Reflection seismology: Difference between revisions

A [[Rayleigh wave]] typically propagates along a free surface of a solid, but the elastic constants and [[density]] of air are very low compared to those of rocks so the surface of the Earth is approximately a [[free surface]]. Low velocity, low frequency and high amplitude Rayleigh waves are frequently present on a seismic record and can obscure signal, degrading overall data quality. They are known within the industry as ‘Ground Roll’ and are an example of coherent noise that can be attenuated with a carefully designed seismic survey.<ref>{{cite web | website = [[Schlumberger]] Oilfield Glossary | title = Ground Roll | url = http://www.glossary.oilfield.slb.com/Display.cfm?Term=ground%20roll | access-date = 8 September 2013 | archive-date = 31 May 2012 | archive-url = https://web.archive.org/web/20120531161347/http://www.glossary.oilfield.slb.com/Display.cfm?Term=ground%20roll | url-status = dead }}</ref> The [[Scholte wave]] is similar to

An event on the seismic record that has incurred more than one reflection is called a ''multiple''. Multiples can be either short-path (peg-leg) or long-path, depending upon whether they interfere with primary reflections or not.<ref>{{cite web | website = [[Schlumberger]] Oifield Glossary | title = Multiples Reflection | url = http://www.glossary.oilfield.slb.com/Display.cfm?Term=multiple%20reflection | access-date = 8 September 2013 | archive-date = 2 June 2012 | archive-url = https://web.archive.org/web/20120602102742/http://www.glossary.oilfield.slb.com/Display.cfm?Term=multiple%20reflection | url-status = dead }}</ref><ref>{{cite journal | last = Pendrel | first = J. | title = Seismic Inversion—A Critical Tool in Reservoir Characterization | journal = Scandinavian Oil-Gas Magazine | issue= 5/6 | year = 2006 | pages = 19–22}}</ref>

*Near-surface applications – an application that aims to understand geology at depths of up to approximately 1&nbsp;km, typically used for [[Engineering geology|engineering]] and [[Environmental geology|environmental]] surveys, as well as [[coal]]<ref>{{Cite journal | doi=10.1190/1.1439738| title=Seismic surveys for coal exploration and mine planning| journal=The Leading Edge| volume=9| issue=4| pages=25–28| year=1990| last1=Gochioco| first1=Lawrence M.| bibcode=1990LeaEd...9...25G}}</ref> and [[mineral]] exploration.<ref>{{cite journal | last1 = Milkereit | first1 = B. | last2 = Eaton | first2 = D. | last3 = Salisbury | first3 = M. | last4 = Adam | first4 = E. | last5 = Bohlen | first5 = Thomas | year = 2003 | title = 3D Seismic Imaging for Mineral Exploration | journal = Commission on Controlled-Source Seismology: Deep Seismic Methods | url = http://www.geophys.geos.vt.edu/hole/ccss/milkereitCCSS.pdf | access-date = 8 September 2013}}</ref> A more recently developed application for seismic reflection is for [[geothermal energy]] surveys,<ref>{{cite web | title = The Role of Geophysics In Geothermal Exploration | website = Quantec Geoscience | url = http://www.quantecgeoscience.com/geothermal-exploration | access-date = 8 September 2013 | archive-date = 5 February 2013 | archive-url = https://web.archive.org/web/20130205030114/http://www.quantecgeoscience.com/geothermal-exploration | url-status = dead }}</ref> although the depth of investigation can be up to 2&nbsp;km deep in this case.<ref>{{cite journal | last1 = Louie | first1 = John N. | last2 = Pullammanappallil | first2 = S. K. | year = 2011 | title = Advanced seismic imaging for geothermal development | journal = New Zealand Geothermal Workshop 2011 Proceedings | url = http://crack.seismo.unr.edu/geothermal/Louie-NZGW11.pdf | access-date = 8 September 2013 | archive-date = 12 July 2012 | archive-url = https://web.archive.org/web/20120712182251/http://crack.seismo.unr.edu/geothermal/Louie-NZGW11.pdf | url-status = dead }}</ref>

*Mineral exploration – The traditional approach to near-surface (<300 m) mineral exploration has been to employ geological mapping, geochemical analysis and the use of aerial and ground-based potential field methods, in particular for greenfield exploration,<ref>{{Cite book|title=Geophysics for the Mineral Exploration Geoscientist|last1=Dentith|first1=Michael|last2=Mudge|first2=Stephen T.|date=2014-04-24|publisher=Cambridge University Press|isbn=9780521809511|doi = 10.1017/cbo9781139024358|bibcode=2014gmeg.book.....D |s2cid=127775731}}</ref> in the recent decades reflection seismic has become a valid method for exploration in hard-rock environments.

Land seismic surveys tend to be large entities, requiring hundreds of tons of equipment and employing anywhere from a few hundred to a few thousand people, deployed over vast areas for many months.<ref>{{cite journal |author= Jon Cocker|year= 2011|title= Land 3-D Seismic Survey Designed To Meet New Objectives|journal= E & P|publisher= Hart Energy|url= http://www.epmag.com/Technology-Contracts/Land-3-D-Seismic-Survey-Designed-Meet-Objectives_80767|access-date= 12 March 2012|archive-date= 19 February 2013|archive-url= https://web.archive.org/web/20130219034357/http://www.epmag.com/Technology-Contracts/Land-3-D-Seismic-Survey-Designed-Meet-Objectives_80767|url-status= dead}}s</ref> There are a number of options available for a controlled seismic source in a land survey and particularly common choices are [[Vibroseis]] and dynamite. Vibroseis is a non-impulsive source that is cheap and efficient but requires flat ground to operate on, making its use more difficult in undeveloped areas. The method comprises one or more heavy, all-terrain vehicles lowering a steel plate onto the ground, which is then vibrated with a specific frequency distribution and amplitude.<ref>{{cite book |title= Petroleum Geoscience|last1= Gluyas|first1= J|last2= Swarbrick|first2= R|year= 2004|publisher= Blackwell Publishing|isbn= 978-0-632-03767-4|page=22 }}</ref> It produces a low energy density, allowing it to be used in cities and other built-up areas where dynamite would cause significant damage, though the large weight attached to a Vibroseis truck can cause its own environmental damage.<ref>{{Cite book| authorfirst1=Sheriff, R.Robert |last1=E., Geldart,Sheriff |first2=L. P. |last2=Geldart | year=1995 | title=Exploration Seismology |edition=2nd | publisher=Cambridge University Press | isbn=0-521-46826-4 |pages= 209–210}}</ref> Dynamite is an impulsive source that is regarded as the ideal geophysical source due to it producing an almost perfect [[impulse function]] but it has obvious environmental drawbacks. For a long time, it was the only seismic source available until weight dropping was introduced around 1954,<ref>{{Cite book| authorfirst1=Sheriff, R.Robert |last1=E., Geldart,Sheriff |first2=L. P. |last2=Geldart | year=1995 | title=Exploration Seismology |edition=2nd | publisher=Cambridge University Press | isbn=0-521-46826-4 |page= 200}}</ref> allowing geophysicists to make a trade-off between image quality and environmental damage. Compared to Vibroseis, dynamite is also operationally inefficient because each source point needs to be drilled and the dynamite placed in the hole.

Unlike in marine seismic surveys, land geometries are not limited to narrow paths of acquisition, meaning that a wide range of offsets and azimuths is usually acquired and the largest challenge is increasing the rate of acquisition. The rate of production is obviously controlled by how fast the source (Vibroseis in this case) can be fired and then move on to the next source location. Attempts have been made to use multiple seismic sources at the same time in order to increase survey efficiency and a successful example of this technique is Independent Simultaneous Sweeping (ISS).<ref>{{cite book |doi= 10.1190/1.3063932|chapter= Independent simultaneous sweeping ‐a-a method to increase the productivity of land seismic crews|title= SEG Technical Program Expanded Abstracts 2008|pages= 2826–2830|year= 2008|last1= Howe|first1= Dave|last2= Foster|first2= Mark|last3= Allen|first3= Tony|last4= Taylor|first4= Brian|last5= Jack|first5= Ian}}</ref>

Marine seismic surveys generate a significant quantity of data <ref>{{Cite book| authorfirst1=Sheriff, R.Robert |last1=E., Geldart,Sheriff |first2=L. P. |last2=Geldart | year=1995 | title=Exploration Seismology |edition=2nd | publisher=Cambridge University Press | isbn=0-521-46826-4 |page= 260}}</ref> due to the size of modern towed streamer vessels and their towing capabilities.

The seismic properties of salt poses an additional problem for marine seismic surveys, it attenuates seismic waves and its structure contains overhangs that are difficult to image. This led to another variation on the NATS survey type, the ''wide-azimuth towed streamer'' (or WAZ or WATS) and was first tested on the [[Mad Dog (oil platform)|Mad Dog field]] in 2004.<ref>{{cite book |doi= 10.1190/1.2370129|chapter= Implementing a wide azimuth towed streamer field trial: The what, why and mostly how of WATS in Southern Green Canyon|title= SEG Technical Program Expanded Abstracts 2006|pages= 2901–2904|year= 2006|last1= Threadgold|first1= Ian M.|last2= Zembeck‐EnglandZembeck-England|first2= Kristin|last3= Aas|first3= Per Gunnar|last4= Fontana|first4= Philip M.|last5= Hite|first5= Damian|last6= Boone|first6= William E.}}</ref> This type of survey involved 1 vessel solely towing a set of 8 streamers and 2 separate vessels towing seismic sources that were located at the start and end of the last receiver line (see diagram). This configuration was "tiled" 4 times, with the receiver vessel moving further away from the source vessels each time and eventually creating the effect of a survey with 4 times the number of streamers. The end result was a seismic dataset with a larger range of wider azimuths, delivering a breakthrough in seismic imaging.<ref name="Barley, B. & Summers, T. 2007 450–458" /> These are now the three common types of marine towed streamer seismic surveys.

Marine survey acquisition is not just limited to seismic vessels; it is also possible to lay cables of geophones and hydrophones on the sea bed in a similar way to how cables are used in a land seismic survey, and use a separate source vessel. This method was originally developed out of operational necessity in order to enable seismic surveys to be conducted in areas with obstructions, such as [[Oil Production Platform|production platforms]], without having the compromise the resultant image quality.<ref>{{cite web | website = [[Schlumberger]] Oifield Glossary | title = Ocean Bottom Cable | url = http://www.glossary.oilfield.slb.com/Display.cfm?Term=ocean-bottom%20cable | access-date = 8 September 2013 | archive-date = 28 July 2012 | archive-url = https://web.archive.org/web/20120728113319/http://www.glossary.oilfield.slb.com/Display.cfm?Term=ocean-bottom%20cable | url-status = dead }}</ref> Ocean bottom cables (OBC) are also extensively used in other areas that a seismic vessel cannot be used, for example in shallow marine (water depth <300m) and transition zone environments, and can be deployed by [[remotely operated underwater vehicle]]s (ROVs) in deep water when [[repeatability]] is valued (see 4D, below). Conventional OBC surveys use dual-component receivers, combining a pressure sensor ([[hydrophone]]) and a vertical particle velocity sensor (vertical [[geophone]]), but more recent developments have expanded the method to use four-component sensors i.e. a hydrophone and three orthogonal geophones. Four-component sensors have the advantage of being able to also record [[seismic wave|shear waves]],<ref>{{cite web | website = [[Schlumberger]] Oilfield Glossary | title = Four-Component Seismic Data | url = http://www.glossary.oilfield.slb.com/Display.cfm?Term=four-component%20seismic%20data | access-date = 8 September 2013 | archive-date = 16 July 2012 | archive-url = https://web.archive.org/web/20120716212613/http://www.glossary.oilfield.slb.com/Display.cfm?Term=four-component%20seismic%20data | url-status = dead }}</ref> which do not travel through water but can still contain valuable information.

OBN has proven to be another very good way to accurately repeat a seismic acquisition. The world's first 4D survey using nodes was acquired over the Atlantis Oil Field in 2009, with the nodes being placed by a ROV in a water depth of 1300–2200 metres to within a few metres of where they were previously placed in 2005.<ref>{{cite book |doi=10.1190/1.3513730|chapter=Atlantis time‐lapsetime-lapse ocean bottom node survey: A project team's journey from acquisition through processing|title=SEG Technical Program Expanded Abstracts 2010|pages=4155–4159|year=2010|last1=Reasnor|first1=Micah|last2=Beaudoin|first2=Gerald|last3=Pfister|first3=Michael|last4=Ahmed|first4=Imtiaz|last5=Davis|first5=Stan|last6=Roberts|first6=Mark|last7=Howie|first7=John|last8=Openshaw|first8=Graham|last9=Longo|first9=Andrew|publisher=Society of Exploration Geophysicists}}</ref>

''Deconvolution'' is a process that tries to extract the reflectivity series of the Earth, under the assumption that a seismic trace is just the reflectivity series of the Earth convolved with distorting filters.<ref>{{Cite book| authorfirst1=Sheriff, R.Robert |last1=E., Geldart,Sheriff |first2=L. P. |last2=Geldart | year=1995 | title=Exploration Seismology |edition=2nd | publisher=Cambridge University Press | isbn=0-521-46826-4 |page= 292}}</ref> This process improves temporal resolution by collapsing the seismic wavelet, but it is nonunique unless further information is available such as well logs, or further assumptions are made. ''Deconvolution'' operations can be cascaded, with each individual deconvolution designed to remove a particular type of distortion.

''CMP stacking'' is a robust process that uses the fact that a particular location in the subsurface will have been sampled numerous times and at different offsets. This allows a geophysicist to construct a group of traces with a range of offsets that all sample the same subsurface location, known as a ''Common Midpoint Gather''.<ref>{{cite web | website = [[Schlumberger]] Oifield Glossary | title = Common-midpoint | url = http://www.glossary.oilfield.slb.com/Display.cfm?Term=CMP | access-date = 8 September 2013 | archive-date = 31 May 2012 | archive-url = https://web.archive.org/web/20120531155223/http://www.glossary.oilfield.slb.com/Display.cfm?Term=CMP | url-status = dead }}</ref> The average amplitude is then calculated along a time sample, resulting in significantly lowering the random noise but also losing all valuable information about the relationship between seismic amplitude and offset. Less significant processes that are applied shortly before the ''CMP stack'' are ''[[Normal moveout|Normal moveout correction]]'' and ''statics correction''. Unlike marine seismic data, land seismic data has to be corrected for the elevation differences between the shot and receiver locations. This correction is in the form of a vertical time shift to a flat datum and is known as a ''statics correction'', but will need further correcting later in the processing sequence because the velocity of the near-surface is not accurately known. This further correction is known as a ''residual statics correction.''

The goal of seismic interpretation is to obtain a coherent geological story from the map of processed seismic reflections.<ref>{{cite book |title= Petroleum Geoscience|last1= Gluyas|first1= J|last2= Swarbrick|first2= R|year= 2004|publisher= Blackwell Publishing|isbn= 978-0-632-03767-4|page=24 }}</ref> At its most simple level, seismic interpretation involves tracing and correlating along continuous reflectors throughout the 2D or 3D dataset and using these as the basis for the geological interpretation. The aim of this is to produce structural maps that reflect the spatial variation in depth of certain geological layers. Using these maps hydrocarbon traps can be identified and models of the subsurface can be created that allow volume calculations to be made. However, a seismic dataset rarely gives a picture clear enough to do this. This is mainly because of the vertical and horizontal seismic resolution<ref>[http://www.epgeology.com/geophysics-seismics-f22/seismic-interpretation-basics-t194.html Basics of Seismic Interpretation]</ref> but often noise and processing difficulties also result in a lower quality picture. Due to this, there is always a degree of uncertainty in a seismic interpretation and a particular dataset could have more than one solution that fits the data. In such a case, more data will be needed to constrain the solution, for example in the form of further seismic acquisition, [[borehole logging]] or [[gravimetry|gravity]] and [[Aeromagnetic survey|magnetic survey data]]. Similarly to the mentality of a seismic processor, a seismic interpreter is generally encouraged to be optimistic in order encourage further work rather than the abandonment of the survey area.<ref>{{Cite book| authorfirst1=Sheriff, R.Robert |last1=E., Geldart,Sheriff |first2=L. P. |last2=Geldart | year=1995 | title=Exploration Seismology |edition=2nd | publisher=Cambridge University Press | isbn=0-521-46826-4 |page= 349}}</ref> Seismic interpretation is completed by both [[geologists]] and [[geophysicists]], with most seismic interpreters having an understanding of both fields.

Seismic attribute analysis involves extracting or deriving a quantity from seismic data that can be analysed in order to enhance information that might be more subtle in a traditional seismic image, leading to a better [[geology|geological]] or [[geophysics|geophysical]] interpretation of the data.<ref>{{cite web | title = Petrel Seismic Attribute Analysis | website = [[Schlumberger]] | url = http://www.slb.com/services/software/geo/petrel/seismic/seismic_multitrace_attributes.aspx | access-date = 8 September 2013 | archive-date = 29 July 2013 | archive-url = https://web.archive.org/web/20130729172415/http://www.slb.com/services/software/geo/petrel/seismic/seismic_multitrace_attributes.aspx | url-status = dead }}</ref> Examples of attributes that can be analysed include mean amplitude, which can lead to the delineation of [[bright spot]]s and [[dim spot]]s, [[Coherence (signal processing)|coherency]] and [[amplitude versus offset]]. Attributes that can show the presence of hydrocarbons are called [[direct hydrocarbon indicator]]s.