Signal Penetration Depth Research Articles

Acoustical characterization of sediments by Parasound and 3.5 kHz systems: Related sedimentary processes on the southeastern Weddell Sea continental slope, Antarctica

Subbottom profiling was performed during four Polarstern expeditions to the southeastern Weddell Sea continental slope, northwest of Lyddan Ice Rise, Antarctica. Analogue records from high resolution, seismic reflection systems (3.5 kHz and Parasound) were used to classify sediment echo types. A comparison between the two systems shows the advantage of the Parasound system. The smaller beam angle produces a higher spatial resolution, which reduces the diffraction hyperbolae considerably. The higher vertical resolution and deeper penetration provide more detailed information concerning seismic stratification. Using four basic echo categories (P = Prolonged, L = Layered D = Diffraction hyperbolae, W = Wedging subbottoms), ten discrete sediment echo patterns were distinguished. Their regional distribution and the depth of acoustic signal penetration into the sediment were mapped. SW-NE trending sediment ridges with channels running parallel on the southeastern side of the ridges were traced in the eastern part of the study area. The prolonged and highly reflective echo types of the channel bottoms indicate deposition of predominantly coarse-grained sediments affected by erosional processes. These channels are interpreted as drainage systems for bottom water currents derived from the shelf and steep upper slope. The flow direction is towards the northeast, counter to the Weddell Gyre. In contrast, the sediment ridges show up to 80 m subbottom penetration and an echo type of distinct, multi-layered reflectors running parallel to subparallel with the surface reflector. The sediments of the ridges are interpreted to be “levee”-sediments, overspilled from the channels and preferentially deposited on the northwestern flank due to the Coriolis force. Sedimentary units separated by wedge shaped reflectors are widespread on the gently inclined terrace in the middle part of the continental slope. These sediments are presumed to be produced by syn-sedimentary slumping and proximal debris flows having a northeastern direction of movement.

Concentration Profiling in Layered Liquids Using a Laser Photoacoustic Probe

An aluminum-coated Pyrex® glass rod is used both as a photoacoustic probe and as a laser light guide to examine a three-layer solution of carbon tetrachloride, hexane, and an aqueous solution of an efficient chromophore, ferroin. The boundaries of the layers show up clearly on a signal amplitude vs. penetration depth graph. A Beer's law plot of the ferroin solution alone shows that the probe is capable of analytical determinations down to 10−7 M. Reflections from the cell walls may cause interference in quantitative analysis if the size of the cell is too small. The sensitivity of the probe is compared with that of a typical photoacoustic cell, and it is found that the probe only attenuates the signal amplitude by ∼35%. Finally, it is found that the signal amplitude transmitted by the photoacoustic probe is roughly proportional to its diameter.

Shuttle Imaging Radar: Physical Controls on Signal Penetration and Subsurface Scattenng in the Eastern Sahara

SIR-A signal penetration and subsurface backscatter within the upper meter or so of the sediment blanket in the Eastern Sahara of southern Egypt and northern Sudan are enhanced both by radar sensor parameters and by the physical and chemical characteristics of eolian and alluvial materials. Interpretation of SIR-A images by McCauley et al. [1], [2] dramatically changed previous concepts of the role that fluvial processes have played over the past 10000 to 30000000 years in shaping this now extremely flat, featureless, and hyperarid landscape. In the present paper we summarize the near-surface stratigraphy, the electrical properties of materials, and the types of radar interfaces found to be responsible for different classes of SIR-A tonal response. The dominant factors related to efficient microwave signal penetration into the sediment blanket include a) favorable distribution of particle sizes, b) extremely low moisture content and c) reduced geometric scattering at the SIR-A frequency (1.3 GHz). The depth of signal penetration that results in a recorded backscatter, here called "radar imaging depth," was documented in the field to be a maximum of 1.5 m, or 0.25 of the calculated "skin depth," for the sediment blanket. Radar imaging depth is estimated to be between 2 and 3 m for active sand dune materials. Diverse permittivity interfaces and volume scatterers within the shallow subsurface are responsible for most of the observed backscatter not directly attributable to grazing outcrops.