Basal Reflector Research Articles

The ocean-continent transition in the Western Central Red Sea

The Red Sea is an important example of a continental rift transitioning slowly to an oceanic basin. However, structures that can inform us of how that transition occurred have been poorly reported because deep seismic reflection data capable of imaging basement under the rift sediments are generally lacking publicly. Three lines of multichannel seismic reflection data have recently been published revealing structures on the Nubian side of the central part of the basin. In this study, we reassess these data in the light of recent studies of the central Red Sea. Over continental crust, the data reveal reflection sequences likely due to strata at or near the base of the evaporites, in two cases with varied dips suggesting the presence of syn-rift growth stratigraphy. Almost all of those reflections dip downwards towards the rift axis, not away as would be expected from tilted fault blocks of bookshelf faulting types. That observation, and low relief of basement, confirm inferences made earlier based on gravity anomalies that this part of the Red Sea lacks large-relief fault escarpments and is most likely a syn-rift sag basin. In the transition to oceanic crust, an abnormally broad magnetic anomaly of estimated Chron 5 age is found not to be associated with structures such as sills, so it likely arises from deeper sources. One of the seismic lines traverses a ridge in Bouguer gravity anomalies that runs across the axis. This feature has previously been interpreted as a volcanic ridge similar to those observed at other ultra-slow spreading ridges. The seismic data reveal diffuse basement reflections and confirm that the record immediately above basement lacks reflections typical of sedimentary strata. Both observations are consistent with the presence of oceanic crust. Modelling of gravity anomalies suggests the ridge is likely underlain by igneous intrusive rocks displacing mantle rocks, as expected for a volcanic ridge. The seismic data, combined with recently updated multibeam and high-resolution sparker seismic results, further suggest how the evaporite movements have been modulated by basement topography. These results add to our knowledge of the evaporite movements and continent-ocean transition structures in the central Red Sea.

Ultrahigh-resolution 9C seismic survey in a landslide prone area in southwest of Sweden

SUMMARY We studied the benefits of a nine-component (9C) seismic survey over a landslide-prone area in southwest of Sweden to retrieve ultrahigh-resolution shear wave reflection images of the subsurface as well as crucial information on physical properties of the sediments. A complete, 1 m shot and receiver spacing, multicomponent 2-D seismic profile was acquired using three-component microelectromechanical-system-based landstreamer receivers, and a 5-kg sledgehammer strike in vertical and horizontal orientations as three-component seismic source. Given the rich number of shear wave reflections observed on all the 9C data, the processing work focused on their retrievals. It revealed three distinct reflections, two of which associated with coarse-grained materials and one with an extremely undulating bedrock surface. Given the extremely slow shear wave velocities on the order of 60–100 m s−1, we obtained ultrahigh-resolution shear wave sections avoiding temporal and spatial aliasing. Imaging results suggest vertical-source and horizontal-radial receiver (V–HR), and horizontal-transverse source–receiver orientations (HT–HT) provided the most optimum images of the subsurface. A non-hyperbolic algorithm was applied to the normal-moveout corrections justified by the traveltime differences of the bedrock reflection in different shear wave sections. The improved images by incorporating the anisotropy term suggest that the data set reveals moderate shear wave anisotropy along some portions of the profile. The Vp/Vs ratio obtained by using bedrock reflection in P- and S-wave sections suggests values ranging 10–16, which implies high water content. Areas with lower Vp/Vs coincides with greater anisotropic parameters and this can indicate disturbed clays or presence of sensitive clays.

Thermophysical arguments against basal melting in the south polar region of Mars

The south polar layered deposits (SPLD) on Mars is primarily composed of H2O ice, with smaller quantities of impurities such as CO2 ice and dust. The presence of these impurities can affect the thermal structure of the SPLD, increasing the basal temperature and potentially leading to basal melting. Recently, bright basal reflectors in radargrams from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) of the SPLD at Ultimi Scopuli have been interpreted as evidence of a briny subglacial liquid water layer. However, this interpretation is difficult to reconcile with the low Martian geothermal heat flow and the frigid surface temperature at the Martian south pole. Here, we conduct a comprehensive thermophysical evolution modeling analysis to investigate the conditions that could enable basal melting at Ultimi Scopuli. Using numerical models that describe time-dependent firn densification and the thermal properties of porous solid mixtures, we first constrain the thermophysical parameters of the SPLD. We then run high-resolution thermal models that account for heat transfer and phase change to ascertain if basal melting can occur. Even when considering SPLD at Ultimi Scopuli to consist of 30% dust, basal melting does not occur. Considering the average surface temperature of the SPLD to be between 160 and 170 K and a maximum surface heat flow of 30 mW m−2, we find that the bulk thermal conductivity of the SPLD needs to be <1.5 W m−1 K−1 for basal melting to occur at Ultimi Scopuli. The combination of H2O ice, CO2 ice, and dust that can yield kbulk <1.5 W m−1 K−1 is at odds with current constraints on the composition of the SPLD. We also model the average dimension of a magmatic intrusion that can enable basal melting at Ultimi Scopuli. We find that an intrusion must be <15 km from the surface and at least 5 km wide to create a 20–30 km wide subglacial liquid water layer. We also find that subglacial liquid water sustained by magmatic intrusion will eventually refreeze over 1–2 million years unless continually sustained by newer intrusions. Given the difficulty in forming and sustaining subglacial liquid water on Mars, we suggest that alternative interpretations of the bright radar reflections merit additional consideration.

Ice Content of Mantling Materials in Deuteronilus Mensae, Mars

AbstractLayers of ice and dust (i.e., “mantle”) are found throughout the mid‐latitudes of Mars, where they drape and mute topography and record the recent climate history. Many of the youngest Late Amazonian “latitude‐dependent mantles” have been characterized, but the full range of mantle characteristics, including variations in ice content and age, has not been well documented. To advance our understanding of these ice‐dust units, we use SHAllow RADar (SHARAD) radar sounding data to constrain the subsurface physical properties of a widespread Middle Amazonian mantle unit within Deuteronilus Mensae. This region hosts a high density of glacial landforms, which allows assessment of the stratigraphic relationship between mantle deposits and glacial ice. SHARAD reflectors at the base of the mantle units are used to determine moderate dielectric constants (∼5.75) and high loss tangents (∼0.038). These radar properties imply relatively limited ice content and high‐loss mantling materials with possible enhanced roughness or subsurface scattering. Further, we interpret deeper reflectors that are continuous with glacier basal reflectors and that suggest materials with lower dielectric constants (∼3.2–4.5) and loss tangents (0.0071–0.0120) to be due to extensions of stagnant, possibly lithic‐rich, glacier ice that are buried by a mantle layer. This study demonstrates the high variability in the origins and evolution of mid‐latitude mantling units on Mars. While the most recent latitude‐dependent mantling deposits may still be ice‐rich, older deposits, such as those preserved in Deuteronilus Mensae and elsewhere in the mid‐latitudes of Mars, may be largely desiccated or contain a more heterogeneous and limited distribution of ice.

Steeply dipping structure imaging with prismatic waves incorporating wavefield decomposition

AbstractPrismatic waves in seismic data can effectively delineate steeply dipping subsurface structures. However, the conventional migration of prismatic wave either requires the exact location of basement reflectors embedded in the accurate migration velocity model or the explicit isolation of prismatic wave from the recorded data to avoid crosstalk artefacts caused by mistaken imaging of primary reflections. Neither of the requirements is a trivial task for field data application with complicated subsurface structures. To image steeply dipping structures with an accurate but smooth enough migration velocity and without the need to extract prismatic waves from the recorded data, we analyse the prismatic imaging condition based on wavefield decomposition. In the up‐ and down‐going wavefield decomposition, the prismatic imaging condition can effectively remove the high‐wavenumber artefacts with incorrect polarity. However, the expected steeply dipping image is buried in low‐wavenumber noises. In contrast, the prismatic imaging condition based on left‐ and right‐going wavefield decomposition can accurately isolate the artefacts induced by the mistaken migration of primary waves from the steeply dipping interfaces in the image and is free of low‐wavenumber noises. Based on this analysis, we develop a reverse time migration workflow using the modified prismatic imaging condition to depict steeply dipping structures: First, we use the primary imaging condition based on the up‐and‐down‐going wavefield decomposition to obtain an image that can approximate the basement reflectors. Then, we use the modified prismatic imaging condition based on left‐ and right‐going wavefield decomposition to recover the steep interfaces. Finally, a combined subsurface image containing both basement reflectors and faults or steeply dipping structures is generated. We verify the feasibility and robustness of the modified prismatic imaging condition based reverse time migration method for delineating steeply dipping structures using several synthetic tests.

Comparable Bulk Radar Attenuation Characteristics Across Both Martian Polar Layered Deposits

AbstractThe dust‐laden and predominantly water ice south polar layered deposits (SPLD) exhibit unique radar behavior at Shallow Radar (SHARAD) frequencies that is not seen in the NPLD. A vertically extensive scattering phenomenon leads to diffuse echoes that obscure radar stratigraphy within the SPLD, referred to as “fog.” Here, we use 2‐D and 3‐D SHARAD data to measure the areal extent in which the SPLD basal interface can be identified. We also measure reflected power of SPLD basal reflectors and compare those values to ice thickness, regional geomorphology, and the presence of radar echo scattering, to constrain any influence these factors may have on the propagation of radio waves and detectability of the base. Approximately 33% of the area in which the basal interface is observed in Mars Advanced Radar for Subsurface and Ionosphere Sounding data is also detected in SHARAD data. As expected, basal power shows a negative correlation with ice thickness, but the effects of radar fog are less clear. Areas of “column” radar fog patterns exhibit higher average basal power values than regions without radar fog, and areas with only near‐surface radar fog have the lowest average basal power values. Our study results suggest that these diffuse echoes may not be associated with radio wave attenuation. Average loss tangent for the SPLD (0.0019 ± 0.0012) indicates no distinguishable difference from previously reported NPLD values in terms of bulk loss properties. We speculate that the SPLD's infrequent reflections compared to the NPLD may be due to the former's dust being less densely packed together to facilitate radar detection.

Partially‐Saturated Brines Within Basal Ice or Sediments Can Explain the Bright Basal Reflections in the South Polar Layered Deposits

AbstractStrong radar reflections have been previously mapped at the base of the Martian South Polar Layered Deposits. Here, we analyze laboratory measurements of dry and briny samples to determine the cause of this radar return. We find that liquid vein networks consisting of brines at the grain boundaries of ice crystals can greatly enhance the electrical conductivity, thereby causing strong radar reflections. A brine concentration of 2.7–6.0 vol% in ice is sufficient to match the electrical properties of the basal reflection as observed by Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). When brine is mixed with sediments, the brine‐ice mixture in the pores must be 2–5 times more concentrated in salt, increasing the brine concentration to 6.3–29 vol%. Our best fit of the median observed MARSIS value suggests a salt‐bulk sample concentration of ∼6 wt%. Thus, salt enhancement mechanisms on the order of a magnitude greater than the Phoenix landing site are needed. To form brine, the basal reflector must reach a temperature greater than the eutectic temperature of calcium perchlorate of 197.3 ± 0.2 K, which may be possible if more complex thermal modeling is assumed. Colder metastable brines are possible, but stability over millions of years remains unclear. Conversely, gray hematite with a concentration of 33.2–59.0 vol% possess electrical properties that could cause the observed radar returns, but require concentrations 2–3 times larger than anywhere currently detected. We also argue that brines mixed with high‐surface‐area sediments, or dry red hematite, jarosite, and ilmenite cannot create the observed radar returns at low temperatures.

Low‐Angle Normal Faults on the NW Shelf of Australia: Implications for Late Paleozoic Rifting

AbstractLate Paleozoic rifting of the NW Shelf of Australia formed a wide basin that fundamentally controlled the Mesozoic continental rifting and passive margin development; however, structural style associated with this extensional process remains poorly constrained. Here, we integrate high‐resolution seismic data with well data from the proximal domain of the NW Shelf to show that the Mermaid fault system in the eastern Dampier Sub‐basin is a large‐scale (∼80 km long), low‐angle (5–20°) normal fault with up to 10 km offset formed during the Late Paleozoic rifting. In its northern and southern domains, the Mermaid fault has a planar geometry with characteristic extensional structures. In its central domain, seaward‐dipping, domino‐style normal faults in the upper crust extend downward into a highly reflective seismic zone that arches upward to form an elliptical dome. Growth strata in the hangingwall half‐graben and footwall basement reflections indicate that the Mermaid fault initiated as a high‐angle normal fault and was rotated by footwall exhumation during extension. Intrabasement deformation along and below the Mermaid fault is characterized by moderate‐ to high‐amplitude, semi‐continuous seismic reflectors that are distinct from transparent reflection character in the overlying basement unit. Comparison of the Mermaid fault system to well‐studied detachment faults in metamorphic core complex settings reveals striking similarities. We therefore suggest that the Mermaid fault developed as part of a nascent metamorphic core complex with a warm to hot footwall that enabled middle‐lower crustal flow during the Late Paleozoic continental rifting, likely induced by southward subduction of the Paleo‐Tethys Ocean.

Geologic Context of the Bright MARSIS Reflectors in Ultimi Scopuli, South Polar Layered Deposits, Mars

AbstractRadar‐bright basal reflectors have been detected below the South Polar Layered Deposits (SPLD), using Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) data and have an exciting but controversial interpretation: liquid water from subglacial lakes. We mapped the surface of the SPLD immediately above and surrounding the putative lakes (1:2M map scale) in order to provide a geologic context for the interpretation of bright basal reflectors. We use Thermal Emission Imaging System daytime IR (100 m/pixel), Context Camera (6 m/pixel), and High Resolution Imaging Science Experiment (25 cm/pixel) data to characterize geologic units and typical surface roughness. We find evidence for multiple geologic units with features due to CO2 and aeolian‐related processes. We do not find evidence for surface modification linked to postulated lake locations. This is not consistent with the interpretation of the MARSIS basal radar reflector as subglacial lakes.

Holocene morpho-stratigraphic evolution of a compound submarine deltaic system in front of the shelf-incising Almanzora and Garrucha Canyons (Palomares margin, southeastern Iberia)

The Holocene morpho-stratigraphic evolution of a compound submarine deltaic system linked with the Antas and Almanzora Rivers on the narrow (< 5 km) shelf along the Palomares margin (southeastern Iberia) has been reconstructed from the integrated analysis of geomorphology, seismo-stratigraphy and sedimentology. The shelf morphology is characterized by a 3–4 km wide deltaic body off the Almanzora River, where seafloor undulations and gullies are present. Larger and more incised gullies, crescent-shaped bedforms and pockmarks are recognizable at or close to the head of shelf-indenting canyons associated with the Almanzora-Garrucha canyon system.The seismo-stratigraphy displays a strong and irregular basal reflector overlain by a sedimentary wedge, showing variable thicknesses (1–25 m) and seismic characteristics along and across the shelf. The main depocenter of this wedge is not located off the Almanzora River mouth (i.e., the main sedimentary input in the study area) but along the southern shelf encompassing the Almanzora and Antas Rivers. Grain-size and sand compositional analyses show that silty sand with high terrigenous content (mostly quartz and mica) is mainly found on the submarine deltaic body. In contrast, sandy silt with variable ratios between terrigenous and biogenic contents is retrieved from the canyon head and associated gullies as well as from the surrounding mid-outer shelf.The evolution of the Antas-Almanzora compound delta has been divided into three evolutionary stages, mainly driven by the interplay between paleo-topography and riverine and marine processes and modulated by sea level changes during the Holocene. The origin of the present-day morphological features is also discussed, again highlighting a complex interaction between sedimentary gravity flows (mainly flash flood-generated hyperpycnal flows), storm-waves and shelf currents, fluid seepage and retrogressive slope failures.

Development of a subglacial lake monitored with radio-echo sounding: case study from the eastern Skaftá cauldron in the Vatnajökull ice cap, Iceland

Abstract. We present repeated radio-echo sounding (RES, 5 MHz) on a profile grid over the eastern Skaftá cauldron (ESC) in Vatnajökull ice cap, Iceland. The ESC is a ∼ 3 km wide and 50–150 m deep ice cauldron created and maintained by subglacial geothermal activity of ∼ 1 GW. Beneath the cauldron and 200–400 m thick ice, water accumulates in a subglacial lake and is released semi-regularly in jökulhlaups. The RES record consists of annual surveys conducted at the beginning of every summer during the period 2014–2020. Comparison of the RES surveys reveals variable lake area (0.5–4.1 km2) and enables traced reflections from the lake roof to be distinguished from bedrock reflections. This allows construction of a digital elevation model (DEM) of the bedrock in the area, further constrained by two borehole measurements at the cauldron centre. It also allows creation of lake thickness maps and an estimate of lake volume at the time of each survey, which we compare with lowering patterns and released water volumes obtained from pre- and post-jökulhlaup surface DEMs. The estimated lake volume was 250 GL (gigalitres = 106 m3) in June 2015, but 320 ± 20 GL drained from the ESC in October 2015. In June 2018, RES profiles revealed a lake volume of 185 GL, while 220 ± 30 GL were released in a jökulhlaup in August 2018. Considering the water accumulation over the periods between RES surveys and jökulhlaups, this indicates 10 %–20 % uncertainty in the RES-derived volumes at times when significant jökulhlaups may be expected.

Seismic stratigraphy of a partially filled incised valley on a semi-arid continental shelf, Northeast Brazil

This study aimed to understand the key surfaces and the sedimentary filling pattern of the Coreau incised valley (CIV) system (NE Brazil) using high-resolution seismic stratigraphy. Six key surfaces (Ss) and five seismic units (SUs) were identified. The difference in the basal reflector (S1) and the paleointerfluve (modern sea floor) was verified to be up to 40 m of incision, of which only 20 m are filled. S1 (sequence boundary—SB) was associated with the acoustic basement (Tibau Formation/Barreiras Group) and presented strong control (antecedent topography) in the incision (re-incisions in the Pleistocene lowstands) and sedimentary stacking pattern. The CIV fill showed lowstand system tract, LST (chaotic facies)–transgressive system tract, TST (horizontal layers to chaotic facies)–highstand system tract, HST (chaotic facies) from the base to top. Thin fluvial deposits above SB correspond to LST. Overlapping this system tract is the first marine flooding surface—transgressive surface. Afterwards, the largest seismic unit (lateral and vertical) with sub-parallel and parallel (onlap) seismic facies, interpreted as related to a low energy system, with shallow gas, and vertical accretion (central basin of estuary) occurred. This environment is related to the drowning of the shelf from the shelf break (a decrease in gradient). A tide ravinement surface truncated the top of SU3 and favored coastal to shallow marine deposition. The transgressive system tract (TST) is thicker in the middle (mainly) and outer sectors in the fluvial troughs (depocenter). The maximum flooding surface appears near the sea floor and favored the development above of carbonate sedimentation in the HST. These seimostratigraphic stacking patterns showed a strong influence of semi-arid climate and antecedent topography/structural inheritance. Thus, it is evident that the filling of the CIV differs from other valleys on semi-arid systems and/or influenced by paleotopography (e.g., South Africa and Texas Shelf), which demonstrates the importance of local settings. The great influence of estuarine to coastal sediments in the valley infilling is also observed in the modern Coreau coastal plain incised valley.

Vp/Vs Ratio of Incoming Sediments Off Cascadia Subduction Zone From Analysis of Controlled‐Source Multicomponent OBS Records

AbstractP‐to‐S‐converted waves observed in controlled‐source multicomponent ocean bottom seismometer (OBS) records were used to derive the Vp/Vs structure of Cascadia Basin sediments. We used P‐to‐S waves converted at the basement to derive an empirical function describing the average Vp/Vs of Cascadia sediments as a function of sediment thickness. We derived one‐dimensional interval Vp/Vs functions from semblance velocity analysis of S‐converted intrasediment and basement reflections, which we used to define an empirical Vp/Vs versus burial depth compaction trend. We find that seaward from the Cascadia deformation front, Vp/Vs structure offshore northern Oregon and Washington shows little variability along strike, while the structure of incoming sediments offshore central Oregon is more heterogeneous and includes intermediate‐to‐deep sediment layers of anomalously elevated Vp/Vs. These zones with elevated Vp/Vs are likely due to elevated pore fluid pressures, although layers of high sand content intercalated within a more clayey sedimentary sequence, and/or a higher content of coarser‐grained clay minerals relative to finer‐grained smectite could be contributing factors. We find that the proto‐décollement offshore central Oregon develops within the incoming sediments at a low‐permeability boundary that traps fluids in a stratigraphic level where fluid overpressure exceeds 50% of the differential pressure between the hydrostatic pressure and the lithostatic pressure. Incoming sediments with the highest estimated fluid overpressures occur offshore central Oregon where deformation of the accretionary prism is seaward vergent. Conversely, landward vergence offshore northern Oregon and Washington correlates with more moderate pore pressures and laterally homogeneous Vp/Vs functions of Cascadia Basin sediments.

Depth-dependent artifacts resulting from ApRES signal clipping

AbstractSeveral autonomous phase-sensitive radio-echo sounders (ApRES) were deployed at Greenland glaciers to investigate ice deformation. Different attenuation settings were tested and it was observed that, in the presence of clipping of the deramped ApRES signal, each setting produced a different result. Specifically, higher levels of clipping associated with lower attenuation produced an apparent linear increase of diurnal vertical cumulative displacement with depth, and obscured the visibility of the basal reflector in the return amplitude. An example with a synthetic deramped signal confirmed that these types of artifacts result from the introduction of harmonics from square-wave-like features introduced by clipping. Apparent linear increase of vertical displacement with depth occurs when the vertical position of a near-surface internal reflector changes in time. Artifacts in the return amplitude may obscure returns from internal reflectors and the basal reflector, making it difficult to detect thickness evolution of the ice and to correctly estimate vertical velocities. Variations in surface melt during ApRES deployments can substantially modulate the received signal strength on short timescales, and we therefore recommend using higher attenuator settings for deployments in such locations.

Investigation of an oceanic plateau formation and rifting initiation model implied by the Caroline Ridge on the Caroline Plate, western Pacific

ABSTRACT The rifting of oceanic plateaus is an important mechanism for initiating lithospheric break-up and subsequent seafloor spreading. In this study, we present the latest multichannel seismic data investigating the Caroline Ridge and provide one of the typical cases for initial oceanic plateau evolution. We reveal that a smooth basement reflector (R2), as the top of the lava flows, is subparallel to the sediments with horizontal seismic reflections over the surface of the Caroline Ridge. Thick layer-parallel lava flows beneath the R2 appear within the crust. Large seamounts in the Sorol Trough possess abundant saucer-shaped intracrustal reflectors, and the overlying sediments were destroyed by intrusive bodies. The overlying sedimentary sequences, basement, and thick lava flows on the Caroline Ridge flanks were faulted by opposing normal fault sets, and the eruptions of the seamounts deformed the strata. A widespread bright horizontal reflector (R1), as an unconformity inside the Caroline Ridge sediments, truncates the lower tilted sediment layers and is itself cut by normal faults in the flank strata. Furthermore, we propose that subaerial lava flows extended laterally from the hotspot magmatism localizing in the Sorol Trough and led to Caroline Ridge formation. The initial rifting of the Caroline Ridge occurred during the Early-Middle Miocene. Limited volcanoes concentrate only in the Sorol Trough due to the attenuated thermal effect. It is suggested that dome uplifting and far-field force could have jointly caused initial rifting process of the Caroline Ridge.

Décollements, Detachments, and Rafts in the Extended Crust of Dangerous Ground, South China Sea: The Role of Inherited Contacts

AbstractWe investigate the crustal structure of the Dangerous Ground (South China Sea) through processing and interpretation of coincident wide‐angle reflection and refraction seismic data. Continental crust of Dangerous Ground has been moderately thinned, down to 15 km, so that most of the structures accompanying the early opening of the South China Sea from Cretaceous to Miocene have been preserved. Subbasement reflectors as well as refraction velocities image an interpreted dismantled Mesozoic metamorphic unit in the southernmost section of our study area. A rollover structure indicates that the reflective base of the unit was used as a décollement where low‐angle normal faults root and blocks rafted. The metamorphic unit is discontinued in a nearby basin located immediately to the north, where the refraction velocity model shows thinning of the crust from 20 to 15 km, with the presence of a 5‐km‐high mantle dome. In this deeper basin, mass transport deposits are found lying on a strong amplitude basement reflector interpreted as the footwall of an ~15 km offset crustal detachment surface that we link down to the mantle dome. We infer that the detachment reactivated an inherited low‐angle contact most probably related to the Yanshanian belt. In map view, the reactivated structure forms a half‐graben basin oriented NNE‐SSW oblique to the generally accepted direction of extension. This orientation follows the general trend of a granitic belt that spanned the South China margin prior to extension, related to the subduction of the Paleo‐Pacific.

Glacier bed surveying with helicopter-borne dual-polarization ground-penetrating radar

ABSTRACTTraditionally, helicopter-borne ground-penetrating radar (GPR) systems are operated with a single pair of bistatic dipole antennas to measure the thickness of glaciers. We demonstrate numerically that the directivity of the radiation pattern of single airborne dipoles do not correspond to an ideal full-space solution if the antennas are employed at typical flight heights. These directionality effects can degrade the quality of the subsurface images significantly, when the GPR antennas are orientated unfavorably. Since an adjustment of the antenna orientation is impractical during flight, we have developed a novel dual-polarization helicopter-borne GPR system consisting of two orthogonal pairs of commercial antennas in broadside configuration. To overcome the image quality deficits of the individual channels, we apply a pseudo-scalar approach in which we combine the data of both polarizations. Results of helicopter-borne GPR surveys on two alpine glaciers in Switzerland reveal more coherent bedrock reflections in the summed data compared with single dipole pair profiles. Generally, the dual-polarization setup is more suitable than a single antenna systems, because it is more versatile and less prone to directional effects caused by the placement of the dipole antennas in relation to undulating subsurface reflectors.

Oceanic-like axial crustal high in the central Red Sea

The Red Sea is an important example of a rifted continental shield proceeding to seafloor spreading. However, whether the crust in the central Red Sea is continental or oceanic has been controversial. Contributing to this debate, we assess the basement geometry using seismic reflection and potential field data. We find that the basement topography from seismically derived structure corrected for evaporite and other sediment loading has an axial high with a width of 70–100 km and a height of 0.8–1.6 km. Basement axial highs are commonly found at mid-ocean ridges affected by hotspots, where enhanced mantle melting results in thickened crust. We therefore interpret this axial high as oceanic-like, potentially produced by recently enhanced melting associated with the broader Afar mantle anomaly. We also find the Bouguer gravity anomalies are strongly correlated with basement reflection depths. The apparent density contrast necessary to explain the Bouguer anomaly varies from 220 kg m−3 to 580 kg m−3 with no trend with latitude. These values are too small to be caused primarily by the density contrast between evaporites and mantle across a crust of uniform thickness and density structure, further supporting a thickened crustal origin for the axial high. Complicating interpretation, only a normal to modestly thickened axial crust is predicted from fractionation-corrected sodium contents (Na8.0), and the basement reflection is rugged, more typical of ultra-slow spreading ridges that are not close to hotspots. We try to reconcile these observations with recent results from seismic tomography, which show modest mantle S-wave velocity anomalies under this part of the Red Sea.

A low-angle normal fault and basement structures within the Enping Sag, Pearl River Mouth Basin: Insights into late Mesozoic to early Cenozoic tectonic evolution of the South China Sea area

The basement structure of the Cenozoic Enping Sag, within the Pearl River Mouth Basin on the northern margin of South China Sea, is revealed by borehole-constrained high-quality 3D seismic reflection data. Such data suggest that the Enping Sag is bounded in the north by a low-angle normal fault. We interpret this low-angle normal fault to have developed as the result of the reactivation of a pre-existing thrust fault part of a pre-Cenozoic thrust system. This is demonstrated by the selective reactivation of the pre-existing thrust and by diffuse contractional deformation recognized from the accurate analysis of basement reflections. Another significant result of this study is the finding of some residual rift basins within the basement of the Enping Sag. Both the thrust system and the residual basins are interpreted to have developed after the emplacement of continental margin arc-related granitoids (J3–K1) that define the basement within the study area. Furthermore, seismic sections show that the pre-existing residual rift basins are offset by the main thrust fault and they are both truncated by the Tg unconformity. These structural relationships, interpreted in the frame of previous studies, help us to reconstruct a six-event structural evolution model for the Enping Sag from the late Mesozoic to the early Cenozoic. In particular, we interpret the residual rift basins to have formed as the result of back-arc extension due to the slab roll-back of the Paleo-Pacific Plate subduction in the early K2. The thrust system has recorded a compressional event in the late K2 that followed the back-arc extension in the SCS area. The mechanism of this compressional event is still to be clarified, and might be related to continuous subduction of the Paleo-Pacific Plate or to the continent-continent collision between a micro-continental block and the South China margin.

Three-component seismic land streamer study of an esker architecture through S- and surface-wave imaging

We deployed a newly developed 3C microelectromechanical system-based seismic land streamer over porous glacial sediments to delineate water table and bedrock in Southwestern Finland. The seismic source used was a 500 kg vertical impact drop hammer. We analyzed the SH-wave component and interpreted it together with previously analyzed P-wave component data. In addition to this, we examined the land streamer’s potential for multichannel analysis of surface waves and delineated the site’s stratigraphy with surface-wave-derived S-wave velocities and [Formula: see text] ratios along the entire profile. These S-wave velocities and [Formula: see text] ratios complement the interpretation conducted previously on P-wave stacked section. Peculiarly, although the seismic source used is of a vertical-type nature, the data inspection indicated clear bedrock reflection on the horizontal components, particularly the transverse component. This observation led us to scrutinize the horizontal component data through side-by-side inspection of the shot records of all the three components and particle motion analysis to confirm the S-wave nature of the reflection. Using the apparent moveout velocity of the reflection, as well as the known depth to bedrock based on drilling, we used finite-difference synthetic modeling to further verify its nature. Compared with the P-wave seismic section, bedrock is relatively well delineated on the transverse component S-wave section. Some structures connected to the kettle holes and other stratigraphic units imaged on the P-wave results were also notable on the S-wave section, and particularly on the surface-wave derived S-wave velocity model and [Formula: see text] ratios. Our results indicate that P-, SV-, and SH-wave energy is generated simultaneously at the source location itself. This study demonstrates the potential of 3C seismic for characterization and delineation of the near-surface seismics.