It is common in some projects to have a few horizons that have dense data in depth and other horizons that have very little data. In these cases, the sparse data may be measured as thickness from the better defined horizons and the user is faced with how to use this in EarthVision. The illustration below shows just such an example, where an intermediate horizon has been created using a thickness grid between two existing horizons.

The use of thickness data can also occur when geologists prefer to create new horizons using independent thickness grids to stack up or down from a reference horizon.

This article discusses the following topics regarding using thickness data in EarthVision:

• Handling Thickness Data In Horizon Gridding
• Special Techniques For Working With Thickness Data
• Creating Thickness Data

Handling Thickness Data In Horizon Gridding

First, it is important to review the use of reference and intermediate horizons in the WorkFlow Manager and the Geologic Structure Builder. While reference horizons in an EarthVision model must be based on input scattered data, intermediate horizons can take as input a scattered data file (representing elevation), a constant (representing thickness), or a 2D grid (representing thickness).

When entering thickness data (either as a 2D grid or as constant) as the input data for an intermediate horizon in the WorkFlow Manager's Horizon Data window, the data values can represent the distance from the reference surface that is either "above" or "below" the intermediate horizon (as indicated on the Reference Horizon Location option menu; the third option "above and below" is discussed next). Positive values represent a vertical distance from the reference horizon. The larger the value, the farther from the reference horizon. Zero or negative values indicate an intersection of the intermediate and the reference horizon.

NOTE: In the Geologic Structure Builder interface, the same selections for intermediate surfaces are found on the Intermediate Horizon window and the reference horizon is set to Top (equivalent to Above in the WorkFlow Manager) or Bottom (equivalent to Below in the WorkFlow Manager).

In the WorkFlow Manager's Horizon Data window, when the Reference Horizon Location is set to Above & Below, a constant value is interpreted as a percentage distance between the two horizons. Values of 0 to 100 represent percentages of 0% to 100%, with 0% representing the below horizon and 100% representing the above horizon. A constant that represents a percentage between the two reference horizons is specified by selecting the %CST icon button and choosing a value along the Proportional Value slider. In the 5.1 version of the WorkFlow Manager (due out October '99), the option of using a 2D grid as a percentage grid is also supported.

NOTE: In the Geologic Structure Builder interface, the same selections for intermediate surfaces are found on the Intermediate Horizon window and the reference horizon is set to Both (equivalent to Above & Below in the WorkFlow Manager). The option of using a 2D grid as a percentage grid is also currently supported in this program.

When an Above or Below relationship to the reference horizon is specified, the horizon gridder handles thickness data (constants or 2D grids) by adding or subtracting these to or from the reference horizon in faulted space at the end of the horizon gridding process. (Remember that the actual horizon gridding is performed in a geometrically restored unfaulted space.) By using the thickness data at the end of the process, the result is that priority is given to the thickness data rather than honoring the fault bounding polygon (which circumscribes a dying fault) in cases where these two pieces of information are in opposition (the extent of a dying fault is initially calculated in the unfaulted space).

It should also be noted that the operations involving thickness constants and grids are isochore operations in that thickness is added in the vertical direction.

Special Techniques for Working with Thickness Data

While the general process of using thickness data in the WorkFlow Manager and Geologic Structure Builder is very straightforward, there are some special caveats and techniques to handle special situations.

1. Before using thickness grids for horizon gridding, it is a good idea to examine them and generate statistics on them. If the values in the grid are all positive, you will see a result similar to the following:

Some thickness grids, however, describe horizons that intersect with the reference horizon, such as sand bodies, etc. In these cases, it is important to ensure that the thickness grids have not been "zeroed out." That is, some thickness grids are produced that have all zeros in the areas where there is no positive thickness. These zeros are fine for making thickness maps, but thickness grids are used to create horizons: the areas where the thickness grid is zero will become co-planar to the reference surface. Co-planar surfaces should be avoided when making Geologic Structure Builder (or WorkFlow Manager) models because of the difficulty in determining an exact line of intersection between these two identical surfaces. Ideally, grids should be created that have positive values where there is thickness and negative values where there is no thickness. If a contour map of this ideal grid is made, the zero contour line should define the line of intersection with the reference horizon. (Normal minimum tension or isopach gridding can be used to create 2D grids from thickness data).

In the example below, both the intermediate horizon and the reference horizon were specified as depositional. With such a configuration, the dashed lines indicate the negative portions of a thickness grid that are eliminated when the model is constructed.

2. Building horizons with thickness grids above a reference horizon is quite simple and the basic rules of how the surfaces are used (depositional, unconformity, and erosional) all work the way one would expect. Often a problem arises, however, when horizons are built using thickness grids specified as below the reference horizon; the construction of the horizon sequence requires some care.

If you are building horizons with thickness grids below the reference horizon and the thickness grids contain only positive values, the horizon sequence should not require any modifications.

In this example, all the horizons are depositional and exist in the sequence file in the following order:
 

Horizon Name	Geologic Operation
Reference Horizon	Deposition
Intermediate Horizon A	Deposition
Intermediate Horizon B	Deposition

After the horizons are gridded, the model building modules assemble the model from the bottom up by depositing Intermediate Horizon B first, then Intermediate Horizon A next, and finally Reference Horizon. Following the rules for depositional surfaces, the horizons only deposit where no other layer currently exists.

In this second example, the thickness grids for the intermediate horizons pinch out against the reference horizon. What we want to see is this:

In this particular example, Intermediate Horizon A is created with a thickness grid measured from the Reference Horizon. Intermediate Horizon B is created with a thickness grid measured from Intermediate Horizon A. It is fine to have an intermediate horizon reference another intermediate horizon as long at no circular dependencies are created (i.e., two intermediate surfaces are not allowed to reference one another).

It might be tempting to use the same sequence as in the above case:
 

Horizon Name	Geologic Operation
Reference Horizon	Deposition
Intermediate Horizon A	Deposition
Intermediate Horizon B	Deposition

But such a geologic sequence would produce the following model:

This model occurs because, according to the rules of depositional surfaces, Intermediate Horizon B is laid down first, then the other two horizons can only deposit where Intermediate Horizon B did not previously exist. In order to achieve the desired model, it is necessary to change the operations of Intermediate Horizon A and Reference Horizon to unconformity (which allows the upper surfaces to both "erode" and "deposit"):
 

Horizon Name	Geologic Operation
Reference Horizon	Unconformity
Intermediate Horizon A	Unconformity
Intermediate Horizon B	Deposition

This sequence of geologic operations allows the Intermediate Horizon A and Reference Horizon surfaces to cut into the lower horizons in order to get the desired surface interaction.

3. Finally, when the Above & Below selection is made and a 2D grid is specified, the values in that grid are considered as percentages between the top and bottom reference horizons (referenced from the bottom grid). If the new horizon lies between the two reference horizon, values in the grid will typically be between 0 and 100. Where the values are below 0 or above 100, a pinchout situation is created. There is no inherent problem in this situation; however, you may need to change the geologic operations as described in the above situation.

Creating Thickness Data

Some modelers wish to create their own thickness grids and modify them before putting them back into horizon gridding. In the situation where a reference horizon has been calculated using horizon gridding and a Z data file for a intermediate horizon is available, it can be difficult to get the thickness between the faulted reference horizon and the scattered data points of the intermediate horizon. To do this, a series of scripts is now available. Information about downloading these scripts is found at the end of this article.

The goal of these scripts is to calculate positive thickness values between a horizon found in a sequence file and a scattered data file. Here are the steps:

a. The initial menu (evthickness) takes as input a sequence file with previously built horizons.

b. The second menu (produced when Continue is selected on the first menu) presents the following selections:

i. Reference Horizon - This is the horizon from which thickness to a data set will be measured.

ii. Target Horizon Data file - This is the data file to which thickness is measured from the reference horizon. Field selection is for the Z-field.

iii. Location of Target Horizon - In the end, the result we want is positive thickness from the reference horizon to the Z-values of the target data set. To calculate these values and to allow for the possibility that at some points the target data may "intersect" the reference horizon, it is necessary to know if the target data set is "above" or "below" the reference horizon. If Above is selected, any thickness calculated where the target data set is ABOVE the reference horizon will be positive. If any point in the target data set is below the reference horizon, it will be marked with a negative thickness value (and vice versa for the Below setting).

iv. Output Data File - This is the name of the output file where the following fields are added:

BAKINT - contains the back interpolated value of the reference horizon at the X,Y locations found in target data set

DIFF - contains the difference between reference horizon and target data set Z-values

THICKNESS - contains the difference between the reference horizon and target data set Z-values taking into account whether the target horizon is ABOVE or BELOW the reference horizon

c. The method of calculation:

i. First the target data set is sorted according to fault blocks to ensure that during the back interpolation the points are back interpolated to grid in the correct fault block. Since the sorting is done referencing the sequence file, the program ev_fbsort is used so that no points in the target data file are thrown out because of fault tolerance values.

ii. The data file is separated into data files that have only points for a particular fault block in them.

iii. Then a back interpolation is done at each target point location in a particular fault block to the reference horizon grid in that fault block.

Note that in more horizontal faults, the target point may be back interpolated to a portion of the reference horizon grid that is not visible in the model, as illustrated in the figure below:

Point M is found in fault block 1, so it will be back interpolated to the Horizon A grid that is associated with fault block 1. In this particular example, point M will be back interpolated to the part of the Horizon A grid that is not visible in fault block 1 in the model. It is not back interpolated to the Horizon A grid in fault block 2 that is most immediately above it (nor would you want it to be).

The following shareware scripts can be downloaded.

evthickness
evthickness2
evthickness.sh

These can be downloaded, given execute permissions:

chmod 666 evthickness evthickness2 evthickness.sh

and then put into the DGI Gifts bin:

cp evthickness $DGIHOME/ev5/dgi_gifts
cp evthickness2 $DGIHOME/ev5/dgi_gifts
cp evthickness.sh $DGIHOME/ev5/dgi_gifts

The program is accessed by typing the following;

evthickness