monofractal and multifractal approaches in investigating scaling properties in temporal patterns of the 1983 2000 seismicity in the western corinth graben greece abstract much recent work has been focused on understanding the statistical properties of time occurrence series of earthquakes investigating into the patterns of seismic sequences reveals evidence of scaling features in temporal behavior this is shown in the clustering properties of seismicity of the western corinth graben greece from 1983 to 2000 power law behavior has been found by means of allan factor analysis and fourier spectra for the earthquake sequence with consistent values for the scaling exponents that decrease with the threshold magnitude the analysis of the temporal variation of the scaling exponent performed with different threshold magnitudes reveals an enhancing of the clusterization in correspondence to larger events the multifractal analysis of the temporal distribution of the events has shown a decrease of the intermittent character with the threshold magnitude

characterization of seafloor pockmark seepage of hydrocarbons employing fractal a case study from the western continental margin of india here we apply quantitative technique to describe the seafloor seepages based on the multibeam backscatter and bathymetric investigations to characterize the pockmark morphology the variable seafloor backscatter strength for coarser seafloor sediments are related to the diagenesis derived from biodegraded seepages in this regard box counting method is used to estimate fractal dimension for backscatter imagery data of 398 blocks these blocks are further sub grouped into six classes depending on the spread of pockmark related seepages the study area lies 102 km west off marmagao along the central west coast of india which contains pre dominantly 70 gas charged sediments comparison between the estimated self similar fractals reveals that there is approximately 97 correlation between the box dbox and information dinfo dimensions box dimension derived fractal dimension values suggest that the seepages are more along the fault trace in deeper waters in comparison to sparsely distributed shallow water seepages besides this poor seepage is confined within the smooth to moderately rough seafloor it is established that the high backscatter strength along the upper slope of the pockmark region having higher fractal dimensions reflects multifractal behavior of seepage distribution entire area indicates patchy seepage patterns as supported by estimated fractal values showing intermittent fluctuations which emphasizes non linear behavior estimated self organizing criticality soc parameters for six representative blocks reveal that the nature of pockmark fault trace sediment nature coupled with slumping of pockmark s wall sediment movement due to bottom currents are controlling the dynamic balance in the area seepage system further our study emphasizing the multifractal behavior of seepage blocks clearly depicts the drift in the seepage pattern

automated feature extraction and spatial organization of seafloor pockmarks belfast bay maine usa article i nfo seafloor pockmarks occur worldwide and may represent millions of m 3 of continental shelf erosion but few numerical analyses of their morphology and spatial distribution of pockmarks exist we introduce a quantitative definition of pockmark morphology and based on this definition propose a three step geomorphometric method to identify and extract pockmarks from high resolution swath bathymetry we apply this gis implemented approach to 25 km 2 of bathymetry collected in the belfast bay maine usa pockmark field our model extracted 1767 pockmarks and found a linear pockmark depth to diameter ratio for pockmarks field wide mean pockmark depth is 7 6 m and mean diameter is 84 8 m pockmark distribution is non random and nearly half of the field s pockmarks occur in chains the most prominent chains are oriented semi normal to the steepest gradient in holocene sediment thickness a descriptive model yields field wide spatial statistics indicating that pockmarks are distributed in non random clusters results enable quantitative comparison of pockmarks in fields worldwide as well as similar concave features such as impact craters dolines or salt pools published by elsevier b v

structural highs on the western continental slope of india implications for regional tectonics abstract integrated geological and geophysical explorations over the last two decades along the central western continental margin of india reveal from east to west a terrace like flat shelf an advancing shelf break a nw se oriented wide shelf margin basin the upper slope ridge the prathap ridge and the laccadive ridge the basin on the continental slope extends between the shelf break and the eastern slope of the laccadive ridge and is divided longitudinally into a relatively narrow eastern and a wide western basin by the prominent upper slope ridge detailed multibeam swath mapping now over an area of little more than 20 300 km 2 within the shelf margin basin located on the upper continental slope now reveals the occurrence of several isolated structural highs of variable height four of these highs are located over the upper slope ridge and a similar number on the prathap ridge of these four highs have summit height 1000 m two have heights between 500 and 999 m while the remaining two have heights less than 499 m the digital terrain model developed from the processed multibeam swath bathymetry data offers a three dimensional perspective of these structural highs the tectonic understanding from multibeam bathymetry findings and satellite gravity data supported by other geophysical information suggest that these structural highs are likely to be the peaks of the upper slope ridge and the prathap ridge we discuss the possible origin and mode of emplacement of these structural highs and ridges in the light of regional tectonics and suggest that both the upper slope and prathap ridges may have been formed during the separation of india from madagascar during the middle cretaceous

australian offshore natural hydrocarbon seepage studies a review and re evaluation abstract surprisingly few natural hydrocarbon seeps have been identified in australia s offshore basins despite studies spanning thirty years early studies of natural hydrocarbon seepage around the australian margin were generally based on the geochemical analysis of stranded bitumens water column geochemical sniffer sampling synthetic aperture radar or airborne laser fluorosensor later studies involved the integration of these remote sensing and geochemical techniques with multi channel and shallow seismic a review of these earlier studies indicates that many seepage interpretations need to be re evaluated and that previous data sets when placed in a global context often represent normal background hydrocarbon levels low recent burial and subsidence rates are not favourable for high rates of seepage there are also difficulties in proving seepage on high energy shallow carbonate shelves where seabed features may be rapidly re worked and modern marine signatures are overprinted on authigenic seep carbonates thus the relatively few sites of proven natural hydrocarbon seepage in australia s offshore sedimentary basins can be reconciled relative to their geological occurrences and the dominantly passive margin setting active thermogenic methane seepage on the yampi shelf the only proven documented occurrence in australia is driven by deposition of a thick late tertiary carbonate succession and late miocene tectonic reactivation therefore to increase the success of detecting and correctly interpreting natural hydrocarbon seepage data need to be analysed and integrated within the context of the local geological setting and with an understanding of what is observed globally

complex pockmarks with carbonate ridges off mid norway products of sediment degassing abstract a new type of pockmark has been discovered off mid norway they contain up to 10 m high ridges of methane derived authigenic carbonate mdac rock the pockmarks were investigated using rovs remotely operated vehicles in 2003 and 2004 the complex pockmarks are located well within the gas hydrate pressure temperature stability field and are associated with a regional bsr bottom simulating reflector our high resolution seismic data demonstrate that the near surface sedimentary bedding dips into vertical pipe structures underlying the pockmarks and that the pipes are deeply rooted the pockmarks are circular in plan view and contain up to 190 m long and 40 m wide ridges formed by up to 4 m wide carbonate rocks piled on top of each other in irregular piles other key features of the complex pockmarks include 1 a distinct fauna with local bacterial mats small tubeworms stalked crinoids and pycnogonids sea spiders and 2 the presence of light hydrocarbon gases c 1 c 5 in clay rich sediments the methane 13 c values range from 54 to 69 pdb suggesting the presence of both bacterial and thermogenic gases during our two day observation periods only micro seepage occurred without ebullition or other visual fluid flow petrography and geochemistry of carbonate blocks from the pockmarks suggest precipitation of methane derived aragonite within the sediments 13 c 52 to 58 pdb we suggest that the complex pockmarks formed by sudden catastrophic fluid flow and that the large carbonate ridges represent dislodged mdacs originally formed subsurface prior to the event subsequent micro seepage is still active and may indicate that the pipes represent fluid flow conduits which transport both bacterial and thermogenic hydrocarbons to the water column our data do not suggest pressure buildup and do not predict future eruptions from the complex pockmarks thus our data rather suggest that the catastrophic formation of the complex pockmarks may be a one time event

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

assessment of groundwater circulation in la gomera aquifers canary islands spain from their hydrochemical features la gomera canary islands spain does not show water scarcity like other islands of the archipelago however the study of its aquifers is paramount as nearly 60 of the water supplies are covered with spring water according to the currently accepted hydrogeological model la gomera presents an upper aquifer consisting of perched groundwater bodies below this hydrogeological unit the general saturated zone or basal groundwater is placed the model also establishes the presence of flows through them many perched groundwater bodies are located under garajonay national park where most of the springs are found therefore if upper and lower aquifers are truly connected and new wells are built the new extractions could affect garajonay ecosystems with the aim of identifying spring groups and related potential areas of water transfer hydrochemical and statistical analyses principal component analysis and cluster analysis have been applied this study shows the great compositional variability of groundwaters precluding the identification of spring groups hydrochemical patterns and therefore the transfer areas with no possibility of assessing the potential impact of a water demand increase on the garajonay national park ecosystems from the present data only the springs belonging to group ii of the cluster analysis could indicate a transfer area the lack of conclusive results could be due to 1 great compositional variability of volcanic materials 2 unequal influence of marine aerosols 3 irregular distribution of rainfall 4 different grades of soils development and 5 the occurrence of partially disconnected water bodies giving as a result a complex hydrogeological system

pockmark like depressions near the goliat hydrocarbon field barents sea morphology and genesis abstract pockmarks are observed worldwide along the continental margins and are inferred to be indicators of fluid expulsion in the present study we have analysed multibeam bathymetry and 2d 3d seismic data from the south western barents sea in relation to gas hydrate stability field and sediment type to examine pockmark genesis seismic attributes of the sediments at and beneath the seafloor have been analysed to study the factors related to pockmark formation the seabed depths in the study area are just outside the methane hydrate stability field but the presence of higher order hydrocarbon gases such as ethane and or propane in the expelled fluids may cause localised gas hydrate formation the selective occurrence of pockmarks in regions of specific seabed sediment types indicates that their formation is more closely related to the type of seabed sediment than the source path of fluid venting such as faults the presence of high acoustic backscatter amplitudes at the centre of the pockmarks indicates harder coarser sediments likely linked to removal of soft material the pockmarks show high seismic reflection amplitudes along their fringes indicating deposition of carbonates precipitated from upwelling fluids high seismic amplitude gas anomalies underlying the region away from the pockmarks indicate active fluid flow from hydrocarbon source rocks beneath which is blocked by overlying less permeable formations in areas of consolidated sediments the upward flow is limited to open fault locations while soft sediment areas allow diffused flow of fluids and hence formation of pockmarks over a wider region through removal of fine grained material

authigenic carbonates from methane seeps of the northern congo fan microbial formation mechanism abstract authigenic carbonates were collected from methane seeps at hydrate hole at 3113 m water depth and diapir field at 2417 m water depth on the northern congo deep sea fan during rv meteor cruise m56 the carbonate samples analyzed here are nodules mainly composed of aragonite and high mg calcite abundant putative microbial carbonate rods and associated pyrite framboids were recognized within the carbonate matrix the 13c values of the hydrate hole carbonates range from 62 5 to 46 3 pdb while the 13c values of the diapir field carbonate are somewhat higher ranging from 40 7 to 30 7 pdb indicating that methane is the predominant carbon source at both locations relative enrichment of 18o 18o values as high as 5 2 pdb are probably related to localized destabilization of gas hydrate the total content of rare earth elements ree of 5 hno3 treated solutions derived from carbonate samples varies from 1 6 ppm to 42 5 ppm the shale normalized ree patterns all display positive ce anomalies ce ce 1 3 revealing that the carbonates precipitated under anoxic conditions a sample from hydrate hole shows a concentric lamination corresponding to fluctuations in 13c values as well as trace elements contents these fluctuations are presumed to reflect changes of seepage flux

quantitative methods for analyzing the roughness of the seafloor the bathymetry of the world ocean has been mapped using a variety of acoustic sounding instruments and traditional contouring methods prior to this study relatively few quantitative studies have been focused on the detailed morphology of the seafloor the work of most investigators has generally dealt with the analysis of seafloor relief without regard to spatial frequency or concern for obvious morphologic anisotropies a brief review of previous work is presented and is accompanied by an identification of the specific applications and limitations of those approaches the availability of more precise digital sounding data coupled with the development of higher resolution sonar mapping instruments e g deeptow seamarc now make it possible to investigate quantitatively many important aspects of seafloor relief and the geological processes responsible for its formation typical bathymetric contour maps represent a low frequency deterministic model of the seafloor to describe the higher frequency variability or roughness of the seafloor requires the development of an appropriate statistical method for generating a valid stochastic model new methods are developed herein which allow valid statistical models of the variability of oceanic depths to be derived from existing digital bathymetric soundings the smooth contoured surface often preserved as a geographic grid of depths when supplemented by such a roughness model can provide an essentially complete statistical description of the relief statistical models of seafloor roughness are also valuable tools for predicting acoustic scattering and bottom loss and in addition contain a wealth of information for more comprehensive interpretations of deep sea relief forming geological processes to allow the variability of depths to be described as a function of scale spatial frequency the amplitude spectrum is employed as the fundamental statistic underlying the model since the validity of the amplitude spectrum depends upon the assumption of a statistically stationary sample space a computer algorithm operating in the spatial domain was developed which delineates geographic provinces of limited statistical heterogeneity within each of these provinces a spectral model is derived by fitting the amplitude estimates with one or two two parameter power law functions using specialized regression techniques the distribution of resultant model parameters is examined for a large test area adjacent to the coast of oregon 42 45 n 130 124 w which includes several contrasting geologic environments the distribution of roughness corresponds generally with the various physiographic provinces observed in the region within some provinces additional complexities are apparent in the roughness model which cannot be inferred by simply studying the bathymetry these patterns are related to a variety of geological processes operating in the region such as the convergence of the continental margin and the presence of a propagating rift on the northern gorda rise in contrast a very large area of the continental margin off the east coast of the united states was found to have in common a single distinct amplitude spectrum this amplitude spectrum is almost identical to that found from the tufts abyssal plain region off the west coast of the united states spectra from both of these areas can be clearly separated into two straight line segments of different slope the two segments of these spectra are interpreted as reflecting two dominant relief forming processes the higher frequency band representing a sedimentary regime and the lower frequency band representing an underlying tectonic volcanogenic regime in many cases the calculated roughness statistics are not constant for data collected along different ship track directions this is due to the anisotropic nature of the seafloor relief a simple model is developed which describes the roughness statistics as a function of azimuth the parameters of this model quantify the anistropy of the seafloor allowing insight into the directionality of the corresponding relief forming processes and the physical meaning of the derived model statistical parameters finally the model is used to successfully predict the roughness of a surface at scales much smaller than those resolvable by surface sonar systems the model regression line derived from a hull mounted sonar is compared to data from deep towed sonars and bottom photographs the amplitude of roughness is predicted to within half an order of magnitude over five decades of spatial frequency and this prediction capability can probably be improved even further the stochastic models presented also demonstrate the potential for closely approximating the full two dimensional nature of some areas of the seafloor with only four statistical parameters these parameters can be estimated from random ship track data given a sufficient number of tracks and range of headings

site specific microzonation study in delhi metropolitan city by 2 d modelling of sh and p sv waves delhi the capital of india lies on a severe earthquake hazard threat not only from local earthquakes but also from himalayan events just 200 250 km apart the seismic ground motion in a part of delhi city is computed with a hybrid technique based on the modal summation and the finite difference scheme for site specific strong ground motion modelling complete realistic sh and e5 e5 sv wave seismograms are computed along two geological cross sections 1 north south from inter state bus terminal isbt to sewanagar and 2 east west from tilak bridge to punjabi bagh two real earthquake sources of july 15 1720 mmi ix m 7 4 and august 27 1960 m 6 0 have been used in modelling the response spectra ratio rsr i e the response spectra computed from the signals synthesized along the laterally varying section and normalized by the response spectra computed from the corresponding signals synthesized for the bedrock reference regional model have been determined as expected the sedimentary cover causes an increase of the signal amplitude particularly in the radial and transverse components to further check the site effects we reversed the source location to the other side of the cross section and recomputed the site amplifications there are only a few sites where a large amplification is invariant with respect to the two source locations considered the rsr ranges between 5 to 10 in the frequency range from 2 8 to 3 7 hz for the radial and transverse components of motion along the ns cross section along the ew cross section rsr varies between 3 5 to 7 5 in the frequency range from 3 5 to 4 1 hz the amplification of the vertical component is considerable at high frequency 4 hz whereas it is negligible in lower frequency range

shallow gas and fluid migration in the northern arafura sea offshore northern australia abstract a variety of shallow gas indicators and fluid migration pathways have been interpreted from new sub bottom profiler multibeam bathymetry side scan sonar and echo sounder data together with geochemical analyses of sampled sediments from the northern arafura sea offshore northern australia this study provides new geological data and a seismic stratigraphy of the youngest units in the money shoal basin in an area that has not been closely studied in over 30 years the shallow gas indicators include pockmarks low frequency enhanced reflectors and acoustic blanking these indicators are supported by gas within shallow cores geochemistry indicates that this gas has a microbial origin but deeper fluid movement is also suggested by the presence of interpreted hydrocarbon slicks based on synthetic aperture radar data a region of likely hydrocarbon seepage is indicated by the clear coincidence of shallow gas indicators with an area of increased faulting and a zone of poor quality seismic data recognised on conventional seismic data together with anomalies interpreted on remote sensing data aligned with mapped deep structures

migration of methane at the diapiric structure of the western continental margin of india insights from seismic data investigations are being carried out to locate the indirect evidences of gas occurrences in the indian continental margins by identifying features like pockmarks shale diapirs venting as well as bottom simulating reflectors bsr we investigate one such case of a shale diapirs occurrence of free gas and gas hydrates is reported using a complex trace analysis to study the variation of reflection strength and instantaneous frequency along individual reflections a frequency analysis over the flank of the shale diapir indicates attenuation of high frequencies possibly representing the migration of fluids probably methane along the fault pathways similarly a plot of reflection strength along an individual reflector shows the migration pathways and significant blanking

anomalous sea floor backscatter patterns in methane venting areas dnepr paleo delta nw black sea the relation between acoustic sea floor backscatter and seep distribution is examined by integrating multibeam backscatter data and seep locations detected by single beam echosounder this study is further supported by side scan sonar recordings high resolution 5 khz seismic data pore water analysis grain size analysis and visual sea floor observations the datasets were acquired during the 2003 and 2004 expeditions of the ec funded crimea project in the dnepr paleo delta area northwestern black sea more than 600 active methane seeps were hydroacoustically detected within a small 3 96 km by 3 72 km area on the continental shelf of the dnepr paleo delta in water depths ranging from 72 m to 156 m multibeam and side scan sonar recordings show backscatter patterns that are clearly associated with seepage or with a present dune area seeps generally occur within medium to high backscatter areas which often coincide with pockmarks high resolution seismic data reveals the presence of an undulating gas front i e the top of the free gas in the subsurface which domes up towards and intersects the sea floor at locations where gas seeps and medium to high backscatter values are detected pore water analysis of 4 multi cores taken at different backscatter intensity sites shows a clear correlation between backscatter intensity and dissolved methane fluxes all analyzed chemical species indicate increasing anaerobic oxidation of methane aom from medium to high backscatter locations this is confirmed by visual sea floor observations showing bacterial mats and authigenic carbonates formed by aom grain size analysis of the 4 multi cores only reveals negligible variations between the different backscatter sites integration of all datasets leads to the conclusion that the observed backscatter patterns are the result of ongoing methane seepage and the precipitation of methane derived authigenic carbonates mdacs caused by aom the carbonate formation also appears to lead to a gradual self sealing of the seeps by cementing fluid pathways horizons followed by a relocation of the bubble releasing locations

stochastic modeling of seafloor morphology inversion of sea beam data for second order statistics at scale lengths less than 100 km or so statistical descriptions of seafloor morphology can be usefully employed to characterize ridge crest processes off ridge tectonics and vulcanism sedimentation and postdepositional transport we seek to develop methods for the estimation of seafloor statistics that take into account the finite precision resolution and sampling obtained by actual echo sounding systems in this initial paper we restrict our attention to the problem of recovering second order statistics from data sets collected by multibeam devices such as sea beam the seafloor is modeled as a stationary zero mean gaussian random field completely specified by its two point covariance function we introduce an anisotropic two point covariance function that has five free parameters describing the amplitude orientation characteristic wave numbers and hausdorff fractal dimension of seafloor topography we formulate the general forward problem relating this model to the statistics of an ideal multibeam echo sounder in particular the along track autocovariance functions of individual beams and the cross covariance functions between beams of arbitrary separation using these second moments as data functionals we then pose the inverse problem of estimating the seafloor parameters from realistic noisy data sets with finite sampling and beamwidth and we solve this inverse problem by an iterative linearized least squares method the inversion method is applied to sea beam transit data from both the pacific and atlantic oceans sea beam system noise stands out as a sharp spike on the along track autocovariance function and can be modeled as a white noise process whose amplitude generally increases with beam angle the five parameters in our second order model can be estimated from the inversion of data sets comprising 100 200 km of track length in general the cross track wave number is the most poorly determined although uncertainties in the assumed sea beam response may bias the values of the fractal dimension using the assumed beamwidth the measured noise values and the seafloor parameters recovered from the inversion we generate sea beam synthetics whose statistical character can be directly compared with raw sea beam data for most of the track segments we have processed thus far the synthetics are similar to the data in the case of one atlantic profile however the comparison clearly indicates the necessity of incorporating higher order statistics the space domain procedures described in this paper can be extended for this purpose

precession phasing offset between indian summer monsoon and arabian sea productivity linked to changes in atlantic overturning circulation results from transient climate modeling experiments indicate an in phase relationship between insolation forcing and indian summer monsoonal precipitation this is in contrast to high resolution radioisotopically dated speleothem oxygen isotope 18o records of china which showed that east asian monsoon maxima lag northern hemisphere peak summer insolation by 2 700 years while an approximately 8 000 year time lag was derived from late pleistocene records of arabian sea sediments here we evaluate the precession phase of the arabian sea signal by comparing a new high resolution productivity and oxygen minimum zone omz intensity record from the arabian sea over the past 450 000 years with the results of a transient climate modeling experiment that includes glacial bound ice volume variations the well established tuning technique between radioisotopically dated north atlantic cold events and the occurrence of deep dwelling planktonic foraminifera in the arabian sea for the last glacial cycle was used to extend the arabian sea chronology independent of orbital tuning cross spectral analysis over the last 224 000 years reveals that arabian sea productivity maxima lag precession minima by 6 900 200 years i e in close agreement with previous reconstructions also our climate modeling simulations are in accord with previous studies indicating an in phase relationship between precession minima and maximum summer monsoon intensity we argue that the summer monsoon is most likely not the main driver of changes in arabian sea biological productivity and omz intensity at the precession frequency band but that changes in the intensity of the atlantic meridional overturning circulation amoc have played the prominent role in controlling the nutrient delivery into the euphotic layer of the northern indian ocean and hence the amount of primary productivity and intensity of the oxygen minimum zone in the arabian sea such a mechanism explains the large precession related time lag between minimum precession and maximum productivity and omz conditions in the arabian sea since intensified amoc occurred during precession maxima

new evidence for enhanced preservation of organic carbon in contact with oxygen minimum zone on the western continental slope of india paropkari a l prakash babu c and mascarenhas a 1993 new evidence for enhanced preservation of organic carbon in contact with oxygen minimum zone on the western continental slope of india mar geol i 11 7 13 in order to evaluate the reason for organic enrichment on the western continental slope of india we provide rock eval pyrolysis data of 25 surficial sediment samples from this area results show that the organic matter on the slope is hydrogenrich type ii kerogen which suggests that the organic carbon there is mostly marine the low productivity of overlying waters rules out surface productivity as a possible cause of organic enrichment the association of higher hydrogen index hi values average 306 and higher organic carbon contents av 3 28 wt with the sediments of the upper slope in contact with the oxygen minimum zone omz clearly indicates enhanced preservation under reducing conditions in the anoxic waters of the omz it is suggested that the sediments of the upper slope in contact with the omz have excellent hydrocarbon generation potential and that these slope beds deserve preferential attention for hydrocarbon exploration

seafloor classification of the mound and channel provinces of the porcupine seabight an application of the multibeam angular backscatter data in this study multibeam angular backscatter data acquired in the eastern slope of the porcupine seabight are analysed processing of the angular backscatter data using the nrgcor software was made for 29 locations comprising different geological provinces like carbonate mounds buried mounds seafloor channels and inter channel areas a detailed methodology is developed to produce a map of angle invariant normalized backscatter data by correcting the local angular backscatter values the present paper involves detailed processing steps and related technical aspects of the normalization approach the presented angle invariant backscatter map possesses 12 db dynamic range in terms of grey scale a clear distinction is seen between the mound dominated northern area belgica province and the gollum channel seafloor at the southern end of the site qualitative analyses of the calculated mean backscatter values i e grey scale levels utilizing angle invariant backscatter data generally indicate backscatter values are highest lighter grey scale in the mound areas followed by buried mounds the backscatter values are lowest in the inter channel areas lowest grey scale level moderate backscatter values medium grey level are observed from the gollum and kings channel data and significant variability within the channel seafloor provinces the segmentation of the channel seafloor provinces are made based on the computed grey scale levels for further analyses based on the angular backscatter strength three major parameters are utilized to classify four different seafloor provinces of the porcupine seabight by employing a semi empirical method to analyse multibeam angular backscatter data the predicted backscatter response which has been computed at 20 is the highest for the mound areas the coefficient of variation cv of the mean backscatter response is also the highest for the mound areas interestingly the slope value of the buried mound areas are found to be the highest however the channel seafloor of moderate backscatter response presents the lowest slope and cv values a critical examination of the inter channel areas indicates less variability within the estimated three parameters financial support of this study was granted by the european commission fifth framework project geomound contract no evk3 ct 1999 00016

subsurface gas offshore of northern california and its link to submarine geomorphology abstract the northern california continental margin contains evidence of abundant subsurface gas and numerous seafloor features that suggest a causative link between gas expulsion and geomorphology analyses of seismic reflection sidescan sonar and high resolution multibeam bathymetric data show that the occurrence of subbottom gas and the migration processes beneath the shelf differ from those beneath the slope subsurface gas inferred from enhanced reflectors and other geophysical indicators is spatially variable and related more to total depth and stratigraphy than to underlying structure with the exception of one band of gas that follows the regional structural trend shallow depressions on the seafloor pockmarks are used to infer expulsion sites the largest zone of acoustically impenetrable subsurface gas occurs between water depths of 100 m and 300 m where expulsion features are rare the upper slope water depths 400 600 m has a high concentration of pockmarks diameter 10 20 m in contrast to a near absence of pockmarks at water depths shallower than 400 m of nearly 4000 pockmarks observed on sidescan sonar records more than 95 are located in water depths deeper than 400 m bottom simulating reflectors bsrs on some seismic reflection profiles indicate the possible presence of gas hydrate we find that gas and pore fluid migration in the offshore eel river basin is 1 correlated to surface morphology 2 a contributor to seabed roughness 3 a significant mode of sediment redistribution on the upper slope and 4 potentially a factor in large slope failures