在做抽水试验时,有时在试验孔中不能观测水位(如因孔径太小,铅钟太大),后者只能在离试验孔不远的副孔中观测。由于在试验孔周围的水力坡度很大,故虽两孔相距不远,但仍不能以副孔中的水位代替试验孔中的,象某些抽水给所做的那样。这样,就产生了对两孔中水位差的估计问题。对这问题的阐 During the pumping test, sometimes the water level can not be observed in the test hole (as the diameter is too small and the lead is too large), the latter can only be observed in the auxiliary hole not far from the test hole. Due to the large hydraulic gradient around the test hole, it is still not possible to replace the test hole with the water level in the secondary hole, as some pumps do. In this way, a problem of estimating the water level difference between the two holes arises. Explain this question