In heavily wooded areas such as Scandinavia or the North American Northeast, or deeply alluviated areas as in the Netherlands, fieldwalking is not always a practical method. Humus and fallen leaves in forested areas, silt on alluvial fans, or turf in built-up neighbourhoods may render near-surface artifacts and sites invisible even at short distances. In such instances, archaeological surveyors may concentrate on above-ground structures such as burial cairns, collapsed field walls and rock art panels, look for unnatural changes in the vegetation and landscape to decide what may be hidden under the vegetation, or survey by subsurface testing (SST). SSTs can consist of a series of shovel-test pits that penetrate the humus layer or turf or, where substantial later sediments may cover archaeological materials, series of auger or core holes. SSTs are much more costly than fieldwalking, and surveys by SST usually have very low probability of intersecting and detecting archaeological remains unless intensity (density of SSTs), and thus cost, is prohibitively high. The various Scandinavian sites and monuments registers mainly list above-ground monuments, not ploughed-out sites with scattered pottery. Narrowing it down Because of the high costs involved in some kinds of surveys, it is often helpful to use "predictive modelling" to narrow down the search for archaeological materials. This is particularly important for purposive surveys, but can also be used to guide sampling surveys by eliminating the need to survey areas where, for geological or other reasons, we can reasonably expect all ancient traces to be destroyed (e.g., by erosion) or far too deeply buried (e.g., by alluvium) to be detectable. Modern predictive models in archaeology employ Geographic Information Systems (GIS). Geophysical survey A geophysical survey is used for subsurface mapping of archaeological sites. In recent years, there have been great advances in this field, and it is becoming an increasingly useful and cost-effective tool in archaeology. Geophysical instruments can detect buried archaeological features when their electrical or magnetic properties contrast measurably with their surroundings. In some cases, individual artifacts, especially metal, may be detected as well. Readings taken in a systematic pattern become a dataset that can be rendered as image maps for interpretation. Survey results can be used to guide excavation and to give archaeologists insight into the patterning of non-excavated parts of the site. Unlike other archaeological methods, geophysical survey is not invasive or destructive. For this reason, it is often used where preservation (rather