Current methods of subbottom sounding use low frequencies for penetration with the subsequent degradation of beam width such that large horizontal areas at the bottom are illuminated. This paper describes a method of subbottom surveying at medium frequencies, 12 kHz, with good penetration and narrow beam width by using a parabolic reflector. Design criteria for the surveyor along With the resultant data obtained from experiments performed at Lake Winnipesaukee, New Hampshire are presented.
Attempts to develop a three dimensional model of the ocean bottom and sub-bottom are made with an acoustic survey.1 Modeling of the ocean floor by remote acoustic sensing is predicated on the fact that reflections in an acoustic wave will occur wherever a change in the mechanical impedance of the medium is encountered. Neither chemical nor physical differences in sub-bottom structures will result in an echo unless these differences also modify the acoustic wave propagation characteristics. While extensive research has been conducted in an effort to relate the physical and acoustic properties of sediments and other mineral deposits, most current resource exploration and petrochemical searches have depended for their interpretation of the distribution and geometry of stratification on extensive past experience with formations which have proven to contain oil or gas deposits.2 The analysis of a time sequence of echoes is complicated by the occurrence of multiple echoes and images within a multiple layer bottom.
Conventional acoustic methods for the determination of bottom and subbottom sediment layer profiles are influenced by the requirements of large scale surveys to be conducted over extended areas as rapidly as possible and at minimum cost.3 This has resulted in the evolution of an extensive technology using "pinger" pulse sources and hydrophone arrays which can be trailed from a survey vessel as it traverses a region to be explored. Primary echo patterns from bottom layers are mixed with multiple higher-order reflections from the air-water interface. Signal conditioning and correlation techniques must be employed to extract the desired geological information. The results represent the most probable interpretation of the characteristics of the bottom averaged over a relatively large lateral area. The horizontal scale of this average is established by the extent of the hydrophone array, the spacing of the survey tracks, the speed of the survey ship and the source pulse repetition rate.4 Since acoustic signal attentuation in the sub-bottom increases with frequency, deep penetration surveys may use a frequency of 25 Hz or less. Due to wave diffraction effects, obstacles or layers with dimensions under. 3 wavelengths have a theoretical resolution limit of 20 to 40 m. The actual detail obtainable in a survey is, in practice, limited by the extent of the region to be scanned and the time and hence cost which can be expended. The success of these methods in locating large sub-bottom formations having deposits of value is evident and well documented.
