Excluding Data Points from Analysis

Excluding Data Points From Analysis

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There are several methods that Super Slug can use to exclude certain data points from the slug test analysis. Several definitions used by Super Slug are important to remember when excluding data points.

Drawdown is the measured elevation of water in the well throughout the test, relative to a fixed datum. This value is entered into Super Slug.

Head is the difference between the static water level and the drawdown.

Head = |static water level - drawdown|

Head ratio is the ratio of head of a given trial to the initial head (Ho) measured during the test.

Initial Head (Ho) is the head at time equal to 0. This is not necessarily the first time value in the data set. When data is first entered into Super Slug, Super Slug looks for the largest value of head and makes that value Ho. Time values are then adjusted so that time is equal to zero at Ho.

There are two ways the user can change Ho.

1. From Options | Graph Options, the value of First Trial indicates the trial used to calculate Ho.

2. From Options | Graph Options, check the box Recalculate New Head Ratio with Each Adjustment of Data. Now when early values are added or excluded from the best fit line calculations, the first value will be Ho. All time values will be adjusted so that Ho occurs at time equal to 0.

Excluding Data with the Graphical Methods.

When either the Hvorslev graphical method, the Bouwer and Rice graphical method, or the Cooper Type Curve method is selected from the View menu, one or two pairs of arrow buttons (shown below) will appear at the bottom of the screen.

$image\BUTTONS.gif$

Two pairs of buttons will appear for the Bouwer and Rice graphical method or the Hvorslev graphical method when the line is not weighted to the first point. One pair will be in the bottom left corner of the screen, and one pair in the bottom right corner of the screen.

One pair of buttons will appear for the Hvorslev graphical method when the best fit line is weighted to the first point, and head ratio is non recalculated with each change in data.

These buttons are used to increase or decrease the number of points in the best fit analysis.

Adding or removing data points from the end of the test:

The buttons in the bottom right corner of the screen add or subtract data from the end of the test. The right arrow increases the number of trials at the end of the test. The left arrow decreases the number of trials at the end of the test. For example, if there are too many measurements at the end of the test, pressing the left arrow will decrease the number of measurements included in the analysis one at a time. The measurements are not removed from memory, but are only excluded from calculations in determining the best fit of the line. Data points not included in the analysis will be displayed with the symbol specified for excluded data. Data points included in the analysis will be displayed with the symbol specified for included data. Use the right arrow to restore the points to the analysis.

From Options | Graph Options, check Show Late Values not Included in Calculations if you would like these late values to be displayed as the graph symbol for excluded data. If this box is not checked, the graph will be readjusted to fit the screen with each addition or removal of a data point.

Adding or removing data points from the beginning of the test:

The left and right arrow buttons in the bottom left of the screen add and remove data points respectively, from the beginning of the analysis. If the option to display excluded data points at the beginning of the test is on, data points not included in the analysis will be displayed with the symbol specified for excluded data points. Data points included with the analysis will be displayed with the symbol specified for included data points. If this option is off, the data points not included in the analysis at the beginning of the test will not be displayed at all.

Data can not be added or removed from the beginning of the test if the Hvorslev best fit line is weighted to the first point, and head is not being recalculated with each data adjustment..

Adding or Removing Data for the Automatic Analysis Methods

If data has been excluded from the analysis during one of the graphical procedures, it will also be excluded in the automatic analysis methods. Before using an automatic method, a graphical method should be selected to exclude undesirable data.

Excluding Data for the Cooper Type Curve method

Excluding data has no effect on the Cooper type curve method because data is matched to the type curve by the user.

Excluding Data and Recalculating Head Ratio

When data is first entered into Super Slug, Super Slug looks for the maximum drawdown value and uses it to calculate head ratio. In some tests, there may be one or several anomalously high drawdown values at the beginning of the test. Using the points to calculate head ratio may cause invalid results. From Options | Graph Options, the user can specify which trial should be used to calculate head ratio. This value is specified as the First Trial in the Graph Options dialog.

Time will be adjusted to zero for the trial equal to First Trial.

Once a change is made to First Trial, use View | Data to see how this change has affected the calculation of head ratio and adjusted time.

The value of First Trial also changes when data adjustments are made with the arrow buttons, and Recalculate New Head Ratio with each Adjustment of Data is checked from Options | Graph Options.

To reset data to the original values before user modification, select Options | Reset Data Exclusions.