Sources of noise on a seismic record. Top-left: air wave; top-right: head wave; bottom-left: surface wave; bottom-right: multiple.In addition to reflections off interfaces within the subsurface, there are a number of other seismic responses detected by receivers and are either unwanted or unneeded:Air wave The airwave travels directly from the source to the receiver and is an example of.
It is easily recognizable because it travels at a speed of 330 m/s, the in air.Ground roll / Rayleigh wave / Scholte wave / Surface wave A typically propagates along a free surface of a solid, but the elastic constants and of air are very low compared to those of rocks so the surface of the Earth is approximately a. 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. The is similar toground roll but occurs at the sea-floor (fluid/solid interface) and it can possibly obscure and mask deep reflections in marine seismic records.The velocity of these waves varies with wavelength, so they are said to be dispersive and the shape of the wavetrain varies with distance.
Refraction / Head wave / Conical wave A head wave refracts at an interface, travelling along it, within the lower medium and produces oscillatory motion parallel to the interface. This motion causes a disturbance in the upper medium that is detected on the surface. The same phenomenon is utilised in.Multiple reflection An event on the seismic record that has incurred more than one reflection is called a multiple.
Diagram of equipment used for marine seismic surveysMarine – The marine zone is either in shallow water areas (water depths of less than 30 to 40 metres would normally be considered shallow water areas for 3D marine seismic operations) or in the deep water areas normally associated with the seas and oceans (such as the Gulf of Mexico).Seismic surveys are typically designed by and who hire service companies such as, and to acquire them. Another company is then hired to process the data, although this can often be the same company that acquired the survey. Finally the finished seismic volume is delivered to the oil company so that it can be geologically interpreted.Land survey acquisition. Receiver line on a desert land crew with recorder truckLand 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. There are a number of options available for a controlled seismic source in a land survey and particularly common choices are 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.
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. Dynamite is an impulsive source that is regarded as the ideal geophysical source due to it producing an almost perfect but it has obvious environmental drawbacks.
For a long time, it was the only seismic source available until weight dropping was introduced around 1954, 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.A land seismic survey requires substantial logistical support. In addition to the day-to-day seismic operation itself, there must also be support for the main camp (for catering, waste management and laundry etc.), smaller camps (for example where the distance is too far to drive back to the main camp with vibrator trucks), vehicle and equipment maintenance, medical personnel and security.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). Marine survey acquisition (streamer). Seismic data collected by the in theTraditional marine seismic surveys are conducted using specially-equipped vessels that tow one or more cables containing a series of hydrophones at constant intervals (see diagram). The cables are known as streamers, with 2D surveys using only 1 streamer and 3D surveys employing up to 12 or more (though 6 or 8 is more common).
The streamers are deployed just beneath the surface of the water and are at a set distance away from the vessel. The seismic source, usually an or an array of airguns but other sources are available, is also deployed beneath the water surface and is located between the vessel and the first receiver. Two identical sources are often used to achieve a faster rate of shooting.
Marine seismic surveys generate a significant quantity of data, each streamer can be up to 6 or even 8 km long, containing hundreds of channels and the seismic source is typically fired every 15 or 20 seconds.A seismic vessel with 2 sources and towing a single streamer is known as a Narrow-Azimuth Towed Streamer (or NAZ or NATS). By the early 2000s, it was accepted that this type of acquisition was useful for initial exploration but inadequate for development and production, in which had to be accurately positioned. This led to the development of the Multi-Azimuth Towed Streamer (MAZ) which tried to break the limitations of the linear acquisition pattern of a NATS survey by acquiring a combination of NATS surveys at different azimuths (see diagram). This successfully delivered increased illumination of the subsurface and a better signal to noise ratio.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 in 2004. 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.
These are now the three common types of marine towed streamer seismic surveys.Marine survey acquisition (ocean bottom seismic (OBS)) 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, without having the compromise the resultant image quality.
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. See also:, andThere are three main processes in seismic data processing:, (CMP) stacking and.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.
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. 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 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.Seismic migration is the process by which seismic events are geometrically re-located in either space or time to the location the event occurred in the subsurface rather than the location that it was recorded at the surface, thereby creating a more accurate image of the subsurface.Seismic interpretation. See also:The goal of seismic interpretation is to obtain a coherent geological story from the map of processed seismic reflections. 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 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,.
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. Seismic interpretation is completed by both and, with most seismic interpreters having an understanding of both fields.In hydrocarbon exploration, the features that the interpreter is particularly trying to delineate are the parts that make up a – the, the reservoir rock, the seal and.Seismic attribute analysis. See also: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 or interpretation of the data. Examples of attributes that can be analysed include mean amplitude, which can lead to the delineation of and,. Attributes that can show the presence of hydrocarbons are called.Crustal studies The use of reflection seismology in studies of and the Earth's crust was pioneered in the 1970s by groups such as the Consortium for Continental Reflection Profiling (COCORP), who inspired deep seismic exploration in other countries such as BIRPS in Great Britain and ECORS in France. The British Institutions Reflection Profiling Syndicate (BIRPS) was started up as a result of oil hydrocarbon exploration in the North Sea.
It became clear that there was a lack of understanding of the tectonic processes that had formed the geological structures and which were being explored. The effort produced some significant results and showed that it is possible to profile features such as that penetrate through the crust to the with marine seismic surveys.
Environmental impact As with all human activities, seismic reflection surveys have some and both the hydrocarbon industry and environmental groups partake in research to investigate these effects.Land On land, conducting a seismic survey may require the building of, for transporting equipment and personnel, and vegetation may need to be cleared for the deployment of equipment. If the survey is in a relatively undeveloped area, significant disturbance may occur and many governments require seismic companies to follow strict rules regarding destruction of the environment; for example, the use of dynamite as a seismic source may be disallowed. Seismic processing techniques allow for seismic lines to deviate around natural obstacles, or use pre-existing non-straight tracks and trails. With careful planning, this can greatly reduce the environmental impact of a land seismic survey. The more recent use of inertial navigation instruments for land survey instead of theodolites decreased the impact of seismic by allowing the winding of survey lines between trees.Marine The main environmental concern for marine seismic surveys is the potential for noise associated with the high-energy seismic source to disturb or injure animal life, especially such as, and, as these mammals use sound as their primary method of communication with one another. High-level and long-duration sound can cause physical damage, such as hearing loss, whereas lower-level noise can cause temporary threshold shifts in hearing, obscuring sounds that are vital to marine life, or behavioural disturbance.A study has shown that migrating will leave a minimum 3 km gap between themselves and an operating seismic vessel, with resting humpback whale pods with cows exhibiting increased sensitivity and leaving an increased gap of 7–12 km. Conversely, the study found that male humpback whales were attracted to a single operating airgun as they were believed to have confused the low-frequency sound with that of.
3d Seismic Survey
In addition to whales, and all showed alarm and avoidance behaviour in the presence of an approaching seismic source. It is difficult to compare reports on the effects of seismic survey noise on marine life because methods and units are often inadequately documented.The will avoid its regular migratory and feeding grounds by 30 km in areas of seismic testing. Similarly the breathing of gray whales was shown to be more rapid, indicating discomfort and panic in the whale.