From: DV (Avatar) 30/11/2001 21:56:57 Subject: re: ROCK LEGEND NOTES post id: 518862 How do they know where to look for… Gold? Gold itself makes up only a tiny fraction of the earth's crust, but there are places where it is concentrated due to hydrothermal activity. Gold is only soluble under very specific chemical and physical conditions: as the hydrothermal channel carries it up, it will eventually reach a point where the solubility of gold decreases sharply, and the gold is deposited. The gold itself does not have a very strong geophysical signature, but fortunately other there are other minerals, mostly metal sulphides, that are often deposited by the same hydrothermal activity, and these have a very strong magnetic and electromagnetic signature. Once a likely area has been found, a ground survey of outcrops and soil chemistry will give a better idea of the nature of the hydrothermal deposit, and may even show a high amount of gold in the soil. Additionally, gold is found in alluvial sediment deposits, like diamonds. Oil (especially under the oceans) The single most important tool in modern marine hydrocarbon exploration is the seismic survey. Seismic waves are basically sound waves; compressive and/or shear waves in materials. Basically, the notion behind the seismic survey is that the geology will interact with incident seismic waves through reflection, refraction, scattering and absorption, in accordance with well understood differential equations, and hence that by examining the seismic output, one can determine some things about the geology, particularly the location of interfaces between different layers of rock and the density and seismic velocities in those layers. For marine work, reflection is the most useful kind of interaction. In order to overcome noise and random near surface features, it is important to use a large number of ray paths per point in the subsurface. Suppose there is an 40000 km2 area of continental shelf that( for reasons of sedimentology and fold history or previous seismic work) seems like a likely area for hydrocarbon traps. A seismic vessel will traverse the entire area in lines separated by 1km, say. (I'll call the direction the boat travels the X-direction. At the back of the boat is a seismic source, basically a specialised airgun. The boat is also towing a set of streamers running in the X-direction. There might be around 12 streamers, each 5 km in length. These streamers might be separated by about 100 metres in the Y-direction. All along the parallel portions of the streamers are hydrophones separated by regular intervals (say, 25 metres). The streamers are actually about 8 metres beneath the water, and these hydrophones are basically like digital microphones designed to work underwater. The boat will slowly move along a single line in the X-direction, firing the airgun at regular space intervals (say, 50 metres). For each "shot", each hydrophrone records maybe 8 seconds of seismic response at a rate of 1000 samples per second, with usually 4 bytes per sample. The information is recorded directly onto tape aboard ship. The ship will then turn around and do the next line, etc. The tapes will be sent back to a processing centre. Using their knowledge of the appropriate seismic equations, along with various shortcuts and algorithmic tricks and assumptions, the geophysicists will use the data to form images of the subsurface layers, and to work out where hydrocarbons might be produced (like in shale layers), and where they might be trapped (like in faults and folds), or even produce an image of the interfaces between different fluids. The survey described above would record over 60000 gigabytes of raw data. You can probably see why the seismic exploration industry is the world's second largest user of IT (after the military).