Gradient surveys, which are more formerly known as "static pressure gradient surveys", are a very accurate and straight-forward way to determine the fluid level in a well. This can become a very helpful data point when analyzing a well test, particularly a drawdown/buildup test. A drawdown/buildup test is performed by flowing a well at a specific rate and/or pressure, and then shutting the well in to monitor the pressure during the "buildup", and this is typically done with bottomhole pressure gauges in the well. This test can reveal all kinds of secrets about the reservoir.
Now, because you are flowing the well, often at large flowrates, there is the potential that you could bring reservoir fluid into the wellbore. This fluid influx can be an extremely valuable data point. This is where gradient surveys come in. It is not good enough to only perform a gradient survey AFTER the test, because you don't know what the starting fluid level was before the test. There could have been fluid downhole before the test, or the wellbore could have been dry. The only way to know is to run a survey before and after, so that you can measure the net change in the wellbore during the test.
If you want to learn how to run a gradient survey, check out our Well Insights topic here.