observed data and the units used. Depending upon whether you are importing gradient data, additional fields may be present. Specify the field and units for each component you wish to import. 4

5 12. Click OK. You are returned to the Specify Gravity Anomaly Profile dialog. 13. Specify the Background density. For Bouguer gravity, this would be the reduction density. 14. Click OK to close the Specify Gravity Anomaly Profile dialog. Your new stations will populate the Gravity tab of the spreadsheet. 15. Click OK to close the spreadsheet. You should see the gravity station locations in the Plan View and Depth Panes as black symbols. Creating Equally-Spaced Stations GM-SYS Profile enables you to create any number of stations at a specified interval and constant elevation. If you don't have actual observed data, but wish to calculate the response for a hypothetical profile over your model, you must create "dummy" stations. Gravity stations are required so that GM-SYS Profile may calculate the response of the model at these locations. To Create Equally-spaced Gravity Stations 1. On the Profile menu, select Manage Anomaly Data. The Manage Anomaly Data spreadsheet appears. 2. On the Gravity Survey menu, select Add New/Replace Existing. The Specify Gravity Anomaly Profile dialog appears. 3. Select Create equally-spaced stations. The Create Stations with Dummy Observations dialog appears. All units are those currently used in displaying the model. 4. Specify the Starting X coordinate in the current display units. 5. Specify the station Spacing. 6. Specify the Number of Stations OR the Last X coordinate. 7. Specify the Reference elevation. 8. From the drop-down list select whether the Elevation is relative to: sea level or topography. 9. Click OK. You are returned to the Specify Gravity Anomaly Profile dialog. 10. Specify the Background density. For Bouguer gravity, this would be the reduction density. 11. Click OK to close the Specify Gravity Anomaly Profile dialog. Your new stations will populate the Gravity tab of the spreadsheet. 5

6 12. Click OK to close the spreadsheet. You should see the gravity station locations in the Plan View and Depth Panes as black symbols. Background Density For Bouguer anomaly data, you would specify the reduction density used in the Bouguer correction. In most other cases, the correct value is "0.0". Filter Gravity Anomaly Use the Filter Gravity Anomaly menu option to select a filter to generate filtered versions of your calculated and observed anomaly profiles. The option is available on the Compute menu, or the context menu in the Gravity pane. Ground units are the distance units defined for the model origin and projection. The 1D FFT filters are called from the Oasis montaj 1D FFT extension. More detailed information about each filter may be accessed by clicking the Help button on the dialog of interest. To Apply Filters to your Anomaly Profile 1. Open your GM-SYS 2D Gravity Model. 2. While hovering your cursor over the Gravity pane, right click and select Filter Anomaly. The Filter Gravity Anomaly Profile dialog appears. 3. Using the Filter to apply dropdown list, select one of the filters provided. 4. As an example, we will select the Bandpass filter. The dialog expands and the Bandpass filter parameters appear. 6

7 5. Specify the Short wavelength cuttoff and the Long cutoff values. For the Long cutoff values, you can enter 0 (zero) to have an infinite cutoff wavelength. The yellow asterisk ( ) beside the parameter names indicates the parameter is required. 6. Using the dropdown list, select whether to Pass or Reject Defined Band. 7. You can click Clear to clear your selection and select a filter again. 8. Click OK and then click the Calculate ( ) button to generate the filtered profiles. You can also access the "Calculate" option from the "Compute" menu. 9. You can toggle the display of the filtered observed and/or calculated profiles by right-clicking in the Gravity pane, and selecting Display Filtered Observed Gravity. For more information about the individual Filters, click the Help button. Note that the help topics will reflect each of the filter selections. Bandpass Filter for Anomaly Profile The Bandpass filter can be used to pass or reject a range of wavenumbers from the data. However, applying such a simple cutoff filter to an energy spectrum almost invariably introduces a significant amount of ringing (otherwise known as Gibb's Phenomena). We recommend that you use a smoother filter such as the Butterworth filter. Butterworth Filter for Anomaly Profile The Butterworth filter is excellent for applying straight forward high-pass and low-pass filters to data because you can easily control the degree of filter roll-off while leaving the central wavelength fixed. If ringing is observed, the degree can be reduced until acceptable. A common, but more complicated alternative is the Cosine filter. Cosine Roll-off Filter for Anomaly Profile The cosine roll-off filter is commonly used for simple high-pass or low-pass operations. This is because an abrupt cut-off filter causes ringing in the resulting output data. The Cosine roll-off mitigates this artifact by introducing a gradual drop over a range. The desired cut-off wavelength resides within this range, but need not be centered on the range. If you observe ringing, you can reduce the artifact by widening the separation between the short and long wavelengths, or decreasing the degree of the filter. Gaussian Filter for Anomaly Profile The Gaussian filter is another smooth filter that is often used for low-pass or high-pass applications. Highpass Filter for Anomaly Profile As with the Band-pass filter, the High-pass filter is seldom used because the results usually suffer from Gibb's Phenomena (ringing). Lowpass Filter for Anomaly Profile As with the Band-pass filter, the Low-pass filter is seldom used because the results usually suffer from Gibb's Phenomena (ringing). How-To Guide Publication Date: 28/01/

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