Time, Adjusted Time, Drawdown, and Head
Time, Adjusted Time, Drawdown, and Head
See Also:
Oscillations in the Water Level
In many cases, the raw data recorded from an automatic measuring device is not suitable for slug test calculations. There are two main reasons for this:
1. The slug is not usually injected or withdrawn at the same instant the recorder reads zero time.
2. The height of the water column or depth to water may not be relative to a static water level of zero.
Super Slug may make modifications to the raw data in preparation of the calculations.
1. The time is adjusted so that the maximum displacement measured is at zero time. Slug test calculations will then begin with adjusted time = 0. Any drawdown values recorded before adjusted time = 0 will not be used in calculations.
2. The "head", or change in water level from the Static Water Level, is calculated based on a static water level value entered by the user. If the user does not enter a value for static water level, Super Slug will calculate static water level.
Here is an example of slug test data showing both the raw data and the transformed values used in the actual calculations.
Static Water Level = 21.50791 meters
Measured Adjusted Measured Head Head
Time Time Drawdown Change Ratio
(seconds) (seconds) (meters) (meters)
0 -0.19998 21.93859 0.43068 0.89771
0.19998 -0 21.98766 0.47976 1
0.40002 0.20004 21.96876 0.46086 0.96061
0.6 0.40002 21.90964 0.40173 0.83736
Measured Time -- This is the raw data time value recorded by the data logger. In this example, the first value is 0, however this is not necessary.
Adjusted Time -- This is the time adjusted so that t = 0 occurs during the maximum drawdown in the test. Note that in this example, Adjusted Time = Measured Time - 0.19998.
Measured Drawdown -- This is the raw data drawdown value recorded by the data logger. Note that in this case, the maximum value occurs at the Adjusted Time of 0. When the Measured Time is equal to zero, the slug has not been fully inserted, the test has not yet begun.
Head -- The head is the change in water level relative to Static Water Level, as a result of the slug test. If the Static Water Level is zero, Head and Measured Drawdown will be equal. Slug test results are actually calculated from the head, not the drawdown.
Head Ratio -- Head ratio is the ratio of the Head at some time = t since the beginning of the test, to the Head at adjusted time = 0. When Adjusted Time is zero, the Head Ratio will always be 1.
The First Trial
For the example above, the First Trial is 2. It is the second drawdown value that is the maximum, and therefore the second value of adjusted time is equal to zero. Head ratio will be calculated with the second drawdown value minus static water level equal to initial head. The First Trial can be changed from Options | Graph Options. For example, if the user changed First Trial to 3, the same data would look like this:
Static Water Level = 21.50791 meters
Measured Adjusted Measured Head Head
Time Time Drawdown Change Ratio
(seconds) (seconds) (meters) (meters)
0 -0.4002 21.93859 0.43068 0.93451
0.19998 -0.20004 21.98766 0.47976 1.0410
0.40002 0 21.96876 0.46086 1
0.6 0.2004 21.90964 0.40173 0.87169
Values occurring before trial 3 would not be included in any of the analyses.
Negative Drawdown or Head Values
Negative values of drawdown or head can occur if the data logger is set to record negative values, or if the water level in the well oscillates beyond the static water level. Here is data from a falling head test where the logger recorded negative values, and the water level oscillated past static water level. This data is from the Oscillating Water Level pre-loaded example file.
Static Water Level = 0.0
Measured Adjusted Measured Head Head
Time Time Drawdown Change Ratio
seconds seconds meters meters
0 0 0.147 0.147 1
0.498 0.498 0.0445 0.0445 0.303
0.994 0.994 0.0241 0.0241 0.164
1.5 1.5 0.00108 0.00108 0.00733
2 2 -0.00559 -0.00559 -0.0381
2.5 2.5 -0.0107 -0.0107 -0.0728
3 3 -0.0031 -0.0031 -0.0211
4 4 0.0011 0.0011 0.00747
4.5 4.5 -0.0011 -0.0011 -0.00747
4.99 4.99 0.0011 0.0011 0.00747
5.5 5.5 0.0011 0.0011 0.00747
6 6 0 0 0
6.5 6.5 0 0 0
7 7 -0.0011 -0.0011 -0.00747
7.5 7.5 0.0011 0.0011 0.00747