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Article

Gas Hydrates, Subsurface Structures and Tectonic Features of the Tuaheni Landslide Complex in the Northern Hikurangi Margin, New Zealand, Revealed by Seismic Attribute Analysis

by
Maheswar Ojha
1,*,
Uma Shankar
2 and
Ranjana Ghosh
1
1
CSIR-National Geophysical Research Institute, Hyderabad 500007, India
2
Department of Geophysics, Institute of Science, Banaras Hindu University, Varanasi 221005, India
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2023, 11(7), 1359; https://doi.org/10.3390/jmse11071359
Submission received: 17 May 2023 / Revised: 13 June 2023 / Accepted: 14 June 2023 / Published: 4 July 2023
(This article belongs to the Section Geological Oceanography)
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Abstract

:
The Tuaheni Landslide Complex, located on the upper slope of the northern Hikurangi Margin in New Zealand, is a unique place to research on slow slip creep-like deformation and seabed failure, as well as their possible relationship with the presence of gas hydrates, cold seeps, and fluid migration. Based on the visual interpretation of seismic data, it is sometimes very difficult to identify various subsurface structures and tectonic features. We study certain seismic attributes, namely the reflection strength, instantaneous frequency, instantaneous phase, and the Hilbert transform, in the Tuaheni Landslide Complex and observe that these attributes play a very important role in identifying and interpreting various subsurface geological features and bed boundaries that are not clearly visible in the seismic sections. In general, these seismic attributes are studied to identify hydrocarbons such as oil and gas. However, in this present study these seismic attributes nicely illustrate the fluid migration pathways, the decollement of the sediment slide, the base of the debris flow, the base of the deformed sediment and gas migration, etc., along two perpendicular seismic profiles crossing the Site U1517 of IODP Expedition 372. The instantaneous phase and Hilbert transform attribute depict the bed boundaries and discontinuities, whereas the reflection strength and instantaneous frequency attributes characterize the various strata in terms of whether they are associated with fluid at their bases. The possible role of tectonic activity and seafloor slope failure due to gas hydrate dissociation and vice versa is clearly visible through fluid-filled weak zones in the seismic attribute volumes. Gas hydrates are dissociating and BSRs are abruptly pinching out towards the seafloor due to the movement of hot fluid and free gas, enhancing seafloor sliding and local tectonic activities together.

1. Introduction

One of the seismically active areas on the Earth is the northern Hikurangi Margin, where the presence of gas hydrates, fluid migration, cold seeps, and creep-like deformation is thought to be responsible for frequent earthquakes, slow slip events, submarine landslides causing tsunamis, and high tides [1,2,3,4,5,6]. The presence of gas hydrates in the Tuaheni Landslide Complex (TLC) of the northern Hikurangi Margin (Figure 1) has been inferred by identifying discontinuous bottom simulating reflectors (BSRs) on the multichannel seismic volume [1,2,3,7,8]; however, the BSRs are not completely following the topography of the seafloor and are rapidly wrapping upward (Figure 2), which may be caused by regional thermal anomalies, cold seeps, and fluid migration [3,4,6]. Various seismic attributes, such as the reflection strength, instantaneous frequency, instantaneous phase, and the Hilbert transform are very useful for interpreting various subsurface features, which are not always possible to see directly from the seismic section [9,10]. The presence of gas hydrates and free gas changes the sediment properties in various ways, which have been characterized by various seismic attributes, such as the reflection strength and instantaneous frequency, in various world margins, such as Blake Ridge, the Vøring Plateau off Norway, Indian offshore, the Shenhu area of the South China sea, the Makran Accretionary Prism, the Ulleung Basin offshore South Korea, etc. [11,12,13,14,15,16,17,18,19,20,21,22,23] Gas hydrates in sediment pore spaces reduce sediment porosity and thus reduce seismic impedance contrasts between units within gas-hydrate-bearing sediments, resulting in lower seismic reflection amplitudes and making the seismic section transparent [24]. Sediment velocity is increased by the presence of gas hydrates, while free gas in the pore spaces beneath BSRs significantly reduces seismic velocity, changing the impedance contrasts across BSRs [16]. On the other hand, gas-charged sediments absorb lots of seismic energy and show low-frequency dominance in the instantaneous frequency section [21].
During the recent drilling at Site U1517 under the International Ocean Drilling Program (IODP) Expedition 372 in 2018, gas-hydrate-bearing reservoirs were documented up to 160 m below the seafloor (mbsf) and at water depths between 600 and 1000 m [24]. One of the objectives of Expedition 372 was to correlate well and seismic data to investigate gradual slip creep-like deformation and seabed failure, as well as their likely relationship to the existence of gas hydrates and their dissociation, cold seeps, and fluid movement. In this study, we interpret the reflection strength, instantaneous frequency, instantaneous phase, and the Hilbert transform of an in-line and cross-line multichannel seismic section crossing the well location in the northern Hikurangi Margin to identify and analyze various subsurface structures/features associated with the existence of gas hydrates, free gas, fluid migration pathways, sediment slides, tectonic activity, etc. Using specific attributes from reflection seismic data, we make an effort to shed light on the interior structures along the top slope of the Hikurangi Margin. This is a generally interesting scientific topic and a good method with which to continue using seismic data to learn more about tectonic structures, the interaction between gas and fluid movement channels, and subsequent mass-wasting events.

2. Study Area

The Hikurangi Margin is situated off the East Coast of the North Island of New Zealand, where the Pacific Plate subducts beneath the Australian Plate (Figure 1) at a rate of 4.5–5.5 cm/year [24]. The plate convergence occurs obliquely towards the southwest [26]. The oblique convergence separates subduction thrust and margin normal as well as parallel components. The Hikurangi margin is further influenced by mass transport deposits and the subduction of various seamounts on the oceanic plate [27,28]. The northern Hikurangi Margin comprises several seamounts on the subducting Pacific plate and is categorized by a combination of tectonic erosion and limited accretion in the north and further south, respectively [2,29].
The Hikurangi Plateau, a vast igneous province with a rough crust and several Cretaceous seamounts, is part of the oceanic subducting plate of 120–90 Ma [5]. The modeling of geophysical data shows that the Hikurangi Margin contains presubduction and Neogene subduction systems [4,30]. The imbricated accretionary wedge comprises an inner foundation of presubduction rocks consisting of late Cretaceous and Paleogene rocks, an outer wedge of Pliocene to Pleistocene accreted trench-filled turbidites, and a deforming cover sequence of Miocene to the recent shelf and slope basin sediments [4,31]. Barnes et al. [4] observed numerous deformed rocks on seismic images, which could be associated with active thrust faulting and folding in the Miocene. The IODP 372 expedition discovered five major litho-units at Site U1517: (i) the top four meters below the seafloor with a silty clay and clayey silt layer, (ii) 4–40 m below the seafloor (mbsf) with alternate sand and mud layers, (iii) 40–67 mbsf with alternating silt and clay layers, (iv) 67–103 mbsf with silty clay and clayey silt, and (v) 103–187 mbsf as a mixture of the above four units [25]. Later, seven litho-units were observed along seismic profiles based on the acoustic impedance and porosity inversion [12].

3. Data

In this study, seismic data from a 3D seismic volume were extracted for scientific investigation purposes in the northern Hikurangi Margin of New Zealand [1]. Seismic data have a high-frequency content of about 220 Hz, with a dominant frequency of about 90 Hz. The seismic datasets were used for site selection and drilling targets for the IODP 372 Expedition before gas hydrate exploration. Figure 2 depicts an in-line seismic section oriented along NW-SE and a cross-line seismic section oriented along SW-NE crossing Site U1517 drilled at a water depth of 720 m [25]. The common depth point (CDP) interval is 12.5 m. The drilling site is located within the 3D seismic survey area in the Tuaheni Landslide Complex, where logging while drilling (LWD) and wireline loggings (WLs) were performed. Mainly, resistivity and velocity logs were used to identify and estimate gas hydrates in the study area. The seismic sections (Figure 2a,b) reveal distinct BSRs in the steep slope region, followed by a high-reflectivity zone at and below the base of the gas hydrate stability zone (BGHSZ) with the decollement of the landslide and the base of the debris flow [25].

4. Seismic Attributes

Seismic attributes are information extracted from seismic data to enhance subsurface geological features. The instantaneous amplitude and instantaneous phase are the two fundamental seismic attributes used in complex seismic trace analyses. Many attributes are derived from these two main attributes—amplitude and phase, through differentiation, averaging, combination, or transformation. Instantaneous attributes are defined at every point on the seismic trace. Since the seismic trace is insufficient for separating the instantaneous amplitude and phase, the quadrature trace is first calculated from the seismic trace using the Hilbert transform. The color displays of seismic attributes are used for the interpretation of geologic structures, stratigraphy, and rock/pore fluid properties. The instantaneous amplitude or trace envelope is a measure of brightness or reflection strength, and it is independent of phase and polarity. It is sensitive to changes in acoustic impedance, and thus to lithology, porosity, hydrocarbons, and thin-bed tuning [23,32]. The presence of gas hydrates increases the acoustic impedance, whereas the presence of even a small amount of free gas decreases the acoustic impedance. The instantaneous phase is used to track reflector continuity for detecting unconformities, faults, and lateral changes in stratigraphy when the phase angle cannot be followed from trace to trace. The instantaneous frequency can be used to identify abnormal attenuation [23] and thin-bed tuning [33]. Since many reflector events are a composite of individual reflections from closely spaced reflectors or the tuning of multiple impedance contrasts, a characteristic reflection frequency pattern is produced. This character of a composite reflection changes gradually as the sequence changes laterally in thickness or lithology. Another useful application of instantaneous frequency is to identify sediments that cause seismic attenuation, such as gas sands, because they highly attenuate seismic P-waves by absorbing high-frequency content, and therefore low-frequency shadows are observed. Gas hydrates also appear to attenuate energy as discussed above, but not as much as gas-bearing sediments [21]. All attributes are generated using the commercial software ProMAX (Version 5000.0.1.0.0, Halliburton, Bengaluru, India 560103).
The conventional seismic trace can be represented as the real component of a complex seismic trace and the imaginary component, which is called a quadrature trace.
The rotated trace under a phase rotation of 90° is the Hilbert transform, resulting in a quadrature trace [23,34]. The quadrature trace does not have any physical meaning. Peaks and troughs on the quadrature trace correspond to a zero-crossing on the seismic trace [23,34]. The trace envelope is defined as the maximum value that a seismic trace can have under a constant phase rotation, which makes it independent of phase. The trace envelope can be described as a function that connects the waveform peaks as well as troughs, since it is independent of polarity. When the trace envelope equals the rotated seismic trace (at maximum), its change with respect to the rotation angle is zero.
The instantaneous phase has a sawtooth appearance, and its values range from (−π, π). They refer to the apparent position along the seismic trace, such that peaks have a 0° phase, troughs have a 180° phase, and downward zero-crossings have a −90° phase. If a seismic trace is represented by a cosine wave with amplitude, the quadrature trace is a sine wave with the same amplitude.
The instantaneous frequency represents the rate of change with the time of the instantaneous phase divided by 2π [23,34]. The instantaneous frequency has units of Hz, but it is not a frequency in the physical sense, as it has negative values at inflection points of the trace and can also exceed the maximum frequency in the signal when the trace is close to zero and crosses it for one or two samples.

5. Results and Discussion

The northern Hikurangi Margin is a very interesting and important place for research, where tectonic activity, mass transport deposits, slope failure, sediment deformation, fractures, fluid migration pathways, gas hydrate deposits, links between gas hydrate dissociation and tectonic activity and vice versa are observed on the shallowest subduction zone on the Earth. Four seismic attributes, namely the reflection strength, instantaneous frequency, instantaneous phase, and the Hilbert transform, were calculated along the in-line and cross-line profiles. These attributes clearly illustrate various subsurface features, such as slipped sediment, decollement, the base of debris flow and deformed sediments, pinch-outs, BSRs, gas-charged sediments, gas migration, etc. in the Tuaheni Landslide Complex on the upper slope of the northern Hikurangi Margin, New Zealand. All interpretations are drawn based on four seismic attributes, where some features are visible on some attributes and others are on other attributes. Instantaneous attributes describe the waveform shape. The seismic section only illustrates the amplitude and polarity of the signal. As the seismic wave propagates through subsurface layers, the amplitude, phase, and frequency change significantly. Seismic attributes are quantities derived from seismic data to highlight specific geological, physical, or reservoir property features. Seismic attributes may sometimes not have any continuous features and, therefore, interpretation should be carried out by observing the whole seismic section and a number of attributes together. Strata or reflectors that are associated with the gas or fluid can be ascertained via analyses of attributes, as discussed below.
At first, we analyze the instantaneous phase and Hilbert transform to interpret bed boundaries and their discontinuities. In general, the phase along a boundary should not change. The instantaneous phase attribute is independent of the amplitude and is used in seismic data interpretation for the lateral continuity and discontinuity of reflection events and sequence boundaries. The Hilbert transform, which produces a 90° phase shift in the signal is generally used to interpret post-stack seismic data by generating an analytic signal [35]. Figure 3 is the instantaneous phase attribute plot, which nicely depicts how and in which direction beds are dipping and being pinched-out towards the sea bottom slope. Figure 4 shows the Hilbert transform of the in-line and cross-line profiles, which clearly delineates various subsurface geological features; however, the instantaneous phase and Hilbert transform attributes do not show whether any strata are associated with gas and fluid.
Next, we analyze reflection strength and instantaneous frequency to characterize whether various strata are associated with fluid at their bases (e.g., slipped sediment, slope failure, the base of debris flow and decollement, etc.).
Reflection strength plots along two seismic lines (Figure 5) show various features demarcated by the contrast between low and high reflection strength. Reflection strength is the strength of producing a strong reflection through lowering or enhancing the impedance (density × velocity) contrast. The presence of gas significantly lowers the impedance and produces very high reflection strength, clearly illustrated by the underlying gas-charged sediments below the BSRs and other strata. The BSRs are associated with a low concentration of gas hydrates above and a high concentration of free gas below [12]. The base of the slipped sediment, debris, deformed sediments, and decollement, which are generally in water-filled zones, are demarcated by low-reflection-strength lineaments and low impedance values. Slipped sediment is not present on the cross-line. Scattered high reflection strengths are observed in the debris flow and deformed sediments within the hydrate stability zone, which is possibly because of the dissociation of gas hydrates due to the tectonic activity and hot fluid migration. The instantaneous frequency plot generally depicts the presence of gas as a low-frequency shadow because of the absorption of high-frequency components in the seismic signal (Figure 6). The presence of free gas within the sediments, below the BSRs, the upward movement of free gas, and velocity pulled-down because of a lowering in velocity due to low-velocity gas are clearly observed in Figure 6. Various features are interpreted through mainly observing the trend/lineament of low frequency. Just below the seafloor, high reflection strength and low frequencies are observed, which may be due to the presence of gas tapped by a low-permeability clay layer on the seafloor. On the other hand, a high-porosity layer is observed in the well log 20 m below the seafloor. These high-porosity layers and free gas below the seafloor may be causing the seafloor deformation. If we observe the right side of Figure 5 and Figure 6, BSRs are being sharply pinched-out to the seafloor due to the upward movement of hot fluid (water) along with free gas (high reflection strength and low-frequency shadow), which are pushing the gas hydrate stability condition up. As cold fluid cannot disturb BSRs, we assume that it should be hot. There are also some other reasons, such as erosion, active thrust faulting, abnormal heat flow, etc., for the lifting up of BSRs [36,37]. This interpretation is consistent with observations from Blake Ridge [21], the Vøring Plateau off Norway [20], the Makran accretionary prism [16], the Ulleung Basin [11,15], and the South China Sea [13].
The average concentration of gas hydrates in this area is estimated to be low [12], and due to the low concentration of gas hydrates we observe very feeble or no signal. Additionally, in the presence of dipping strata, the continuity of the BSR signal becomes distorted. Therefore, by interpreting reflection strength and instantaneous frequency the presence of BSRs can be ascertained. Gas from dissociated gas hydrates and free gases below the gas hydrate layers are migrating up through several paths (Figure 5 and Figure 6) and may be triggering the slope failure. Our results of seismic attribute analyses reveal details of various subsurface features in the northern Hikurangi Margin, which is a unique place with tectonic activity, mass transport deposits, slope failure, sediment deformation, fractures, fluid migration pathways, and gas hydrate deposits on the shallowest subduction zone on Earth.
There are many pitfalls associated with interpreting seismic attributes. Most attributes have a noisy character since they are derived through differentiation, which introduces less coherent high frequencies and suppresses more coherent low frequencies. Additionally, because the instantaneous amplitude and phase have inflection points and discontinuities, their differentiation results in spurious spikes. The instantaneous attributes are also spiky and noisy because of the noise in the data. The most anomalous values of instantaneous frequency are most often associated with small amplitudes and are the least reliable, whereas the ones associated with larger amplitudes are more stable [36]. To remove the high-frequency noise and spikes to ease interpretation through emphasizing the frequency of stronger reflectors, the instantaneous attributes are filtered. The most common filters are median filtering, the selection of instantaneous attributes at envelope peaks, and averaging weight via the instantaneous power or envelope squared [37]. There are also some uncertainties associated with interpreting tectonic features using seismic attributes such as seismic data quality, acquisition footprint, processing artifacts, and velocity pull-up/push-down [10]. Most of these pitfalls can be avoided by observing shallow to deep sections of the whole seismic section, because these artifacts diminish with depth as fold-in data increase with depth during stacking.

6. Conclusions

In this study, we analyze four seismic attributes—the reflection strength, instantaneous frequency, instantaneous phase, and Hilbert transform—to interpret various subsurface features in the Tuaheni Landslide Complex on the upper slope of the northern Hikurangi Margin, New Zealand. We demonstrate that the seismic attributes that are generally used to analyze hydrocarbon deposits can be used to identify various subsurface tectonic features on the seismic attribute plots, which are not visible on the seismic data. The instantaneous phase and Hilbert transform plots illustrate bed boundaries and discontinuities, such as submarine landslides, decollement, the base of debris flow and deformed sediments, pinch-outs, and the strong parts of the BSRs, whereas the reflection strength and instantaneous frequency plots illustrate hydrocarbon depositional features, such as gas hydrates as well as free gas and their distribution patterns and pathways, in addition to the strata associated with fluid flow at their bases. It is observed that the different upward dipping strata (right to left) are slowly becoming horizontal and then dipping downward along the seafloor slope, and further pinching out to the seafloor. Dipping downward and being pinched-out may be due to the sliding of upper-sediment mass (debris). The migration of hot fluid pushes the BSRs up and pinches BSRs sharply to the seafloor at the right side of the seismic section. Due to the movement of hot fluid and free gas, gas hydrates are also dissociating as well as enhancing seafloor sliding and local tectonic activity together.

Author Contributions

M.O.: conceptualization, methodology, software, and writing—reviewing and editing. U.S.: data curation and writing—original draft preparation. R.G.: validation and writing—reviewing and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research is funded by National Centre for Polar and Ocean Research (NCPOR), Goa, India, (grant number: NCAOR/ IODP/20.25/201(4)) and by the Ministry of Earth Sciences (MoES), New Delhi, India (grant number: MOES/GH/REL-NIOT/17/2017-PC-II).

Informed Consent Statement

Not applicable.

Data Availability Statement

The seismic data that support the findings of this study are publicly available on the New Zealand Petroleum and Minerals website https://www.gns.cri.nz/data-and-resources/petroleum-basin-explorer-pbe/, accessed on 7 May 2023) and through https://doi.pangaea.de/10.1594/PANGAEA.945774 [8], accessed on 7 May 2023).

Acknowledgments

We are thankful to the Directors of the CSIR-National Geophysical Research Institute (NGRI), Hyderabad and Banaras Hindu University, Varanasi for their kind permission to publish the work. The authors are thankful to IODP Expedition 372/375 scientists and other participants. The Ministry of Earth Science (MoES), New Delhi, India is duly acknowledged for providing research grant to CSIR-NGRI. Uma Shankar is grateful to the NCPOR, Goa, India for allowing him to participate in IODP Expedition 372/375 and for a research grant. The collection and processing of the P-Cable 3D volume which the seismic lines in this paper are extracted from was jointly funded by the New Zealand Ministry for Business Innovation and Employment (MBIE), NIWA and GNS Science Core funding, and the Deutsche Forschungsgemeinschaft (DFG-Grant BI 404/7|KR 2222/18). This research used data and samples provided by the International Ocean Discovery Program (IODP).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (A) Tectonic setup of the northern Hikurangi Margin, New Zealand, (B) zoomed part of the study area (white box). Two perpendicular seismic lines used in this study cross the drilling Site U1517 (yellow circle) of IODP expedition 372 in the Tuaheni Landslide Complex on the upper slope of the northern Hikurangi Margin. The black line indicates the track line for the seismic reflection profile crossing Site U1517 (After [25]).
Figure 1. (A) Tectonic setup of the northern Hikurangi Margin, New Zealand, (B) zoomed part of the study area (white box). Two perpendicular seismic lines used in this study cross the drilling Site U1517 (yellow circle) of IODP expedition 372 in the Tuaheni Landslide Complex on the upper slope of the northern Hikurangi Margin. The black line indicates the track line for the seismic reflection profile crossing Site U1517 (After [25]).
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Figure 2. (a) In-line and (b) cross-line seismic section. The seafloor, BSRs, decollement of sediment slide, and base of the debris deposit are shown with arrows on seismic sections in the Tuaheni Landslide Complex on the upper slope of the northern Hikurangi Margin, New Zealand. Sonic P-wave impedance is superimposed on seismic sections. The CDP interval is 12.5 m.
Figure 2. (a) In-line and (b) cross-line seismic section. The seafloor, BSRs, decollement of sediment slide, and base of the debris deposit are shown with arrows on seismic sections in the Tuaheni Landslide Complex on the upper slope of the northern Hikurangi Margin, New Zealand. Sonic P-wave impedance is superimposed on seismic sections. The CDP interval is 12.5 m.
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Figure 3. Instantaneous phase plots along the in-line (a) without interpretation and (b) with interpretation, and along cross-line (c) without interpretation and (d) with interpretation. Instantaneous phase plots show the various bedding planes. The seafloor, BSRs, decollement, slipped sediment, base of debris flow and deformed sediment, pinch-outs, etc., are marked with dashed lines and arrows. Sonic P-wave impedance is superimposed on seismic sections (red color line).
Figure 3. Instantaneous phase plots along the in-line (a) without interpretation and (b) with interpretation, and along cross-line (c) without interpretation and (d) with interpretation. Instantaneous phase plots show the various bedding planes. The seafloor, BSRs, decollement, slipped sediment, base of debris flow and deformed sediment, pinch-outs, etc., are marked with dashed lines and arrows. Sonic P-wave impedance is superimposed on seismic sections (red color line).
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Figure 4. Hilbert transform plots along the in-line (a) without interpretation and (b) with interpretation, and along cross-line (c) without interpretation and (d) with interpretation. Hilbert transform plots show the geophysical signatures of different subsurface features. The seafloor, BSRs, decollement, slipped sediment, base of debris flow and deformed sediment, pinch-outs, etc., are marked with dashed lines and arrows. Sonic P-wave impedance is superimposed on seismic sections (red color line).
Figure 4. Hilbert transform plots along the in-line (a) without interpretation and (b) with interpretation, and along cross-line (c) without interpretation and (d) with interpretation. Hilbert transform plots show the geophysical signatures of different subsurface features. The seafloor, BSRs, decollement, slipped sediment, base of debris flow and deformed sediment, pinch-outs, etc., are marked with dashed lines and arrows. Sonic P-wave impedance is superimposed on seismic sections (red color line).
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Figure 5. Reflection strength plots along the in-line (a) without interpretation and (b) with interpretation, and along cross-line (c) without interpretation and (d) with interpretation. Reflection strength plots show loss in amplitude above the BSRs and enhanced reflection across gas-charged sediments below BSRs. The seafloor, BSRs, decollement, slipped sediment, base of debris flow and deformed sediment, etc., are marked with dashed lines and arrows. Sonic P-wave impedance is superimposed on seismic sections (red color line).
Figure 5. Reflection strength plots along the in-line (a) without interpretation and (b) with interpretation, and along cross-line (c) without interpretation and (d) with interpretation. Reflection strength plots show loss in amplitude above the BSRs and enhanced reflection across gas-charged sediments below BSRs. The seafloor, BSRs, decollement, slipped sediment, base of debris flow and deformed sediment, etc., are marked with dashed lines and arrows. Sonic P-wave impedance is superimposed on seismic sections (red color line).
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Figure 6. Instantaneous frequency plots along the in-line (a) without interpretation and (b) with interpretation, and along cross-line (c) without interpretation and (d) with interpretation. Instantaneous frequency plots show low-frequency shadows (cyan) due to the presence of free gas. The seafloor, BSRs, decollement, slipped sediment, base of debris flow and deformed sediment, gas migration, velocity being pulled down, etc., are marked with dashed lines and arrows. Various features are interpreted by observing the trend/lineament of low frequency. Sonic P-wave impedance is superimposed on seismic sections (red color line).
Figure 6. Instantaneous frequency plots along the in-line (a) without interpretation and (b) with interpretation, and along cross-line (c) without interpretation and (d) with interpretation. Instantaneous frequency plots show low-frequency shadows (cyan) due to the presence of free gas. The seafloor, BSRs, decollement, slipped sediment, base of debris flow and deformed sediment, gas migration, velocity being pulled down, etc., are marked with dashed lines and arrows. Various features are interpreted by observing the trend/lineament of low frequency. Sonic P-wave impedance is superimposed on seismic sections (red color line).
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Ojha, M.; Shankar, U.; Ghosh, R. Gas Hydrates, Subsurface Structures and Tectonic Features of the Tuaheni Landslide Complex in the Northern Hikurangi Margin, New Zealand, Revealed by Seismic Attribute Analysis. J. Mar. Sci. Eng. 2023, 11, 1359. https://doi.org/10.3390/jmse11071359

AMA Style

Ojha M, Shankar U, Ghosh R. Gas Hydrates, Subsurface Structures and Tectonic Features of the Tuaheni Landslide Complex in the Northern Hikurangi Margin, New Zealand, Revealed by Seismic Attribute Analysis. Journal of Marine Science and Engineering. 2023; 11(7):1359. https://doi.org/10.3390/jmse11071359

Chicago/Turabian Style

Ojha, Maheswar, Uma Shankar, and Ranjana Ghosh. 2023. "Gas Hydrates, Subsurface Structures and Tectonic Features of the Tuaheni Landslide Complex in the Northern Hikurangi Margin, New Zealand, Revealed by Seismic Attribute Analysis" Journal of Marine Science and Engineering 11, no. 7: 1359. https://doi.org/10.3390/jmse11071359

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