Most of the present knowledge about the subsurface of the Netherlands comes from boreholes drilled over the past hundred years. Since in most places in the country the subsurface is characterised by thick upper units of unconsolidated gravel, sand, clay and peat, we are routinely able to sample to depths of more than 100 metres, using relatively light drilling equipment. We have a large amount of sample material, including some from much greater depths that has been collected with heavier equipment. The borehole log provides a written record of samples from the drilled part of the subsurface in terms of successive layers of sediment and rock.
Drilling can be done in two ways: using muscle or machine. Depending on the composition of the soil and subsoil, we can use a hand drill to drill down to a depth of around twelve metres. Mechanical drilling can reach a depth of around 100 metres (bailer method) or several hundred metres (suction method). A single borehole can take several days to complete. Casing is commonly used to keep these deep boreholes from caving in. Boreholes may be fitted with filters to measure groundwater levels and groundwater quality.
DINOloket currently offers access to logs of boreholes chiefly penetrating the shallow subsurface, enabling detailed characterisation of layers to a maximum depth of about 500 metres.