Talk:Reflection seismology

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Re: The equation for the Transmission coefficient: I have checked up on the equations posted on this page, and I am sorry to conclude that they have not been correct for quite som time. The equation is T + R = 1. This does not present a problem regarding the aforementioned reflection coefficient -1 at the sea-air interface (from below). It is true that the incoming energy is almost entirely reflected. It is, however, important to remember that the amount of energy in the reflected and the transmitted waves does not only depend on the amplitude of the waves, but also on the geophysical parameters of the respective materials. The energy of a pressure wave in material 1 is given as E1 = α1ρ1ω2A2, where α1 is the P-wave velocity in the material, ρ1 is the density, ω is the angular frequency and A is the pressure wave amplitude. Assuming energy conservation between the incoming wave and the reflected and transmitted wave it can be shown that the fraction of reflected energy is ER = R2 while the fraction of transmitted energy is E_T = \frac{Z_1}{Z_0}T^2. Considering the extremely small acoustic impedance of air it is no surprise that the transmitted energy is negligible in the case of pressure wave reflection in the air water interface. Geologos (talk) 23:28, 25 November 2007 (UTC)

I have reverted the edits of Geologos because the equations were correct as they were. The precise form of the transmission coefficient depends on whether it is for pressure or particle velocity. The versions quoted are for pressure. Thunderbird2 (talk) 11:26, 26 November 2007 (UTC)
I apoligize for altering the content of the page so impolitely, but I believe that the section on reflection and refraction coefficients need altering to avoid misunderstandings of this sort. It is only at the second formula (the transmission coefficient) that it is told which parameter is actually dealt with, and it is put in parenthesis "(the ratio of transmitted to incident pressure)". I therefore suggest that it is clarified to avoid this potential ambiguity. A good name could be pressure reflection and transmission coefficients. A further possibility is to present both pressure and displacement R/T coefficients under separate headers and present their respective relations to the reflected and transmitted energy (To show how surprisingly little energy is actually reflected at real lithological interfaces). Geologos (talk) 20:32, 26 November 2007 (UTC)
Hi Geologos. There's no need to apologise. You corrected what you honestly believed to be an error, and I did the same, only in reverse. That's how Wikipedia works (see wp:bold) :-) Thunderbird2 (talk) 07:09, 27 November 2007 (UTC)

Re: the equation for transmission coefficient: it is T = 1 + R. For those of you familiar with marine seismic data, the reflection coefficient at the ocean surface is, for all practical purposes, -1, and (practically) no energy is transmitted to the air, the impedance contrast being so great. Thus, the equation for T cited above gives the correct value for T, namely 0. Note that R = 1 occurs in the opposite situation (air to water), as air has an impedance of effectively 0 compared to water or rock. This gives T = 2. I think the question is "what exactly is the amplitude we are measuring"? I will try to clarify that. Gwimpey 03:57, Jan 28, 2005 (UTC)

You are right, it depends on what you are measuring. If you measure pressure, then pressure is zero on the free surface, hence reflected wave must have opposite sign to the incident wave to yield that zero, so that R=-1. Thus for pressure T=0, as you say. Conversely, at a very hard surface, displacement is zero, while pressure has a maximum. Thus for pressure we have R=1, the resultant pressure being twice that of the incident wave. Since pressure is continuous across the interface, this creates the wave in the second medium which has twice the pressure amplitude of the incident wave. Thus T=2 is correct. It is reverse for displacement.

Now I have an entirely different question. When talking about environmental impact, shouldn't we mention that in the past explosive sources were widely used offshore, but have been banned for some years now? Unfortunately, I do not know legal or historical aspects of this. Can anyone comment?BorisG 13:47, 13 November 2005 (UTC)

That would be a good idea. As with many resource activities, impact is much less today than in the past. But finding the specifics may be hard. I don't know where this would have been documented. Gwimpey 18:07, 28 December 2005 (UTC)

Contents

[edit] Direct Hydrocarbon Indicators

'Seismic technology does not allow the direct detection of hydrocarbons, but it is used by geologists and geophysicists who interpret the data to distinguish the presence of structural traps that could potentially contain hydrocarbons' This sentence is not strictly true. Firstly, in seismic data processing we attempt to search for and enhance images artefacts called 'direct hydrocarbon indicators'. usually seen as a 'bright spot' or 'dim spot' on a stcaked seismic section. Secondly, seismic data processing has long since ceased to be solely about structural interpretation. Nowadays a significant amount of effort is made to search for various amplitude anomalies to act as lithology and property (egs. porosity, permeability etc) identifiers. If no-one objects I will consider a re-write over the next few days.

You are absolutely correct that an independent branch of seismic technology has emerged using attributes of seismic data to directly identify presence of hydrocarbons. Go ahead and edit - I will contribute if required.

My comments (not'draft'): Bright spots, flat-spots, etc have been recognised (and drilled, sometimes successfully) for decades but direct targeting really took off in the late 90's with emergence of relatively inexpensive 3D data and the high quality of deep-water data eg West Africa. The emergence of more powerful computers also made analysis of pre-stack data practical eg Amplitude-Versus-Offset or 'AVO' which shows variation in amplitude with offset in the pre-stack gather (important to note that DHI's are often pre-stack attributes). We should distinguish between structural/stratigraphic attributes (basics), lithology (poro-perm) and DHI (fluids).--Okram999 (talk) 17:26, 4 January 2008 (UTC)

[edit] Spelling correction

Made on minor spelling correction while reading this article. Shanneranner 20:33, 22 September 2006 (UTC)

[edit] Introduction defining the topic

“Reflection seismology or seismic reflection is a branch of seismology that uses reflected seismic waves to produce images of the Earth's subsurface.” When reading this article, it gives more an impression of describing reflection seismology as a method – with applications, to collect subsurface information (data). To interpret the data, scientists make use of seismology (“the scientific study of earthquakes and the movement of waves”) and Exploration geophysics among other disciplines. --Roarjo 10:11, 12 November 2006 (UTC)

Would it be better to say that "Reflection seismology is an application of seismology that uses..." rather than a branch? Or maybe "Reflection seismology is a method of exploration geophysics that uses seismology and reflected seismic waves to produce..."? I will try a variation on one of these and see how it sounds. GeoGreg 20:49, 31 January 2007 (UTC)
After some experimenting, I decided that producing "images" is not correct; reflection seismology estimates properties, a more general statement that is true of all geophysical methods. But anyone feel free to hack away. GeoGreg 20:57, 31 January 2007 (UTC)

[edit] Applications

I'm thinking the application part could be updated to include some of the newer applications of reflection seismic, such as reservoir monitoring through time-lapse and characterisation through inversion. I don't know much about inversion, but I could try to write something about time-lapse. Any thoughts on this? Asbjørn L. Johansen 14:43, 7 August 2007 (UTC)