Submarine High-resolution Acoustic Detection And The Application

Submarine high-resolution acoustic detection technique is an important method to investigate the sedimentary strata and resources in the offshore area. It is well-known that the acoustic detection has characteristics of high efficiency and high resolution. However, besides of the inadequate experimental investigations for data acquisition, it still suffers the problems lacking in key processing techniques such as effective noise elimination, velocity analysis and migration. This may seriously affect the quality of sub-bottom profile (SBP), and even lead to wrong geological interpretations.Focusing on the sub-bottom profile, this thesis makes a comprehensive analysis of shallow water acoustic wave propagation characteristics, the producing mechanism of various kinds of noises and their abatement approaches in acquisition. The key techniques of combined noise attenuation, deconvolution, migration imaging and trace integration are integrated to be suitable for SBP data processing. These techniques are then applied in the detections of sea sands and submarine hydrocarbon seepages.(1) Based upon the systematical analysis of noise and affection of natural conditions to data acquisition in sub-bottom profile detections, the specific measures are proposed for some key aspects such as survey vessels, navigation, equipment operation and the conditions of transducer installation.(2) There usually exist the direct waves, refractions, multiples and low frequency noise on the shallow water sub-bottom profile. Besides, the high frequency absorption, uneven horizontal energy and poor amplitude consistency are usually occurred in the raw data. These problems may be well overcome by using the combined noise attenuation techniques and the main energy impulse deconvolution approach. The processing is aimed at ensuring the reliability of SBP resolution by expanding the spectrum in the effective frequency range and keeping the phase characteristics of impulse deconvolution at the same time.(3) Based upon the analysis to the features of raw SBP data, the corresponding modules of MBP seismic software package are integrated to form a processing flow which is suitable for SBP data processing. The migration velocity can be obtained by the image scanning of Kirchhoff integration migration. Thus the SBP migration imaging can be made using the optimum velocity field. This may be the first time obtaining the migrated image in the sub-bottom profile detection.(4) The trace integrating technique which is independent on logging data is applied in the data processing. This relative impedance profiles processed by trace integrating may be in more reality to reveal the physical property of sedimentary formations. These profiles make not only the comparison and interpretation relatively easier to the geological features of targets and larger strata units, but also more reliable for the division of the stratigraphic sequence and analysis of seismic facies units.(5) The submarine high-resolution acoustic detection and processing techniques are then applied to marine sand investigation in the Pearl River Estuary offshore. Based on the topographical feature and sedimentary types of surface layer, the seafloor in the study area can be divided into 3 zonations of different topographic level according to morphological and genetic classification principle. The inner shelf of northern South China Sea belongs to the first level topography. The second-level topography includes modern marine accumulation plains and modern-residue mixed accumulation of plains. In the modern marine accumulation plain there develops the third-level landform such as shallow groove, erosion gully group, sand waves and dunes, ancient dike, convex, the scarp and depression, and in the modern-residue mixed accumulation of plains there develops shoals, shelf trough, and ridges, buried channels and buried shells. The bottom sediment in the study area can be classified into 5 types, i.e. gravel substrate sand (SG), medium-coarse sand (MCS), fine sand (FS), clayey silt (YT) and silty clay (TY). From seabed surface down to the detectable depth, the sequence stratigraphy can be divided into transgressive half cycle of deglaciation (sequence I), transgressive-regressive cycle of the middle-late glaciation (sequence II) and regressive half cycle of the early glaciation (sequence III). The shallow strata are divided into 9 interfaces of acoustic reflection (including sub-interfaces) and 8 units of seismic facies (including sub-units). The study reveals that there exist 4 genetic types of sea sand deposits in the Pearl River Estuary offshore area, which are the sands of ancient residual sand, sands of ancient tide ridge, sands of the ancient channel sand (or ancient tidal creek type) and sands of modern tide ridge sand. It is found that the ancient residual sand is directly exposed on seabed, widely distributed and has high-grade quality. Therefore it should be the main deposit for the sand exploration in study area.(6) It may be the first time that submarine acoustic detection results reveal the presence of submarine hydrocarbon seepage in selected block of the northern depression in the South Yellow Sea Basin. The landforms formed by seepages are predominant in pockmarks, domes and faults (scarp). The pockmarks are roughly spread in SN. Two domes on seabed are found. The bigger one is about 1600m in diameter and about 6m high. The study shows that submarine seepages in the area are associated with the faults penetrating or closing to the seabed. It is the faults that provide channels and sources of seepages. It can be deduced that, from analysis of the formation mechanism and characterization, the hydrocarbon seepage in study area is currently in micro-seepage stage.