Currently, almost all oil and gas exploratory wells are preceded by 3-D seismic surveys. It is instructive to review the method, but it has fallen into disuse because of the decreased cost of CDP surveys and the difficulty of quantitative interpretation in most cases. After the 11th voyage of deep-sea drilling, this anomalous reflector was named the bottom simulating reflector (BSR), which was thought associated with the existence of submarine gas hydrate (Hollister et al., 1972). The following provides an example of 3-D seismic imaging of VMS deposits in the Flin Flon mining camp in Manitoba, Canada (Malinowski et al., 2012; White et al., 2012). Figure 11.28. Upon arrival at the detectors, the amplitude and timing of waves are recorded to give a seismogram (record of ground vibrations). Acute heterogeneity and concentrations of low-velocity upwellings are imaged in the upper mantle wedge above subducting slabs, with the best detail being provided by regional-scale tomographic inversions (e.g., Zhao et al., 1992, 1994, 1995, 1997). At greater distances, however, the seismic pulse travels faster by the refraction path because its velocity is greater along the top of the lower, denser layer than it is through the upper layer; in this case, the refracted wave arrives first. We will discuss this issue in the next chapter. However, this should be balanced against the number of channels available and the required horizontal resolution. Gravity surveys are carried out to map subsurface density contrasts caused by geological structures. Travel-time anomalies for P waves beneath other hotspots have been interpreted as being caused by plumes extending through the transition zone (Nataf and VanDecar, 1993). Figure 4.10. The type of wave most often used for seismic investigation is a low-frequency sound wave. Seismic methods can provide valuable information of the subsurface, such as the seismic velocity structure of the geology (e.g. Any mechanical vibration is initiated by a source and travels to the location where the vibration is recorded. using seismic refraction, surface-wave methods) and the presence of geological layers due to their seismic reflectivity (e.g. mechanism used in a seismic survey, seismic waves are grouped primarily into direct, reflected, refracted, and surface waves (Fig. This is usually called a P wave; during its passage, individual particles oscillate backwards and forwards in the direction that the wave is travelling, so that the wave consists of alternating compressions and rarefactions. In order to develop these methods, one should study carefully the physics of seismic waves. Deep structures that may correspond to frozen-in plume features have also been imaged by tomography under South America (Van Decar et al., 1995). In the submarine stratum containing gas hydrate, the sediment pore is filled and cemented by gas hydrate, and its seismic velocity increases, while the underlying stratum without hydrate often has free gas and smaller wave velocity, so the stratum containing hydrate and the underlying stratum without hydrate often cause a negative strong reflection. Ring in the new year with a Britannica Membership, This article was most recently revised and updated by, https://www.britannica.com/science/seismic-survey. Global tomography provides limited resolution of presumed upwelling regions at this point, in part due to the spatially localized nature of most upwellings. This is especially true of the large three-dimensional surveys that are now routinely being acquired. Corrections? Reflection and Refraction are the most commonly used seismic techniques. 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