scholarly journals Temporal Variability of Thermohaline Fine-Structure Associated With the Subtropical Front Off the Southeast Coast of New Zealand in High-Frequency Short-Streamer Multi-Channel Seismic Data

2021 ◽  
Vol 8 ◽  
Author(s):  
Joanna K. Cooper ◽  
Andrew R. Gorman ◽  
M. Hamish Bowman ◽  
Robert O. Smith
Keyword(s):  
Fine Structure ◽  
New Zealand ◽  
Seismic Data ◽  
Temporal Variability ◽  
Water Masses ◽  
Seismic Reflection Data ◽  
Near Surface ◽  
Subtropical Front ◽  
Southeast Coast ◽  
Seismic Lines

Seismic oceanography generally makes use of multi-channel seismic reflection data sourced by air gun arrays and long hydrophone streamers to image oceanographic water masses and processes—often piggybacking on surveys that target deeper geological features below the seafloor. However, due to the acquisition methods employed, shallow (upper 200 m or so) regions of the ocean can be poorly imaged with this technique, and resolution is often lower than desirable for imaging fine-structure within the water column. In 2012, we collected a set of higher-resolution seismic lines off the southeast coast of New Zealand, with a generator-injector airgun source and hydrophone streamer configuration designed to improve images of shallower water masses and their boundaries. The seismic lines were acquired with coincident expendable bathythermograph deployments which provides direct ties between physical oceanographic data and seismic data, allowing for definitive identification of the Subtropical Front and associated water masses in the subsurface. Repeat acquisition along the same transect shows significant temporal variability on the scale of hours, illustrating the highly dynamic nature of this important ocean boundary. Comparisons to conventional low-frequency seismic data in the same location show the value of high-resolution acquisition methods in imaging the near-surface of the ocean and allowing subsurface features to be linked to their expressions at the surface.

3D constraints and finite-difference modeling of massive sulfide deposits: The Kristineberg seismic lines revisited, northern Sweden

Geophysics ◽  
2012 ◽  
Vol 77 (5) ◽  
pp. WC69-WC79 ◽  
Author(s):  
Mahdieh Dehghannejad ◽  
Alireza Malehmir ◽  
Christopher Juhlin ◽  
Pietari Skyttä
Keyword(s):  
Finite Difference ◽  
Seismic Data ◽  
Massive Sulfide ◽  
Northern Sweden ◽  
Seismic Reflection Data ◽  
Sulfide Deposits ◽  
Time Migration ◽  
Massive Sulfide Deposits ◽  
Prestack Time Migration ◽  
Seismic Lines

The Kristineberg mining area in the western part of the Skellefte ore district is the largest base metal producer in northern Sweden and currently the subject of extensive geophysical and geologic studies aimed at constructing 3D geologic models. Seismic reflection data form the backbone of the geologic modeling in the study area. A geologic cross section close to the Kristineberg mine was used to generate synthetic seismic data using acoustic and elastic finite-difference algorithms to provide further insight about the nature of reflections and processing challenges when attempting to image the steeply dipping structures within the study area. Synthetic data suggest processing artifacts manifested themselves in the final 2D images as steeply dipping events that could be confused with reflections. Fewer artifacts are observed when the data are processed using prestack time migration. Prestack time migration also was performed on high-resolution seismic data recently collected near the Kristineberg mine and helped to image a high-amplitude, gently dipping reflection occurring stratigraphically above the extension of the deepest Kristineberg deposit. Swath 3D processing was applied to two crossing seismic lines, west of the Kristineberg mine, to provide information on the 3D geometry of an apparently flat-lying reflection observed in both of the profiles. The processing indicated that the reflection dips about 30° to the southwest and is generated at the contact between metasedimentary and metavolcanic rocks, the upper part of the latter unit being the most typical stratigraphic level for the massive sulfide deposits in the Skellefte district.

Seismic images of the Grenville Orogen in Ontario

1994 ◽  
Vol 31 (2) ◽  
pp. 293-307 ◽  
Author(s):  
D. J. White ◽  
R. M. Easton ◽  
N. G. Culshaw ◽  
B. Milkereit ◽  
D. A. Forsyth ◽  
...  
Keyword(s):  
Shear Zone ◽  
Seismic Data ◽  
Lower Crust ◽  
Wide Band ◽  
The South ◽  
Tectonic Zone ◽  
Seismic Reflection Data ◽  
Near Surface ◽  
Crustal Shortening ◽  
The North

In 1990, Lithoprobe acquired 240 km of seismic-reflection data across parts of the Central Gneiss Belt (CGB) and the Central Metasedimentary Belt (CMB) within the western Grenville Province of southern Ontario. Interpretation of these data in conjunction with geological constraints provided by bedrock mapping supports a model of northwest-directed thrusting and crustal shortening for the Grenville Orogen. Within the CGB, the Parry Sound shear zone is imaged as a 3 km wide zone of reflections dipping southeastward at 20–25° and soling at depths < 7 km in the north and < 3 km in the south beneath Parry Sound domain. Parry Sound domain and the immediately adjacent domains are underlain by a gently southeast-dipping reflective zone at 4.5–12.0 km depth interpreted as a detachment surface, likely associated with the central Britt shear zone. This zone may have accommodated northwesterly transport of Parry Sound, southern Britt, and northwestern Rosseau domains over Britt domain during Grenvillian thrusting.Within the CMB, the seismic data indicate that crustal shortening and imbrication have not been confined to domain and terrane boundaries, as presently defined. A 6 km wide band of reflections dips south at ~20° from the surface within Bancroft terrane, soling into a mid-crustal décollement beneath Elzevir terrane. Beneath and to the north of this planar reflective zone is a complex pattern of strong, south-dipping (10–40°) reflections that extends from the near surface to the lower crust above a less reflective wedge-shaped zone. The zone of complex reflectivity projects updip to the CMB boundary zone and into the CGB; together with the linear band of reflections affiliated with Bancroft terrane, they form the tectonized boundary between the CGB and the CMB. To the south of the linear reflective zone, prominent reflective packages are restricted to the middle and upper crust. The generally nonreflective uppermost crust beneath Elzevir terrane is underlain by a series of gently southeast-dipping, antiformal reflections that appear to sole into the mid-crustal décollement beneath Mazinaw terrane. These observations suggest that the collision between the CMB and the CGB resulted in a sequence of relatively thin (15–20 km thick) allochthonous terranes within the CMB being transported along a regional décollement and thrust northwestward over footwall rocks of the CGB along a penetratively deformed tectonic zone, while a lower crustal wedge may have delaminated the CMB lower crust. Crustal thickness where defined by the seismic data is 42.0–43.5 km in both the CGB and the CMB.

scholarly journals Dimethylsulfoniopropionate (DMSP) and dimethyl sulfide (DMS) cycling across contrasting biological hotspots of the New Zealand subtropical front

Ocean Science ◽  
2017 ◽  
Vol 13 (6) ◽  
pp. 961-982 ◽  
Author(s):  
Martine Lizotte ◽  
Maurice Levasseur ◽  
Cliff S. Law ◽  
Carolyn F. Walker ◽  
Karl A. Safi ◽  
...  
Keyword(s):  
New Zealand ◽  
Dimethyl Sulfide ◽  
Degradation Products ◽  
Strong Association ◽  
Bacterial Biomass ◽  
Global Ocean ◽  
Potential Contribution ◽  
Microbial Conversion ◽  
Near Surface ◽  
Subtropical Front

Abstract. The oceanic frontal region above the Chatham Rise east of New Zealand was investigated during the late austral summer season in February and March 2012. Despite its potential importance as a source of marine-originating and climate-relevant compounds, such as dimethyl sulfide (DMS) and its algal precursor dimethylsulfoniopropionate (DMSP), little is known of the processes fuelling the reservoirs of these sulfur (S) compounds in the water masses bordering the subtropical front (STF). This study focused on two opposing short-term fates of DMSP-S following its uptake by microbial organisms (either its conversion into DMS or its assimilation into bacterial biomass) and has not considered dissolved non-volatile degradation products. Sampling took place in three phytoplankton blooms (B1, B2, and B3) with B1 and B3 occurring in relatively nitrate-rich, dinoflagellate-dominated subantarctic waters, and B2 occurring in nitrate-poor subtropical waters dominated by coccolithophores. Concentrations of total DMSP (DMSPt) and DMS were high across the region, up to 160 and 14.5 nmol L−1, respectively. Pools of DMSPt showed a strong association with overall phytoplankton biomass proxied by chlorophyll a (rs  =  0.83) likely because of the persistent dominance of dinoflagellates and coccolithophores, both DMSP-rich taxa. Heterotrophic microbes displayed low S assimilation from DMSP (less than 5 %) likely because their S requirements were fulfilled by high DMSP availability. Rates of bacterial protein synthesis were significantly correlated with concentrations of dissolved DMSP (DMSPd, rs  =  0.86) as well as with the microbial conversion efficiency of DMSPd into DMS (DMS yield, rs  =  0.84). Estimates of the potential contribution of microbially mediated rates of DMS production (0.1–27 nmol L−1 day−1) to the near-surface concentrations of DMS suggest that bacteria alone could not have sustained DMS pools at most stations, indicating an important role for phytoplankton-mediated DMS production. The findings from this study provide crucial information on the distribution and cycling of DMS and DMSP in a critically under-sampled area of the global ocean, and they highlight the importance of oceanic fronts as hotspots of the production of marine biogenic S compounds.

scholarly journals Model studies of shallow common‐offset seismic data

Geophysics ◽  
1990 ◽  
Vol 55 (4) ◽  
pp. 394-401 ◽  
Author(s):  
Thomas H. Wilson
Keyword(s):  
Seismic Response ◽  
Seismic Data ◽  
Acoustic Properties ◽  
Seismic Reflection Data ◽  
Near Surface ◽  
Model Studies ◽  
Reflection Data ◽  
Full Waveform ◽  
Geophysical Logs ◽  
Zero Offset

For simplicity, optimum‐window common‐offset data‐acquisition procedures are frequently employed to collect near‐surface, high‐resolution, seismic reflection data. However, because of large incidence angles, interpretations of the data often cannot be evaluated accurately using zero‐offset simulations alone. Common‐offset hammer seismic data collected in the central Appalachian plateau province of West Virginia are examined in this paper. Synthetic shot records using a minimum‐phase wavelet estimated from the data and subsurface acoustic properties derived from full‐waveform and other geophysical logs are used to simulate the offset seismic response of near‐surface, coal‐bearing Pennsylvanian aged rocks. Zoeppritz equations are used to model amplitudes. This study indicates that offset simulations may be required to determine the origins of events observed at a given offset. Offset simulations also help determine whether amplitude variations with offset have a significant effect on the appearance of events observed at the optimum offset. The offset seismic response is significantly different from the zero‐offset response for reflections arising from depths less than about two‐thirds of the offset distance; for greater depths, zero‐offset simulations adequately approximate the offset response.

Characterization of gas hydrate systems on the Hikurangi margin (New Zealand) through geostatistical seismic and petrophysical inversion

Geophysics ◽  
2021 ◽  
pp. 1-60
Author(s):  
Francesco Turco ◽  
Leonardo Azevedo ◽  
Dario Grana ◽  
Gareth J. Crutchley ◽  
Andrew R. Gorman
Keyword(s):  
New Zealand ◽  
Gas Hydrate ◽  
Seismic Data ◽  
Effective Means ◽  
Rock Physics ◽  
Seismic Inversion ◽  
Inversion Method ◽  
Seismic Reflection Data ◽  
Reflection Data

Quantitative characterization of gas hydrate systems on continental margins from seismic data is challenging, especially in regions where no well logs are available. However, probabilistical seismic inversion provides an effective means for constraining the physical properties of subsurface strata in such settings and analyzing the variability related to the results. We apply a workflow for the characterization of two deep-water gas hydrate reservoirs east of New Zealand, where high concentrations of gas hydrate have been inferred previously. We estimate porosity and gas hydrate saturation in the reservoirs from multi-channel seismic data through a two-step procedure based on geostatistical seismic and Bayesian petrophysical inversion built on a rock physics model for gas hydrate-bearing marine sediments. We found that the two reservoirs together host between 2.45 × 105 m3 and 1.72 × 106 m3 of gas hydrate, with the best estimate at 9.68 × 105 m3. This estimate provides a first-order assessment for further gas hydrate evaluations in the region. The two-step statistically based seismic inversion method is an effective approach for characterizing gas hydrate systems from long-offset seismic reflection data.

scholarly journals Dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) cycling across contrasting biological hotspots of the New Zealand Subtropical Front

2017 ◽  
Author(s):  
Martine Lizotte ◽  
Maurice Levasseur ◽  
Cliff S. Law ◽  
Carolyn F. Walker ◽  
Karl A. Safi ◽  
...  
Keyword(s):  
New Zealand ◽  
Strong Association ◽  
Austral Summer ◽  
Bacterial Biomass ◽  
Global Ocean ◽  
Potential Contribution ◽  
Microbial Conversion ◽  
Near Surface ◽  
Subtropical Front ◽  
Bacterial Protein Synthesis

Abstract. The oceanic frontal region above the Chatham Rise east of New Zealand was investigated during the late austral summer season in February and March 2012. Despite its potential importance as a source of marine-originating and climate-relevant compounds, such as dimethylsulfide (DMS) and its algal precursor dimethylsulfoniopropionate (DMSP), little is known of the processes fuelling the reservoirs of these sulfur (S) compounds in the water masses bordering the Subtropical Front (STF). This study focused on the two opposing fates of DMSP-S following its uptake by microbial organisms: either its conversion into DMS, or its assimilation into bacterial biomass. Sampling took place in three phytoplankton blooms (B1, B2 and B3) with B1 and B3 occurring in relatively nitrate-rich, dinoflagellate-dominated Subantarctic waters, and B2 occurring in nitrate-poor Subtropical waters dominated by coccolithophores. Concentrations of total DMSP (DMSPt) and DMS were high across the region, up to 160 nmol L−1 and 14.5 nmol−1, respectively. Pools of DMSPt measured in this study showed a strong association with overall phytoplankton biomass proxied by chlorophyll a (rs = 0.83) likely because of the persistent dominance of dinoflagellates and coccolithophores, both DMSP-rich taxa. Heterotrophic microbes displayed low S assimilation from DMSP (less than 5 %) likely because their S requirements were fulfilled by high DMSP availability. Rates of bacterial protein synthesis were significantly correlated with concentrations of dissolved DMSP (DMSPd, rs = 0.86) as well as with the microbial conversion efficiency of DMSPd into DMS (DMS yield, rs = 0.84). Estimates of the potential contribution of microbially-mediated rates of DMS production (0.1–27 nmol L−1 d−1) to the near-surface concentrations of DMS suggest that bacteria alone could not have sustained DMS pools at most stations, indicating an important role for phytoplankton-mediated DMS production. The findings from this study provide crucial information on the distribution and cycling of DMS and DMSP in a critically under-sampled area of the global ocean, and they highlight the importance of oceanic fronts as hotspots of the production of marine biogenic S compounds and as potential sources of aerosols particularly in regions of low anthropogenic perturbations such as the frontal waters of the Southern Hemisphere.

A possible new hydrocarbon play, offshore central West Greenland

1998 ◽  
pp. 28-30
Author(s):  
Nina Skaarup ◽  
James A. Chalmers
Keyword(s):  
Seismic Data ◽  
Mineral Resources ◽  
Source Rocks ◽  
Bright Spot ◽  
Offshore Area ◽  
West Greenland ◽  
The Government ◽  
Structural Closure ◽  
Multichannel Seismic Data ◽  
Seismic Lines

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Skaarup, N., & Chalmers, J. A. (1998). A possible new hydrocarbon play, offshore central West Greenland. Geology of Greenland Survey Bulletin, 180, 28-30. https://doi.org/10.34194/ggub.v180.5082 _______________ The discovery of extensive seeps of crude oil onshore central West Greenland (Christiansen et al. 1992, 1994, 1995, 1996, 1997, 1998, this volume; Christiansen 1993) means that the central West Greenland area is now prospective for hydrocarbons in its own right. Analysis of the oils (Bojesen-Koefoed et al. in press) shows that their source rocks are probably nearby and, because the oils are found within the Lower Tertiary basalts, the source rocks must be below the basalts. It is therefore possible that in the offshore area oil could have migrated through the basalts and be trapped in overlying sediments. In the offshore area to the west of Disko and Nuussuaq (Fig. 1), Whittaker (1995, 1996) interpreted a few multichannel seismic lines acquired in 1990, together with some seismic data acquired by industry in the 1970s. He described a number of large rotated fault-blocks containing structural closures at top basalt level that could indicate leads capable of trapping hydrocarbons. In order to investigate Whittaker’s (1995, 1996) interpretation, in 1995 the Geological Survey of Greenland acquired 1960 km new multichannel seismic data (Fig. 1) using funds provided by the Government of Greenland, Minerals Office (now Bureau of Minerals and Petroleum) and the Danish State through the Mineral Resources Administration for Greenland. The data were acquired using the Danish Naval vessel Thetis which had been adapted to accommodate seismic equipment. The data acquired in 1995 have been integrated with the older data and an interpretation has been carried out of the structure of the top basalt reflection. This work shows a fault pattern in general agreement with that of Whittaker (1995, 1996), although there are differences in detail. In particular the largest structural closure reported by Whittaker (1995) has not been confirmed. Furthermore, one of Whittaker’s (1995) smaller leads seems to be larger than he had interpreted and may be associated with a DHI (direct hydrocarbon indicator) in the form of a ‘bright spot’.

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