Fréchet kernels, which can be regarded as a measure for how an anomaly in T influences the drawdown at a particular observation well, are used to examine the sensitivity of the drawdown to a heterogeneous transmissivity distribution. The corresponding equations have already been discussed in previous chapters and are state again briefly. For pumping a well at a constant rate in an infinite domain, the Fréchet kernel for transmissivity is given
Sensitivity of the Drawdown 67
by Equation (4.1) [Knight and Kluitenberg , 2005]
FT(x, x′, t) = −q(r2− a2/4)
with distances relating to a Cartesian coordinate system as shown in Figure 4.2a, diffusivity D being defined by D = T0/S0, and, K1the modified Bessel function of the second kind, of order one. For a BCH-domain, the corresponding Fréchet kernel is given by Equation (4.3)
FTB(x, x′, t) = − q
with distances according to Figure 4.2b. In order to illustrate the sensitivity of the draw-down with respect to heterogeneity, this section compares the Fréchet kernel in BCH-domains FTB to its counterpart in infinite domains FT.
The influence of particular anomalies in T on the drawdown at the pumping well (OW=PW) is exemplified in Figure 5.4. Each plot depicts FTB for three selected anomalies at equal distances from OW=PW but in different directions, i.e., at different distances from the IW. In addition, Figure 5.4 shows the corresponding Fréchet kernels for infinite domains FT, which are solely a function of the distance between OW=PW and the anomaly.
From Figure 5.4, it can be seen that for early pumping times FTB equals FT in an infinite
Figure 5.4: Fréchet kernel for infinite domains FT, compared to the Fréchet kernel for BCH-domains FTB, for selected anomalies at equal distances of a) L/2 and b) 2L from OW=PW. Geometry plots illustrate locations of considered anomalies, with the BCH located East of the PW.
domain. The farther an anomaly in T from OW=PW and the closer it is to the boundary, the shorter this period. As pumping continues, FTB starts to deviate from FT, with the deviation occurring later the farther an anomaly form the BCH or the IW, respectively.
Furthermore, by considering the given geometry with the BCH East of the PW at x = 0 (Figure 4.2b), FTB is monotonically increasing if the anomaly is located at x > −L. For anomalies at x < −L, FTB peaks at a particular time, characteristic for every anomaly, before decreasing to a constant value under steady-state conditions.
Figures 5.5 to 5.7 examine the spatial distribution of FT in infinite domains (plots a to c) and FTB in BCH-domains (plots d to f), for observations at different wells at selected times, including steady-state conditions. Additionally, Figures 5.5 to 5.7 show the ratio FTB/FT (plots g to i) to illustrate the area where deviations between FT and FTB are most pronounced. In general, Figures 5.5 to 5.7 show that for very early pumping times (t=0.1), the magnitudes of FT and FTB are equal, indicated by a ratio of FTB/FT=1. That is, the drawdown at a particular observation is not affected by the BCH during very early times, which was already concluded from the discussion on γbch in Section 5.2. With continuous pumping FTB in the vicinity of the BCH generally exceeds FT.
For a single-well pumping test configuration, Figure 5.5 shows that FT and FTB are most pronounced in the vicinity of OW=PW. Regarding the ratio FTB/FT, anomalies in T located between OW=PW and the BCH gain pronounced impact on the drawdown in the BCH-domain as pumping continuous (Figure 5.5i). Anomalies in the rest of the domain show a weaker impact on the drawdown if the domain is bounded. However, since the absolute magnitudes of FTB within the sub-domain of pronounced sensitivity is considerably smaller than FTB in the vicinity of OW=PW, the impact of the corresponding anomalies on the drawdown is rather weak. Thus, for single-well pumping tests in BCH-domains, the drawdown is most sensitive to the local transmissivity in the direct vicinity of the well TPW, even under steady-state conditions.
Figure 5.6 shows the kernels and their ratio for an OW located between the PW and the BCH. In the infinite domain, FT is most pronounced at the wells, though much smaller than for the case of OW=PW. The spatial distribution of the kernels is characterized by a circle which intersects both wells. Inside this circle the kernels are positive, and negative outside. For very early times FT and FTB are equal, with FTB/FT = 1. As pumping continues, FTB starts to exceed FT in a sub-domain between the wells, the BCH and the mentioned circle. Interestingly, as t → ∞, FTB in the close vicinity of the OW even exceeds FTB in the vicinity of the PW (Figure 5.6f). That is, the local transmissivity in the vicinity of the OW has a more pronounced impact on the corresponding drawdown than TPW. This is contrary to FT in infinite domains, where the local transmissivities in the vicinity of the OW and the PW influence the drawdown at the OW by equal parts (Figure 5.6c).
Furthermore, the magnitude of FTB exceeds FT significantly in a quite narrow sub-domain
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between the OW, the PW and the BCH (Figure 5.6i). Since the magnitude of FTB in this sub-domain does not deviate much from FTB in the vicinity of the PW, anomalies in T located within this sub-domain have pronounced influence on the drawdown at the OW.
Moreover, the sensitivity for anomalies behind the wells is generally small. For anomalies on the circle, where FTB and FT change from positive to negative, the ratio FTB/FT also deviates significantly from 1. However, as FTB for these anomalies is generally small, their impact on the drawdown is negligible.
Figure 5.5: Comparison of the spatial distribution of the Fréchet kernels in infinite domains FT(a to c) and bounded domains FTB (d to f), for a single-well pumping test with OW=PW, at different times. The Fréchet kernels are depicted in terms of log10|F |. Plots g to i show the ratio FTB/FT. The BCH is East of the PW, which is indicated by a circle. Distances are normalized by L.
Figure 5.7 now shows the spatial distribution of the kernels and their ratio for an OW located behind the PW, opposite of the BCH. Considering the unbounded case, the local transmissivities in the vicinity of the wells influence the drawdown at the OW again by equal parts (Figure 5.7a to c). With regard to the bounded case, the PW is now closer to the BCH. Thus, FTB in the vicinity of the PW exceeds FTB in the vicinity of the OW as pumping continues, yet just slightly (Figure 5.7f). The ratio FTB/FT shows that deviations
Figure 5.6: Comparison of the spatial distribution of the Fréchet kernels in infinite domains FT(a to c) and bounded domains FTB (d to f), for observations at an OW located between the PW and the BCH, at different times. The Fréchet kernels are depicted in terms of log10|F |. Plots g to i show the ratio FTB/FT. The BCH is East of the PW, which is indicated a circle. The location of the OW is marked by a cross. Distances are normalized by L.
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between both kernels are again most pronounced in a sub-domain between the OW, the PW and the BCH, though, deviations are smaller than for the case of an OW located close to the BCH. Moreover, the corresponding sub-domain is considerably larger if the OW is located behind the PW. For anomalies located behind the OW, FTB/FT < 1. Consequently, the drawdown at the OW is most sensitive to the local transmissivities at the wells, at which the influence of TPW slightly prevails, and, some average of the point transmissivities in
Figure 5.7: Comparison of the spatial distribution of the Fréchet kernels in infinite domains FT
(a to c) and bounded domains FTB (d to f), for observations at an OW located behind the PW, opposite of the BCH, at different times. Fréchet kernels are depicted in terms of log10|F |. Plots g to i show the ratio FTB/FT. The BCH is East of the PW, with a circle indicating PW. The location of the OW is marked by a cross. Distances are normalized by L.
the sub-domain where FTB/FT> 1.
In summary, the above discussion illustrates some important characteristics of BCH-domains: (1) During early pumping times the existence of the BCH does not affect the drawdown at an observation well. (2) After the BCH is affecting, the drawdown at an OW is basically sensitive to the local transmissivities at the OW and the PW, and, to a sub-domain located between the wells and the BCH. With regard to the local transmissivities at the wells, the transmissivity at the well which is closest to the BCH prevails. Regarding the sub-domain of pronounced sensitivity between the wells and the BCH, the closer an OW to BCH, the more narrow this sub-domain becomes, and, the more pronounced the impact of this sub-domain on the drawdown at the OW. (3) Under steady-state conditions, the sensitivity of the drawdown at the OW to point transmissivities located behind the OW and the PW, opposite of the BCH, is almost negligible.