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Some limitations of shallow water geophysical devices imposed by different oceanographic and geological conditions
Ist Teil von
2013 IEEE/OES Acoustics in Underwater Geosciences Symposium, 2013, p.1-8
Ort / Verlag
IEEE
Erscheinungsjahr
2013
Link zum Volltext
Quelle
IEEE Xplore Digital Library
Beschreibungen/Notizen
Applied shallow water seismic-reflection methods are an important component in designing engineering projects to address navigability, port construction and dredging activities, among others. The more these techniques are applied, the more that is known about the environment in question resulting in an increased potential to minimize costs. It is crucial to choose the correct techniques to get the desired responses, since each device has its own frequencies and operational requirements. When site-specific physical parameters and characteristics are taken into consideration when planning survey operations, high quality data can be acquired. However, this study presents some limitations of shallow water geophysical devices imposed by different oceanographic and geological conditions. Examples of a Side Scan Sonar and sub-bottom profilers were taken from four (4) survey sites along the Brazilian territory: (1) a ria-like coastal environment along the southern coast of Rio de Janeiro State; (2) the coast of Bahia State; (3) a rocky river in the central region of the country; and (4) a macro tide-dominated estuary in the Northeast. The equipment was selected and operated with specific project goals in mind including: definition of stratigraphy, depth to crystalline basement, and sediment thickness. The first site is an example of classic shallow sub-seafloor gas occurrences off the coast of Rio de Janeiro, where those gas accumulations were mapped with an integrated system consisting of a boomer (0.5 - 2.0), chirp (2.0 - 8.0) and a pinger (24 kHz), with the seismic signal being limited by strong reflections that caused phase inversions. In a similar situation, a boomer survey was conducted off the coast of Bahia State to define the depth of consolidated rock, although the signal penetration was limited by a layer of soft sediments. This layer was sampled with percussion cores and revealed very soft clay content that the cores easily penetrated. This layer also caused phase inversion in the seismic data, not very common in sediments without gas accumulations. The third site is a narrow river in which the boomer source was employed. The presence of solid rock, plus the characteristics of this impulsive source, resulted in a strong sideswipe effect, which can cause some confusion in the interpretation process. The last site is known to have high current speeds due to the tidal amplitude. The estuary was surveyed with a Side Scan Sonar operating with dual, simultaneous frequencies of 300 and 600 kHz. The high frequency swath was dispersed and attenuated within in the water column in a way that the signal could not reach the bottom, thus resulting in no acquisition at all. The 300 kHz was used without any issue; water samples (Niskin Bottle) were taken which demonstrated a high concentration of particulate material on the water column. Along with the Side Scan, a chirp survey was conducted operating in a frequency range of 0.5 - 8.0 kHz. The chirp signal penetration was seriously degraded by a thin layer of consolidated clay, verified through surficial samples where the clay outcropped. These examples were selected to demonstrate the importance of choosing the appropriate geophysical system to achieve individual project goals at highly-variable site-specific locations However, it is important to note that there are some specific environmental conditions that limit the effectiveness of some of these methods, but that may be used for the benefit of the interpreter, depending on its particular attributes.