Physical property measured in seimic surveys?

[marcoskids]

Does anyone know what the physical property measured in seismic surveys is?

Seismic surveys used in searching for hydrocarbons to be more specific. I was thinking it would be the rocks density but I'm not sure, can anyone help please? :)

 

[davenn]

welcome to the forums

quick response .. others may add more ;)

primarily density. different layers have varying densities
so the velocity will vary through each layer according to density
from that the thickness and composition of the layers can be determined.

Dave

 

[billiards]

Seismic surveys measure the time it takes for energy to travel through the rocks, but also the size and shape of the wavelet.

The time it takes for energy to travel through the rock tells you something about the velocity of the wave through rock. This is not simply a function of density, but also of other elastic parameters. In the simple isotropic case there are only two elastic parameters you need to worry about, however in more complex cases there are more elastic parameters you need to worry about and in these cases the medium is anisotropic. Anistropy means that the velocity depends on the direction of particle motion of the associated wave (whether that be P waves or S waves). Another major complication is the presence of heterogeneity -- waves in the Earth don't usually simply travel through the same rock, but through multiple different rock types -- so what you actually measure is a combination of lots of different rocks.

The size (amplitude) of the wavelet can give you information if you know what it is you are looking at. For example if you are looking at a reflection, the size of the wavelet is determined by the acoustic impedance contrast at the interface which caused the reflection. The acoustic impedance for a layer is simply the velocity times the density, the impedance contrast difference is the change in this property from one layer to the next -- big changes reflect more energy. The phase of the wavelet can give you information of the dispersion properties of the medium. This is an anelastic effect whereby high frequencies are attenuated more readily than low frequencies. However, dispersion can also be caused by the effect that low frequency energy has a wider Fresnel zone than high frequency energy and so samples different bits of rock which will have different velocities and therefore will cause a phase shift (shape change) in the wavelet. All of these difficulties are compounded by the fact of interference, whereby the signal is contaminated not only by noise but by other parts of the wavefield itself. To some extent these difficulties can be handled by array processing, but the tricks used to decompose the wavefield are usually based on highly abstract geometric considerations and are often not particularly well grounded in the underlying physics. This second paragraph on wavelet shape is by far the more complex of the two paragraphs and to a large extent previous workers in the oil industry have only worried about the travel time of the seismic wave.

So in summary: what you actually measure in the oil industry is the wave propagation (of a controlled explosion) in space and time. This information is a complex function of elastic parameters, aneslatic parameters, and density (which in principle could be inverted for) -- in reality these are kind of bulked together and the velocity is inverted for. If you know the velocity of the medium and the travel time of the ray paths taken through the medium then it is actually possible to extract the depth of the reflectors -- this is increbibly useful for the geologist who can tell the driller exactly where he wants the well in 3D space. Detailed studies which make the right kind of measurements can in principle unravel parameters of even more interest to the oil tycoon, such as temperature, composition, porosity, pore fluid properties (is it oil, gas, or water?). However these things don't simply drop out of the seismic data, a lot of work must go into extracting this information.