Long-range sonar and photographic studies of the median valley in the FAMOUS area of the Mid-Atlantic Ridge near 37°N

Laughton, A. S.; Rusby, J.S.M.

doi:10.1016/0011-7471(75)90070-4

Cited by 14 publications

(12 citation statements)

References 4 publications

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“…The measured 10-to-15-dB variation was mostly due to the variation of the seafloor slope over the small study area (+20 ø) rather than to a variation in the angle of depression of the incident sonar (<10ø). Some semblance of relief occurs where abyssal hills cast acoustic shadows, which also increase the contrast in the images; these impressions of relief correspond to narrow beam echo sounder measurements [Laughton and Rusby, 1975;Mitchell, 1989, Section 4.2.1.1]. 1.…”

Section: Tim Interpretation Of Gloria Imagesmentioning

confidence: 82%

“…These systems record the backscattered signal as a function of travel time (Figure 2c), which is traditionally converted to a function of horizontal range by using the measured water depth beneath the sonar and assuming that the seafloor is flat [e.g., Teleki et al, 1981]. Because side scanning radars record backscatter amplitudes with travel time or range, the effect in radar images is to shorten slopes which face the sensor and to lengthen slopes which face away, resulting in a parallax opposite to that found in optical systems [Laughton and Rusby, 1975]. In the use of airborne side-imaging radars (which operate in a geometrically similar fashion to side scan sonars), the effects of distortions (foreshortening) are well known [e.g., Trevett, 1986].…”

Section: Posmoning Gloria Data Using Sea Beam To•3•atd'hvmentioning

confidence: 99%

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Improving GLORIA images using Sea Beam data

Mitchell¹

1991

J. Geophys. Res.

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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”.

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Section: Tim Interpretation Of Gloria Imagesmentioning

confidence: 82%

Section: Posmoning Gloria Data Using Sea Beam To•3•atd'hvmentioning

confidence: 99%

Improving GLORIA images using Sea Beam data

Mitchell¹

1991

J. Geophys. Res.

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“…We interpret these as indicating the presence of axial volcanic ridges (e.g. Laughton and Rusby, 1975).…”

Section: N O R T H E R N S P R E a D I N G C E N T R Ementioning

confidence: 98%

Morphology and tectonics of the Romanche Transform and its environs

1994

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Abstract.We describe the results of GLORIA surveys of the Romanche Transform and neighbouring areas of the Equatorial Atlantic Ocean. These surveys have imaged the whole of the Romanche Transform Fault, the eastern end of the Saint Paul Transform, the spreading centre joining them, and the northern part of the spreading centre between Romanche and Chain transforms. They have also imaged representative parts of the off-axis seafloor structure in these areas, and have facilitated preparation of a new bathymetric map, which is presented here. Contrary to previous descriptions of the area, we have found that there is 'normal' spreading fabric (volcano-tectonic lineaments trending NNW-SSE orthogonal to the spreading direction) between these major equatorial fracture zones. This implies that normal seafloor spreading processes have operated there. The Romanche Transform Domain, within which normal spreading fabric is either absent or heavily modified, and transform-related structures predominate, is about 100 km wide. There are multiple transform faults within Saint Paul Fracture Zone, of which we have imaged the active traces of the two southernmost ones. These transforms are joined by short but otherwise normal spreading segments. The Romanche Transform is shorter than previously thought, with a length of 840 km. This is equivalent to an age offset of about 50 Ma. The transform has a complex system of short normal and oblique spreading centres at its eastern intersection, forming an overall oblique section of ridge which may have recently cut off the corner of an earlier, simple, orthogonal ridge-transform intersection. The recent complex system appears to have developed at about 1.5 Ma. An oblique valley is mapped south of the western end of Romanche Transform, and is thought to be the fracture zone trace of an old transform offset between Romanche and Chain transforms. This transform disappeared at about chron 8 (26.9 Ma), as its trace is not seen between 20°W and the spreading centre.

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“…Until recently, this portion of the MAR had been subject to only a few detailed studies (Searle 1980;Luis et al 1994;Detrick et al 1995), concentrated on segments FAMOUS (e.g. Laughton & Rusby 1975;Whitmarsh & Laughton 1976;Ballard & van Andel 1977;Macdonald & Luyendyk 1977;ARCYANA 1978;Gould & Karson 1985;Detrick et al 1995) and Lucky Strike (e.g. Langmuir et al 1993;Fouquet et al 1994).…”

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confidence: 99%

Segmentation of the Mid-Atlantic Ridge south of the Azores, based on acoustic classification of TOBI data

Blondel

1996

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The European MARFLUX/ATJ project has investigated volcanic, tectonic and hydrothermal processes along 500 km of the Mid-Atlantic Ridge near the Azores. During cruises CD89-CD90, multibeam bathymetry and high-resolution TOBI sidescan sonar data were acquired and added to the existing Geographical Information System. From the Azores Triple Junction, between 35 ~ and 38~ the ridge runs 227 ~ oblique to the 100 ~ spreading direction. It is partitioned into several short ridge segments dominated by tectonism rather than volcanism, and is characterised by abundant hydrothermal activity. Textural analysis quantifies the second-order statistics of TOBI imagery, enabling the detection of details invisible to the human eye. The results of this analysis are used to quantify the relationship between tectonism and volcanism along-axis, and suggest declining neovolcanic activity away from the Triple Junction. They show that the discontinuities are heavily sedimented, whereas the segments are mostly volcanic and the segment ends are more densely faulted. Textural analysis quantifies the differences in style and activity of the different spreading segments, and the evolution of the discontinuities.

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Long-range sonar and photographic studies of the median valley in the FAMOUS area of the Mid-Atlantic Ridge near 37°N

Cited by 14 publications

References 4 publications

Improving GLORIA images using Sea Beam data

Improving GLORIA images using Sea Beam data

Morphology and tectonics of the Romanche Transform and its environs

Segmentation of the Mid-Atlantic Ridge south of the Azores, based on acoustic classification of TOBI data

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