Contents preface (VII) introduction 1—37

4.6.4. Test Drilling
All geophysical exploration methods – surface as well as subsurface – and remote sensing methods are quicker and economic but yield results which may be interpreted in more than one way. Test drilling, however, provides the most positive information about the subsurface conditions. Test drilling can predict the true geohydrologic character of subsurface formations by drilling through them, obtaining samples, recording geologic logs, and conducting aquifer tests (2). The following data are usually obtained in test drilling:
(i) Identification, location, and elevation of the site of each hole, (ii) Geologic log of the strata penetrated,

(iii) Representative samples of strata penetrated,

(iv) Depth to static water level in each permeable stratum, and
(v) Water quality samples and aquifer test data from water-bearing formations.
Rotary drilling and cable tool drilling are commonly used methods of drilling wells. The rotary drilling method is fast and is the most economical method of drilling wells in unconsolidated formations. However, accurate logging of cuttings is relatively difficult and the depth of water level cannot be predicted accurately unless electric logs have been taken for this purpose. Care should be taken in distinguishing between valid cuttings carried in mud suspension and the cuttings which have been delayed in reaching the surface. Cable tool drilling is suitable for drilling to moderate depths. Sampling of geologic materials is relatively more accurate and presents fewer difficulties. Cable tool drilling is, however, a more time-consuming method.
A good lithologic well log presents variation in the geohydrologic character of subsurface formation with depth and also the depth of the water table.