An ideal world for an exploration geologist would be one like this 3D model of subsurface strata, a world in which the geologist could move freely into and around the earth's subsurface, looking for oil and gas. However, they like the rest of us, are stuck at the earth's surface most of which is covered by water. Thus, in order to find oil and gas, geologists first rely on geophysicists to collect and process indirect images of subsurface earth properties. The broadest of these images are provided by satellites, which among other things, can measure subtle changes or anomaly in the Earth's gravity and magnetic fields, caused by sub surface variations in rock type and structure. This for example, is a satellite derived gravity anomaly map. The bright red to purple regions reflect sub surface features that in some instances might trap oil or gas. Once a subsurface feature of interest has been identified, geophysicists will conduct more detailed mapping over it using energy or seismic waves. This is done in marine settings with a survey ship equipped with energy sources, known as guns, that repeatedly fire pressure waves into the water column. Over land, the mapping is done with trucks specially equipped with energy sources on their undersides in the form of plates that vibrate the ground beneath the trucks. In both cases the energy waves produced by the source travel down into the sub surface reflecting off interfaces where rock type properties change. These reflections are then recorded by rays of pressure sensors or geoforms that extend away from the seismic source. The images produced by seismic reflection mapping differ from what the rock type actually looks like. What the geologist is after is understanding where potential reservoirs for oil and gas might be. Which, in this cross-section of sedimentary strata, would be the sandy white beds. The seismic reflection profile of the strata looks like this. You can make out the bedded nature of the strata, but in this image the bedding is much coarser and often does not correspond with actual bedding in the strata. Furthermore, there is no apparent indication of where the sand beds are. While the geologist skilled in interpreting seismic data could already begin to develop a model of where the sands are, their model will be much more accurate if there is an existing well that the seismic profile crosses. Whether the well produced oil or gas, or came up dry, chances are that it was logged, meaning that one or more geophysical sensors were lowered down and then raised back up the drill hole, recording the surrounding rock types. And in some instances there might even be core samples of the actual rock. Note, though, that both the well logs and any core samples are for a small lateral area, while the seismic profile can extend for tens of kilometers. Therefore, the geologist will supplement the data he has access to with experience, studying the makeup and geometry of analogous sedimentary systems exposed at the Earth's surface. These range from modern river and coastal environments to ancient rocks originally formed in a similar setting to their exploration area, but which now outcrop above ground. Given this collection of information, the geologist can develop a detailed model of the subsurface feature structure and rock composition, and make an educated guess as to the location of potential oil and gas reservoirs.