GLORIA Data Research Articles

Oblique Propagation and Refraction of Gravity Waves Over the Andes Observed by GLORIA and ALIMA During the SouthTRAC Campaign

AbstractGravity waves (GW) carry energy and momentum from the troposphere to the middle atmosphere and have a strong influence on the circulation there. Global atmospheric models cannot fully resolve GWs, and therefore rely on highly simplified GW parametrizations that, among other limitations, account for vertical wave propagation only and neglect refraction. This is a major source of uncertainty in models, and leads to well‐known problems, such as the late break‐up of polar vortex due to the “missing” GW drag around 60°S. To investigate these phenomena, GW observations over Southern Andes were performed during SouthTRAC aircraft campaign. This paper presents measurements from a SouthTRAC flight on 21 September 2019, including 3‐D tomographic temperature data of the infrared limb imager GLORIA (8–15 km altitude) and temperature profiles of the ALIMA lidar (20–80 km altitude). GLORIA observations revealed multiple overlapping waves of different wavelengths. 3‐D wave vectors were determined from the GLORIA data and used to initialize a GW ray‐tracer. The ray‐traced GW parameters were compared with ALIMA observations, showing good agreement between the instruments and direct evidence of oblique (partly meridional) GW propagation. ALIMA data analysis confirmed that most waves at 25–40 km altitudes were indeed orographic GWs, including waves seemingly upstream of the Andes. We directly observed horizontal GW refraction, which has not been achieved before SouthTRAC. Refraction and oblique propagation caused significant meridional transport of horizontal momentum as well as horizontal momentum exchange between waves and the background flow all along the wave paths, not just in wave excitation and breaking regions.

Halokinetics and other features of GLORIA long-range sidescan sonar data from the Red Sea

The Red Sea is an unusual example of a rift basin that transitioned from its evaporitic stage to fully open-ocean conditions at the end of the Miocene (∼5.3 Ma), much more recently than older Mesozoic margins around the Atlantic and Gulf of Mexico. The patterns of halokinetic deformation occurring in the Red Sea are potentially of interest for understanding more generally how evaporite deposits deform during this early stage. Relevant to this issue, a line of reconnaissance sidescan sonar data (GLORIA) collected along the Red Sea in 1979 is re-evaluated here. We first interpret the data with the aid of newly compiled bathymetry from multibeam sonars in the central and southern Red Sea. Features in the acoustic backscatter data are associated with ridges, valleys and rounded flow fronts produced by halokinetic deformation. Some areas of higher acoustic backscattering from the evaporites are suggested to relate to roughness produced by deformation of the evaporite surface. Within the volcanic (oceanic) axial valleys, areas of differing high and low backscattering suggest varied sediment cover and/or carbonate encrustations. With the benefit of the above experience, we then interpreted data from the northern Red Sea, where there are fewer multibeam data available. Rounded fronts of halokinetic deformation are present in the Zabargad Fracture Zone, a broad, shallow valley crossing the Red Sea obliquely. The presence of halokinetic deformation here is evidence that subsidence has occurred along the fracture zone. Elsewhere in the northern Red Sea, the GLORIA data reveal folds in the evaporite surface, suggesting local areas of convergence, like those implied by multibeam data from inter-trough zones further south. Some linear features are observed, many of which are likely to be ridges overlying salt walls. Interestingly, several such features are oriented along an accommodation zone that is oriented parallel to the plate spreading direction. Several rounded, corrugated features are interpreted as possible evaporite flow fronts. Overall, the impression from the data is of a strongly mobile seabed in the Red Sea because of halokinetic deformation, involving both vertical and horizontal movements. However, salt walls appear more common than in the central and southern axial Red Sea, where horizontal movements instead tend to dominate.

Diversidad florística en cimas de la Sierra Madre Occidental, México, y su relación con variables ambientales

Las partes más altas de la Sierra Madre Occidental presentan comunidades vegetales con algunas características de vegetación alpina. Estos sitios son ecológicamente importantes, pues además de sus altos índices de diversidad, son de las primeras zonas terrestres donde pueden detectarse efectos del cambio climático. El objetivo del trabajo fue aportar información acerca de la composición florística y su cobertura en las cimas de tres de los picos más altos de la Sierra Madre Occidental: Cerro Gordo, Huehuento y Las Antenas, comparar su riqueza específica y medir la similitud florística entre las cimas e identificar las variables ambientales que más influyen sobre la vegetación. Los muestreos se basaron en la metodología propuesta por el Proyecto GLORIA. Se realizó un Análisis de Correspondencias Canónicas, el cual mostró una estrecha relación entre la presencia de las especies y las variables de precipitación, temperatura, continentalidad, superficie de afloramiento rocoso, perturbación y altitud. Se registraron 175 taxa y una similitud florística baja, con únicamente dos compartidos entre los tres cerros. Aunque en la cima de Las Antenas fue donde se registró el mayor número de taxa, tanto en el inventario florístico como en las unidades de muestreo, fue en el cerro Huehuento donde se registró el mayor número de especies/m2. El elemento alpino está escasamente representado en la Sierra Madre Occidental, compuesto por Juniperus blancoi var. huehuentensis, Draba implexa, Micranthes mexicana, Bromus richardsonii y varias especies de Sedum, Galinsoga y Muhlenbergia .

Massive edifice failure at Aleutian arc volcanoes

Along the 450-km-long stretch of the Aleutian volcanic arc from Great Sitkin to Kiska Islands, edifice failure and submarine debris-avalanche deposition have occurred at seven of ten Quaternary volcanic centers. Reconnaissance geologic studies have identified subaerial evidence for large-scale prehistoric collapse events at five of the centers (Great Sitkin, Kanaga, Tanaga, Gareloi, and Segula). Side-scan sonar data collected in the 1980s by GLORIA surveys reveal a hummocky seafloor fabric north of several islands, notably Great Sitkin, Kanaga, Bobrof, Gareloi, Segula, and Kiska, suggestive of landslide debris. Simrad EM300 multibeam sonar data, acquired in 2005, show that these areas consist of discrete large blocks strewn across the seafloor, supporting the landslide interpretation from the GLORIA data. A debris-avalanche deposit north of Kiska Island (177.6° E, 52.1° N) was fully mapped by EM300 multibeam revealing a hummocky surface that extends 40 km from the north flank of the volcano and covers an area of ∼ 380 km 2. A 24-channel seismic reflection profile across the longitudinal axis of the deposit reveals a several hundred-meter-thick chaotic unit that appears to have incised into well-bedded sediment, with only a few tens of meters of surface relief. Edifice failures include thin-skinned, narrow, Stromboli-style collapse as well as Bezymianny-style collapse accompanied by an explosive eruption, but many of the events appear to have been deep-seated, removing much of an edifice and depositing huge amounts of debris on the sea floor. Based on the absence of large pyroclastic sheets on the islands, this latter type of collapse was not accompanied by large eruptions, and may have been driven by gravity failure instead of magmatic injection. Young volcanoes in the central and western portions of the arc (177° E to 175° W) are located atop the northern edge of the ∼ 4000-m-high Aleutian ridge. The position of the Quaternary stratocones relative to the edge of the Aleutian ridge appears to strongly control their likelihood for, and direction of, past collapse. The ridge's steep drop to the north greatly increases potential runout length for slides that originate at the island chain.

A seismic reflection and GLORIA study of compressional deformation in the Gorringe Bank region, eastern North Atlantic

Seismic reflection and GLORIA side-scan sonar data obtained on RRS Charles Darwin cruise CD64 reveal new information on the styles of deformation in the Gorringe Bank region, at the eastern end of the Azores–Gibraltar plate boundary. Previous studies suggest that Gorringe Bank was formed by the overthrusting of a portion of the African plate upon the Eurasian plate. The new seismic data show, however, that the most intensely deformed region is located south of Gorringe Bank, on the northern flanks of a NW–SE-trending submarine ridge which includes the Ampere and Coral Patch seamounts. The deformation is expressed as long-wavelength (up to 60 km), large-amplitude (up to 800 m) folds in the sediments and underlying acoustic basement, which in places are associated with one or more reverse faults, and as a fabric of short-wavelength folds (up to 3 km) with a NE trend. In contrast, the same sedimentary units when traced beneath the flanking plains are undeformed, except for some faults with a small throw (~30 m), some of which offset the seafloor. GLORIA data show that recent deformation is broadly distributed over the region. Structural trends rotate from 45° in the west to 70° in the east of the region, nearly perpendicular to the NW-verging plate motion vectors as determined from plate kinematic models. Flexure modelling suggests that a portion of Gorringe Bank has loaded 152 Ma oceanic lithosphere and that a maximum of 50 km of shortening has occurred at Gorringe Bank since the mid-Miocene. Our observations support a model in which there is no single plate boundary in the region, rather that the deformation is distributed over a 200–330 km wide zone.

The Canary Debris Flow: source area morphology and failure mechanisms

The morphology of the source area of the Canary Debris Flow has been mapped using both GLORIA reconnaissance and TOBI high‐resolution sidescan sonar systems. West of ≈19°W, the seafloor is characterized by a strongly lineated downslope‐trending fabric. This fabric can be interpreted as being caused by streams of debris separated by longitudinal shears. Multiple flow pulses are indicated by a series of asymmetrical lateral ridges which mark the northern boundary of the flow. East of ≈19°W, GLORIA data show only a weak fabric of irregular patches and alongslope lineaments. The TOBI data show the patches to be coherent sediment blocks up to 10 km across, surrounded by debris flow material. These are interpreted as in situ areas of seafloor sediment which have survived the slope failure and debris flow event rather than transported fragments of a failed sediment slab. TOBI data from the best developed area of alongslope lineaments show a series of small faults downstepping to the west. This area of seafloor is interpreted as one of partial sediment failure, where the failure process became ‘frozen’ before total mobilization of the seafloor sediments could occur. The overall morphology of the failure area indicates removal of a slab‐like body of sediment, although we cannot distinguish between retrogressive and slab‐slide failure mechanisms. If the latter mechanism is applicable, fragmentation of the failing ‘slab’ must have commenced concurrently with the onset of downslope transport. Immediately upslope from the debris flow source area, a seafloor of characteristic rough blocky texture is interpreted as the surface of a debris avalanche derived from the slopes of the island of El Hierro. The debris flow and avalanche appear to be simultaneous events, with failure of the slope sediments occurring while the avalanche deposits were still mobile enough to fill and disguise the topographic expression of the debris flow headwall. Loading of the slope sediments by the debris avalanche most probably triggered the Canary Debris Flow.

Merged GLORIA sidescan and Hydrosweep pseudo-sidescan: Processing and creation of digital mosaics

We have replaced the usual band of poor-quality data in the near-nadir region of our GLORIA long-range sidescan-sonar imagery with a shaded-relief image constructed from swath bathymetry data (collected simultaneously with GLORIA) which completely cover the nadir area. We have developed a technique to enhance these “pseudo-sidescan” images in order to mimic the neighbouring GLORIA backscatter intensities. As a result, the enhanced images greatly facilitate the geologic interpretation of the adjacent GLORIA data, and geologic features evident in the GLORIA data may be correlated with greater confidence across track. Features interpreted from the pseudo-sidescan may be extrapolated from the near-nadir region out into the GLORIA range where they may nt have been recognized otherwise, and therefore the pseudo-sidescan can be used to ground-truth GLORIA interpretations. Creation of digital sidescan mosaics utilized an approach not previously used for GLORIA data. Pixels were correctly placed in cartographic space and the time required to complete a final mosaic was significantly reduced. Computer software for digital mapping and mosaic creation is incorporated into the newly-developed Woods Hole Image Processing System (WHIPS) which can process both low- and high-frequency sidescan, and can interchange data with the Mini Image Processing System (MIPS) most commonly used for GLORIA processing. These techniques are tested by creating digital mosaics of merged GLORIA sidescan and Hydrosweep pseudo-sidescan data from the vicinity of the Juan Fernandez microplate along the East Pacific Rise (EPR).

A history of the Easter microplate, 5.25 Ma to present

GLORIA and other geophysical data are used to interpret the morphology and tectonic pattern of the Easter microplate. We model magnetic anomaly data and calculate spreading rates for use in Euler pole calculations. We calculate three‐plate closure and best fit poles for Nazca‐Pacific, Nazca‐Easter, and Pacific‐Easter plate motions and compare the results with earthquake focal mechanisms and seismicity. We use the three‐plate closure poles to rotate digital GLORIA and magnetic isochron data of the microplate and surrounding major plates back in time since the microplate's East Rift first started to propagate northward at around 5.25 Ma. The microplate has evolved in two stages. The first is from 5.25 Ma to around 2.5 Ma, and involved continuous northward propagation of the East Rift, while the West Rift became segmented along new transform faults as it adjusted to keep the overall Nazca‐Pacific velocities constant. The transpressive northern boundary migrated with the East Rift tip, while the transtensional southern boundary remained more or less still. At some time after 2.5 Ma the East Rift ceased northward propagation and the Southwest Rift began to open up along the transtensional Pacific‐Easter transform. A convergent northern boundary formed involving southward thrusting of the Nazca plate over the microplate. No further Nazca plate lithosphere was transferred from north of the microplate to the microplate interior, and the Pito Deep region opened by pervasively rifting apart preexisting Nazca plate lithosphere.

Fluid expulsion sites on the Cascadia accretionary prism: Mapping diagenetic deposits with processed GLORIA imagery

Point‐discharge fluid expulsion on accretionary prisms is commonly indicated by diagenetic deposition of calcium carbonate cements and gas hydrates in near‐surface (<10 m below seafloor; mbsf) hemipelagic sediment. The contrasting clastic and diagenetic lithologies should be apparent in side scan images. However, sonar also responds to variations in bottom slope, so unprocessed images mix topographic and lithologic information. We have processed GLORIA imagery from the Oregon continental margin to remove topographic effects. A synthetic side scan image was created initially from Sea Beam bathymetric data and then was subtracted iteratively from the original GLORIA data until topographic features disappeared. The residual image contains high‐amplitude backscattering that we attribute to diagenetic deposits associated with fluid discharge, based on submersible mapping, Ocean Drilling Program drilling, and collected samples. Diagenetic deposits are concentrated (1) near an out‐of‐sequence thrust fault on the second ridge landward of the base of the continental slope, (2) along zones characterized by deep‐seated strikeslip faults that cut transversely across the margin, and (3) in undeformed Cascadia Basin deposits which overlie incipient thrust faults seaward of the toe of the prism. There is no evidence of diagenetic deposition associated with the frontal thrust that rises from the dècollement. If the dècollement is an important aquifer, apparently the fluids are passed either to the strike‐slip faults which intersect the dècollement or to the incipient faults in Cascadia Basin for expulsion. Diagenetic deposits seaward of the prism toe probably consist dominantly of gas hydrates.

Acoustic scattering from the seafloor: Modeling and data comparison

An existing model of seafloor backscattering [D. R. Jackson, D. P. Winebrenner, and A. Ishimaru, J. Acoust. Soc. Am. 79, 1410–1422 (1986)] was implemented with the additions of volume scattering from a random inhomogeneous continuum and scattering from subbottom interfaces. Results of computer simulations with the extended model were compared with values of scattering strength obtained from processed GLORIA data from the Monterey Fan off the coast of California. Regions of well-delineated high and low backscatter are seen in the GLORIA imagery. The geoacoustic input parameters for the simulation runs were either taken directly from or estimated from core data obtained by ground truth sampling in the image area. From the model simulation results it appears that the main factor causing the observed dichotomous character of the strength of returns in the Fingers Area is the presence or absence of a topmost, inhomogeneous silt-clay layer (with a thickness of around 1–1.5 m). In the high backscatter areas, inhomogeneous continuum volume scattering from the silt-clay layer predominates. In the low backscatter areas interface scattering from an acoustically hard, shallow subbottom sand interface (0–10 cm below the water–sediment interface) predominates. These two additions to the scattering model allow use of the ground truth measurements to constrain the input parameter values.

Toward remote seafloor classification using the angular response of acoustic backscattering: a case study from multiple overlapping GLORIA data

While the average seafloor backscatter strength within a narrow range of grazing angles can be used as a first-order classification tool, this technique often fails to distinguish seafloors of known differing geological character. In order to resolve such ambiguities, it is necessary to examine the variation in backscatter strength as a function of grazing angle. For this purpose, a series of multiply overlapping GLORIA sidescan sonar images (6.5 kHz) have been obtained in water depths ranging from 1000 to 2500 m. To constrain the placement of acoustic backscatter measurements and to measure the true impinging angle of the incident wave, the corresponding seafloor was simultaneously surveyed using the Seabeam multibeam system. As a result of the multiple overlap, the angular response of seafloor backscatter strength may be derived for regions much smaller than the swath width. By using the derived angular response of seafloor backscatter strength in regions for which sediment samples exist, an empirical seafloor classification scheme is proposed based on the shape, variance, and magnitude of the angular response. Because of the observed variability in the shape of the angular response with differing seafloor types, routine normalization of single-pass swath data to an equivalent single grazing angle image cannot be achieved. As a result, for the case of single-pass surveys, confident seafloor classification may only be possible for regions approaching the scale of the swath width.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Near real‐time merging of Gloria and Hydrosweep pseudo‐sidescan images

A method has been developed for merging GLORIA and Hydrosweep data into single combined sidescan‐type images, by adding a shaded relief representation of the seafloor topography to the near‐nadir part of the GLORIA image, which is usually poorly insonified. Several examples are presented, taken from a variety of seafloor terrains.

Interference fringes on GLORIA side-scan sonar images from the Bering Sea and their implications

GLORIA side-scan sonographs from the Bering Sea Basin show a complex pattern of interference fringes sub-parallel to the ship's track. Surveys along the same trackline made in 1986 and 1987 show nearly identical patterns. It is concluded from this that the interference patterns are caused by features in the shallow subsurface rather than in the water column. The fringes are interpreted as a thin-layer interference effect that occurs when some of the sound reaching the seafloor passes through it and is reflected off a subsurface layer. The backscattered sound interferes (constructively or desctructively) with the reflected sound. Constructive/destructive interference occurs when the difference in the length of the two soundpaths is a whole/half multiple of GLORIA's 25 cm wavelength. Thus as range from the ship increases, sound moves in and out of phase causing bands of greater and lesser intensity on the GLORIA sonograph. Fluctuations (or ‘wiggles’) of the fringes on the GLORIA sonographs relate to changes in layer thickness. In principle, a simple three dimensional image of the subsurface layer may be obtained using GLORIA and bathymetric data from adjacent (parallel) ship's tracks. These patterns have also been identified in images from two other systems; SeaMARC II (12 kHz) long-range sonar, and TOBI (30 kHz) deep-towed sonar. In these, and other cases world-wide, the fringes do not appear with the same persistence as those seen in the Bering Sea.

GLORIA and other geophysical studies of the tectonic pattern and history of the Easter Microplate, southeast Pacific

Abstract In October–November 1988 the RRS Charles Darwin surveyed the Easter Microplate using the GLORIA (long-range sidescan) sonar system and also obtained magnetic, bathymetric and gravity data. The aim of the cruise was to produce a sidescan mosaic covering most of the microplate, in order that a greater understanding of the region could be achieved. Previous geophysical studies of the area had enabled most of the microplate boundaries to be determined to a reasonable extent, but still little was known about the interior of the microplate, or its tectonic history. The major objectives of the GLORIA survey were to image the whole microplate including its boundary, discover the type of plate boundary element throughout (e.g. fast or slow spreading, transform or compressional zone), calculate the spreading directions and rotation poles, and determine the complex history of the microplate from the tectonic spreading fabric, fracture zone and propagating rift traces, and magnetic anomaly data. The GLORIA survey imaged virtually all of the Easter microplate in just 18 days, and revealed unprecedented detail and complexity in its tectonic fabric and history. There is clear evidence of rapid microplate rotation, of rapidly evolving plate boundaries and of complex deformation near the northern and southern microplate boundaries. Examination of GLORIA and magnetic data to the west of the East Rift of the Easter Microplate indicate that spreading may have been taking place for over 6 Ma and the calculation of poles of rotation for different stages of the Easter Microplate evolution have enabled a simplified history to be reconstructed.

Shallow structure and active tectonism of an offshore continental back-arc spreading system: the Taupo Volcanic Zone, New Zealand

The Taupo Volcanic Zone (TVZ), the predominant active volcano-tectonic structure within the New Zealand sub-continent, marks the zone of Quaternary back-arc spreading and calc-alkaline volcanism associated with the convergence of the Pacific and Australian plates at the Hikurangi plate-boundary margin, North Island, New Zealand. New bathymetric, shallow seismic reflection, magnetic anomaly, and GLORIA data presented here accurately define for the first time the extent, structure and tectonism of the poorly known offshore portion of the TVZ. These data show that nearly half of the entire ca. 40 km wide TVZ lies offshore, extending northeast for some 150 km seaward to 36°45′S. The southeastern and northwestern boundaries of the offshore TVZ are significant tectonic and bathymetric features, which comprise inward facing normal fault zone (the White Island and Tauranga Fault Zones, respectively). Three, essentially parallel, tectono-morphological units are identified along the length of the offshore TVZ. They comprise (from southeast to northwest) a frontal non-volcanic graben, a volcanic ridge, and a volcanic “back-arc” graben. The spatial position of these units with respect to the Hikurangi subduction margin is not a simple “arc” and “back-arc” relationship. Tectonism within the offshore TVZ, although pervasive, increases from northwest to southeast, so that the area of highest extension and subsidence of the non-volcanic graben occurs in front of the volcanic ridge. Geophysical data also suggest the possible tectonic ablation of blocks of the forearc hinterland into the offshore TVZ along its southeastern margin. The southern Havre Trough and northern TVZ are offset sinistrally by 45–50 km. They overlap by 40 km, and differ in orientation by ca. 7°. Dextral displacement between these two extensional regimes is accomodated by a series of dextrally oblique and en-échelon “bookshelf” faults. The orientation of extensional structures within the offshore TVZ is also dextrally oblique, by 10°–15° to the regional 030° trend of the bounding faults, and agree with axes of maximum extension determined elsewhere from geodetic and seismological data.

Modeling acoustic backscatter from the seafloor by long-range side-scan sonar.

An existing model of seafloor backscattering [Jackson etal., J. Acoust. Soc. Am. 79, 1410–1422 (1986)] was extended to include volume scattering from a random homogeneous continuum and scattering from subbottom interfaces. Results of computer simulations with the extended model were compared with values of scattering strength obtained from processed gloria data from the Monterey Fan off the coast of California. Regions of well delineated high and low backscatter are seen in the gloria imagery. The geoacoustic input parameters for the simulation runs were either taken directly from or estimated from core data obtained by ground truth sampling in the image area. From the model simulation results, it was found that the low backscatter region is dominated by interface scattering from a single subbottom interface over a thick homogeneous sand layer. The high return regions are dominated by scattering from the random inhomogeneous continuum. These two additions to the scattering model allow use of the ground truth measurements to constrain the input parameter values. No free parameters are required to fit the scattering strength data. [Work supported by Naval Research Laboratory, Code 5100.]

Fluid expulsion from the Cascadia accretionary prism: evidence from porosity distribution, direct measurements, and GLORIA imagery

Fluid expulsion from the Cascadia accretionary prism off Oregon results from porosity reduction by compaction, and by cementation as methane-rich pore waters precipitate diagenetic carbonate deposits near the sediment-water interface. Porosity changes suggest that dewatering begins 5-6 km west of the base of the slope, in a proto-deformation zone, GLORIA imagery of surficial carbonate deposits confirms that fluid is actively expelled from this zone; there is no such evidence further west in Cascadia Basin. Within the uncertainties of the data, porosities do not decrease landward beneath the prism. This pattern is consistent with imbricate thrust faulting on the slope which provides the vertical load to induce compactive dewatering, and may physically import as much as 50% of the total fluid volume in the section. A simple vertical compaction model suggests that significant pore water volumes have been expelled from the lower slope, but at flux rates (10 -11 -10 -12 m 3 m -2 s -1 ) which are orders of magnitude less than those measured at individual vent sites (10 -6 m 3 m -2 s -1 ). Faulting clearly controls some fluid expulsion, but GLORIA data suggest that repeated local discharge, cementation, and abandonment lead to dispersed accumulations of diagenetic carbonate.

Improving GLORIA images using Sea Beam data

GLORIA and Sea Beam data are processed together with a number of benefits. The Sea Beam data allow an improved slant‐ to ground‐range projection of the imagery that results in less of the distortions, such as layover and foreshortening, that are produced when the seafloor is assumed to be flat. A calculation originally intended to quantify side scan sonar data has also been found to improve the images in an image enhancement sense. This calculation involves estimating the seafloor's backscatter strength from GLORIA side scan sonar data using a simple physical model and the characteristics of the GLORIA instrument. The backscatter strength is the ratio of backscattered to incident sonar intensity on the sea bottom, normalized to the surface area covered by the acoustic pulse. It is computed by correcting the GLORIA data for propagation losses, beam directivity pattern, time‐varied amplifier gain and other factors. The calculation is improved by accounting for refraction in the water column and for some topographic effects using Sea Beam data. A GLORIA image from a mid‐ocean ridge crest is shown to have a large 60‐dB range that is due to the effect of the system response, such as the beam pattern, on the images. The corresponding image of the seafloor backscatter strength has an almost Gaussian distribution of picture element digital numbers with a narrow range, 20 dB, showing that most of the system effects have been removed. Cross sections through the backscatter strength image show flat trends superimposed with finer variations due to the seafloor geology. The conversion of GLORIA data into seafloor backscatter strength serves two purposes; it provides quantitative information that can be used to determine the likely causes of particular features in the images and it performs what is effectively a deterministic “shading correction”.

Geology along the Fairweather-Queen Charlotte Fault System off Southeast Alaska and British Columbia from GLORIA Images and Seismic-Reflection Data: ABSTRACT

GLORIA images collected in 1989 along southeast Alaska and British Columbia strikingly show the active trace of the Fairweather-Queen Charlotte transform fault system beneath the outer shelf and slope; seismic-reflection data are used to track the fault system across the continental shelf where GLORIA data are not available. From Cross Sound to Chatham Strait, the fault system is comprised of two sets of subparallel fault traces separated by 3 to 6 km. The fault system crosses the shelf from Icy Point to south of Yakobi Valley, then follows the shelf edge to Chatham Strait. Between Chatham Strait and Dixon Entrance, a single, sharply defined active fault trace underlies the upper and middle slope. This fault segment is bounded on the seaward side by a high, midslope ridge and by lower slope Quaternary( ) anticlines up to 35 km wide. Southeast of Dixon Entrance, the active fault trace trends back onto the outer shelf until midway along the Queen Charlotte Islands, then cuts back to and stays at midslope to the Tuzo Wilson Knolls south of the Queen Charlotte Islands. The fault steps westward at Tuzo Wilson Knolls, which are likely part of a spreading ridge segment. Major deep-sea fans along more » southeast Alaska show a southeastward age progression from older to younger and record both point source deposition at Chatham Strait and Dixon Entrance and subsequent (Quaternary ) offset along the fault system. Subsidence of ocean plate now adjacent to the Chatham Strait-Dixon Entrance fault segment initiated development of both Mukluk and Horizon Channels. « less

Acoustic image‐processing software

Several algorithims that display, enhance and analyze side‐scan sonar images of the seafloor, have been developed by the University of Washington, Seattle, as part of an Office of Naval Research funded program in acoustic image analysis. One of these programs, PORTAL, is a small (less than 100K) image display and enhancement program that can run on MS‐DOS computers with VGA boards. This program is now available in the public domain for general use in acoustic image processing.PORTAL is designed to display side‐scan sonar data that is stored in most standard formats, including SeaMARC I, II, 150 and GLORIA data. (See image.) In addition to the “standard” formats, PORTAL has a module “front end” that allows the user to modify the program to accept other image formats. In addition to side‐scan sonar data, the program can also display digital optical images from scanners and “framegrabbers,” gridded bathymetry data from Sea Beam and other sources, and potential field (magnetics/gravity) data. While limited in image analysis capability, the program allows image enhancement by histogram manipulation, and basic filtering operations, including multistage filtering. PORTAL can print reasonably high‐quality images on Postscript laser printers and lower‐quality images on non‐Postscript printers with HP Laserjet emulation. Images suitable only for index sheets are also possible on dot matrix printers.