Any second-level command in block > OBSERVATION
Subtracts the value calculated at the first calibration time from the values calculated at subsequent calibration times, allowing the use of temporal changes rather than absolute values as calibration data. The first calibration point by definition has a residual of zero and is thus not included in the objective function and statistical analyses. Alternatively, a reference value for the first calibration point can be provided through command >>>> REFERENCE, in which case the difference to this reference value is calculated, and the first calibration point is included in the analysis.
Note that the measured data are expected to reflect relative changes to the initial value (rather than absolute values), which can be accomplished by subtracting the value measured at the first calibration time using command >>>> SHIFT.
This keyword cannot be combined with third-level keywords PROFILE, SECTION, or MAP, i.e., it can only be used with time-series data, not spatial data.
(pumping starts at t=3600)
>> TIMES: 50 LOGARITHMICALLY SPACED
>>> ELEMENT: WEL_1
>>>> ANNOTATION : DRAWDOWN
>>>> REFERENCE P. : 1.4875E5 (dP(t) = P(t) - Pref)
>>>> SHIFT Pmeas : -1.4875E5 (pressure measured at t=3600)
>>>> DATA on FILE : Pmeas.dat