Low-fold seismic reflection data acquired along the axis of the Hikurangi Trough, New Zealand

Two-dimensional seismic reflection data were acquired along the axis of the Hikurangi Trough, New Zealand, by the former National Institute of Water and Atmospheric Research (NIWA; now Earth Sciences New Zealand) during R/V Tangaroa voyage TAN1213 in 2012 (Wysoczanski et al., 2012). The voyage was funded by New Zealand Government core funding to NIWA and managed under the internal projects Deepsea Resources: Origin, Detection & Environmental Management (COPR1301) and Consequences of Earth-Ocean Change: Underpinning Geological Processes (COPR1302). The data were collected to support research aimed at characterising the outer reaches of the Hikurangi subduction margin and the incoming Pacific Plate (Hikurangi Plateau), as well as to support ocean floor drilling associated with the International Ocean Discovery Program Expeditions 372 and 375, completed in 2017 and 2018 (Wallace et al., 2019). Together, seismic profiles TAN1213-2a and TAN1213-2b extend > 420 km from north of IODP Site U1520 offshore Poverty Bay to offshore of southeastern Wairarapa, North Island (Figure 1). TAN1213 Data Acquisition: Profiles TAN1213-2a and TAN1213-2b were collected along a route specifically planned to maintain the best possible stratigraphic continuity along the Hikurangi Trough, where possible avoiding disruption of key horizons by seamounts, the Hikurangi Channel, and major faults of the deformation front (Figure 1). Seismic lines were acquired using a source comprising two Sercel (formerly Sodera) 45/105 GI airguns operated in GI mode (Tables 1, 2 and 3). The guns were deployed 35 m behind the vessel at a nominal water depth of 5 m. The shot interval was 21.6 seconds (~50 m sailing at 4.5 knots). Data were recorded on a Geometrics GeoEel 48-channel seismic streamer comprising 6 X 100 m active sections, with a group interval of 12.5 m. Streamer depth control was maintained with a CSMX depth control system including three DigiCourse 5011 compass birds. Shot data were written to disk. The record length was 8 s and the sample rate 1 ms. TAN1213 Data Processing: The multichannel seismic reflection data were processed to post-stack time-migrated SEGY sections using GLOBE CLARITAS software (Table 4). The raw data were recorded in SEG-D format. IBM Claritas Extended SEG-Y data were written to disk after geometry was added, after stack, and after migration. Shots were sorted into CDP gathers during the stacking process. Post-stack finite difference migration was applied to the stacked sections to produce dip-true images. The seismic data were written to disk as processed sections in SEG-Y format. Migrated two-way travel time (TWTT) sections of lines TAN1213-2a and TAN1213-2b are provided in SEGY format. The geographic coordinate system and projection are WGS 84 Transverse Mercator (UTM Zone 60S). Merger with seismic line PEG09-06 and depth conversion of composite line TAN1213-2-PEG09-06 Offshore of southeast Wairarapa line TAN1213-2b overlaps and co-aligns with the high-fold seismic line PEG09-06 of the Pegasus Basin PEG09 survey (Figure 1) (Geotrace, 2010; Uruski & Bland, 2011; Plaza-Faverola et al., 2012; Bland et al., 2015). Line PEG09-06 extends a further 200 km southwestward along the edge of the Hikurangi Trough near base of the southern Cook Strait and northeastern Marlborough continental margins. We spliced lines TAN1213-2a, TAN1213-2b, and PEG09-06 into a single composite seismic line TAN1213-2-PEG09-06 (in TWTT) extending over 600 km along the axis of the Hikurangi Trough, and subsequently depth converted the line. Interval velocities were evaluated at six anchor points where published depth-migrated seismic sections and drilling data intersect the composite line. The locations of the anchor points are shown on Figure 1. The depth sections and drilling data included lines MGL1708-48, MGL1708-42 (Gase et al. 2022), PEG09-19 (Plaza-Faverola et al. 2012), PEG09-23 (Crutchley et al. 2020) and 05CM-38 (Plaza-Faverola et al. 2016), together with P-wave velocity data from International Ocean Discovery Program (IODP) Site U1520 (Barnes et al. 2019). Key mapped horizons were identified in both TWTT and depth to determine representative interval velocities between horizons (Table 5). These horizons were picked along the full extent of the composite line in two-way time, and, together with the anchor points, were used to generate a smooth spatially varying two-dimensional velocity model. Interval velocities between anchor points were linearly weighted according to the relative distance between two adjacent anchor points. For example, at an equidistant point between the two anchor points MGL1708-48 and MGL1708-42, the interval velocity (m/s) between Horizon R1 and Horizon R3 would be 0.5 x (1736 + 1657) (see Table 5). The resulting velocity field was then then down-sampled (lateral cell size of 60 m, vertical cell size of 20 ms), exported it as a text file, and then converted it to a GLOBE Claritas velocity file. This model was used in GLOBE Claritas to convert the composite TWTT section to depth, using the spatially variant velocity field. While uncertainties arise from differences among different published velocity models and from interpolation between anchor points, we estimate the overall uncertainty in the final composite velocity model to be approximately ± 10 %. List of files Figure 1. Map showing the location of seismic lines TAN1213-2a, TAN1213-2b, and PEG09-06, and six the anchor points used to derive a smoothed velocity model for depth conversion. Table 1. Summary of TAN1213 recording parameters. Table 2. Summary of TAN1213 lines acquired. Table 3. Summary of TAN1213 line coordinates. Table 4. Summary of TAN1213 seismic processing sequence. Table 5. Interval velocities determined between key horizons at six anchor points along the composite seismic section TAN1213-2–PEG0906, ordered from southwest (left) to northeast (right). Processed SEGY seismic data Migrated two-way travel time (TWTT) sections of lines TAN1213-2a and TAN1213-2b TAN1213-Line02a_MIG_CDP.sgy TAN1213-Line02b_MIG_CDP.sgy Depth converted composite seismic line TAN1213-2-PEG09-06 PEG0906-1213b-1213a_Depth_v2.segy