New Downhole Variogram

Overview

Function: VG COMPOSITE DOWNHOLE

You can use this function to create a downhole variogram directly from the database. This is the same as the File > New > Database variogram function, with the exception that distances between samples are measured down each drillhole. This function composites the selected drillholes as described in Composite Downhole.

Procedure

Choose File > New > Downhole variogram.

Other options

You will see the

Composite Downhole form.

Form Feature	Description
Composite length	Enter the length of the composites that you wish to generate.
Determine composite length by	The composite length can be determined by either fixed length or best fit.
Minimum % of sample to be included	Enter the minimum percentage of the composite length for which samples must be present. Any composites with less than this amount of sample data available will be written to the nominated string+1 (e.g. if nominated string is 1, composites with less than this amount will be written to string 2). For example, if the composite length is 2 m and the minimum percentage of the sample to be included is 75%, then there must be at least 1.5 m of sample data in the interval to form a composite. These short composites will occur at the end of a hole or immediately before a gap in sampling.
Define the zone selection method	You can restrict the bulking of grades to be within particular geological zones by choosing one of the following: NO SELECTION, do not select by geology. MULTIPLE ZONES, define the zone by entering one or more geology codes. ZONE FROM TO, define the zone as being from the top or bottom of one geology code to the top or bottom of another.
Dilute negative samples	Select the check box to cause negative samples to be treated as zero. Clear the checkbox to cause negative samples to be ignored, in which case the composited grade will be calculated over the non-negative assays in the composite.
Table name	Enter the name of the table containing the sample data to be used for defining the sample fields which are to be extracted.
Fields to be composited, Field name	Enter the name of each of the sample fields which are to be included in the string file containing the composite samples.
Optional weighting fields - Field Name, Default	If required, the calculation of the grades/qualities in the composite samples may be weighted by some other field/s in the sample table in addition to the length weighting which is already performed. Typically, additional weighting fields used would be specific gravity and/or recovery. A maximum of 5 additional weighting fields may be used. Enter the field name/s which are to be used to provide the additional weighting criteria for the creation of the composite intervals. Under some circumstances it is likely that no values may be available for the weighting field for some samples. The Default input field permits a reasonable default value to be defined to cover this circumstance. The default value is used in preference to the stored value in the table when either a null value is present or when the value present is less than zero.
Include Limit	Just as rejection criteria based on length can be defined, when using the optional weighting criteria, additional optional rejection criteria can be defined for each of the weighting fields which are being used. The composite samples which are rejected are saved to the defined string number + 1 and are known as Short samples. A Short sample may be determined by length criteria alone or may use 0 or more of the optional weighting values to determine if a sample is Short or not. Enter an appropriate value for each weighting field to determine if a composite sample provides sufficient support for subsequent geostatistical evaluation. If a weighting field of recovery (%) is used, an appropriate cutoff value might be 90%. This is then used with the Minimum % of sample length to determine if a composite sample is Short or not.

Fill in the fields on the Composite Downhole form.
Click Apply.

You will see the Closed

Define Query Constraints form.

Form Feature	Description
Field Name	Enter the name of the field upon which you wish to impose field constraints.
Operand	Enter the operation that will define the field constraints. Valid entries for the operand are: < less than, <= less than or equal to, > greater than, >= greater than or equal to, <> not equal to, = equal to.
Constraint Value	Enter the value of the constraint that you wish to apply to the named field. When entering a character constraint value, e.g. station_id, it is important to enter the name exactly as entered earlier when setting up the database.

Fill in the fields on the Define Query Constraints form.
Click Apply.

You will see the Closed

Basic tab on the Downhole Variogram form.

Form Feature	Description
Minimum value,Maximum value	You may enter minimum and/or maximum values by which to constrain the sample selection. If no minimum and/or maximum value are specified, no constraints will be applied. For example, if neither a minimum nor a maximum value is specified, all samples will be used.
Lag, Maximum distance	The gamma(h) function is calculated for h given by integral multiples of the specified lag up to a given maximum distance (Max Dist). A lag tolerance of half the lag distance is used. This means if a lag of 2 is used the tolerance is 1 to 3. 1. spread 2. h=n*lag 3. spread limit 4. azimuth 5. samples used to calculate ɣ(h) 6. lag tolerance = lag/2
Output report file name, Output report file format	Provide the name and format of the file to which to write information regarding the variogram calculations. If the file name is not specified no file will be written.
Display report	Select this check box if you want the report to be displayed after creation or clear it to just create the report file.

Fill in the fields on the Basic tab of the Downhole Variogram form.
Click the Advanced tab.

You will see the Closed

Advanced tab of the Downhole Variogram form.

Form Feature	Description
Lag Slider Minimum, Maximum, Increment	The left (minimum) and right (maximum) values of the lag slider are set using these values. The increment value is the amount which the lag is moved when the buttons either side of the lag slider are used. Minimum lag slider values less than one are possible, but are not recommended - this can result in very slow refresh rates when moving the lag slider.
Treat negative values as	ZEROS: All negative samples are transformed to zero before statistical calculations are performed. UNDEFINED: Any negative sample is not included in the statistical calculations. NEGATIVE: All negative samples are included in the statistical calculations with their original value.
Save composited string file	If you wish to save the composites to a string file, select either TEXT or BINARY format. The composites will be saved to a string file based on the Output report file name. If no Output report file name was specified, the composites will not be saved. The string file will contain the composites that are greater than the minimum length criteria specified in the creation of the variogram. The string file can either be a binary file or text file. The binary format allows the user to accurately recreate the variogram while the text format allows for easy reading of the selected data but will create a slightly different variogram due to the rounding of decimals in the third decimal place.

Fill in the fields on the Advanced tab of the Downhole Variogram form.
Click Apply.

On completion of the variogram creation, the Experimental Variogram report is displayed on the screen.