Integrated Seismic Study of Weathering in Hawaiian Volcanic Flows
Johnathan R. YaedeS. J. NelsonJ. A. FloresMarion WeberStephen J. TurnbullDavid G. TingeyC. ParkJohn H. McBride
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Along with the increasing demands in Denmark for new and deeper groundwater resources, the application of shallow reflection seismic methods has been intensified. While the traditionally low-priced and extensively used electric and electromagnetic methods are used successfully for mapping of the more shallow geology, reflection seismic is an ideal method for mapping of deep (up to several hundreds of metres) buried quaternary valleys and deep tertiary (Miocene) aquifers as well as faults. The cost for carrying out shallow onshore reflection seismic surveys has, however, been a limiting factor towards the use of seismic data in mapping of groundwater resources. Previously, results using a towed land streamer with gimbal mounted geophones and a pipe gun or sledgehammer as seismic source, has been presented by van der Veen and Green, 1998 and van der Veen et. al. 2001. RAMBOLL has developed a new method for shallow reflection seismic data acquisition called Pulled Array Seismic or PAS using a towed trail of conventional geophones and a seismic vibrator as energy source. The new method is considerably faster and more cost-effective compared to traditional shallow reflection seismic data acquisition, and the data quality is fully comparable also to deeper conventional seismic data. Since the first commercial data acquisition with this new method took place in August 2000 there has been a substantially interest for the method. RAMBOLL has thus acquired 106 km of Pulled Array Seismic on 28 lines for 6 different clients in Denmark and southern Sweden.
Geophone
Vertical seismic profile
Reflection
Synthetic seismogram
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Summary For sub-salt modelling offshore, a combined seismic and non-seismic approach can help reduce uncertainty for deeper structures such as volcanics and crustal geometry/type. Due to the presence of thick and complex salt in the area, the seismic imaging is sub-optimal, and therefore gravity and magnetics modelling is required to de-risk the interpretation, especially for the pre-salt sediment fill and for crustal thickness and type. All these factors play a prominent role in hydrocarbon prospectively in the region. The work presented here was first calibrated on one regional 2D seismic line from offshore Uruguay and the interpretation techniques extended to seven regional 2D seismic lines offshore Brazil. Using seismic interpretation along with gravity and magnetic modelling with relative weight applied on one or another depending on the quality of the data in the area resulted in an integrated earth model and mapping of volcanic structures. The clear SDR's signature from offshore Uruguay enabled regional mapping of these structures offshore Brazil in areas that are unclear in the seismic data alone. In addition, using this approach, exhumed mantle and volcanic intrusions in the crusts could also be mapped offshore Brazil which will help during hydrocarbon exploration in the area.
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A bstract In the Gulf of Cadiz, a Tertiary basin became filled by clastic series during Miocene and Pliocene times. This terrigenous influx, derived from the Iberic Meseta in the north, is characterized by a sandy episode during the Tortonian and Messinian. The sand deposits were probably connected with uplift and major erosion of the Meseta during the sliding of the olistostrome, which occupied the south of the basin from late Helvetian to middle Tortonian. High resolution seismic techniques produced a good picture of the stratigraphy and of the depositional environment of the sands. A further study, using the amplitude of the reflections, inversion of seismic traces into acoustic impedance traces, and modeling, provides a remarkable example of the possibilities of seismic stratigraphy for depicting the lateral evolution of facies and localizing hydrocarbon occurrences. Out of seven exploratory wells based upon seismic information, six encountered gas‐bearing sands with economic potential.
Terrigenous sediment
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Multichannel deep seismic reflection data across a passive continental margin in the northern Gulf of Mexico have been acquired, processed, and interpreted together with three-dimensional gravity modeling. The central Gulf basin is structurally asymmetric from north and south. The northern Gulf is underlain by a 8 to 16 km thickness of sedimentary rocks, significantly greater than the southern Gulf. The top of continental crust occurs at a depth of about 8 km beneath the upper Mississippi-Alabama continental shelf and is characterized by horst and graben structures. The top of oceanic crust occurs at a depth of about 12 km below sea level in the deep Gulf of Mexico. The oceanic crust-transitional crust boundary is interpreted around 27$\sp\circ$ 16$\sp\prime$ N latitude in the profile. The seismic section of continental shelf and continental slope shows four distinct shelf edges; Jurassic, early Cretaceous, mid-Oligocene, and present. Sequence stratigraphic study defines ten seismic sequences since the time of opening of the Gulf of Mexico. The correlation of the sequence boundaries defined in the circum-Gulf region indicates that unconformities with mid-Miocene (10.5 Ma), mid-Oligocene (30 Ma), and mid-Cretaceous (97 Ma), and at the top of the Jurassic (131 Ma) are commonly found as major regional unconformities. The depositional history of this part of the northern Gulf margin can be divided into three main depositional periods: (1) shallow marine deposition from the opening of the Gulf to mid-Cretaceous time, (2) deep marine deposition from Cretaceous to mid-Oligocene, and a return to (3) shallow marine deposition since the mid-Oligocene. The depositional history indicates that characterization of the northern Gulf of Mexico continental margin as a terrigenous sediment wedge province was initiated in late Cretaceous time. Comparison of the location of the seismically defined oceanic crust--transitional crust boundary and the location of steep gravity gradients in the central Gulf of Mexico suggests the existence of outer marginal highs, 20 to 50 km wide. This observation constrains the northern limit of the oceanic crust to 20-50 km south of the steep gravity gradient belt in the north-central Gulf to the west where allochthonous salt inhibits seismic imaging of the deeper structures.
Continental Margin
Horst
Passive margin
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<p>Deep volcanic processes and magma intrusion episodes through the crust are typically accompanied by a variety of seismic signals, including volcano-tectonic (VT) seismicity, very long period (VLP) signals and deep low-frequency (DLF) events. These signals can reveal the migration of magma batches and the resonance of magma reservoirs and dikes. The recent 2018-2019 unrest offshore the island of Mayotte, Comoros archipelago, represents the first case of a geophysically monitored magmatic intrusion from a deep sub-Moho reservoir through the whole crust reaching the surface. At Mayotte, a huge magma movement and the following drainage of a deep reservoir were accompanied by a complex seismic sequence, including a massive VT swarm and energetic long-duration very long period (VLP) signals recorded globally. The identification and characterization of ~7000 VTs and ~400 VLPs by applying waveforms-based seismological methods allowed us to reconst the unrest phases: early VTs, migrating upward, were driven by the ascent of a magmatic dike, and tracked its propagating from Moho depth to the seafloor, while later VTs marked the progressive failure of the reservoir&#8217;s roof, triggering its resonance and the generation of long-duration VLPs. At the Eifel, Germany, weak DLFs earthquakes have been recorded over the last decades and located along a deep channel-like structure, extending from sub-Moho depth (~40-45 km) to the upper crust (~5-10 km). While not showing any clear migration, they reveal a different way of fluid transfer from depth towards the surface, possibly marking intermediate small reservoirs along a feeding channel. Here, brittle failure occurring in the vicinity of the reservoirs may cause their resonance. The Mayotte and Eifel observations are example of end member models for deep fluid transfer processes through the crust. These examples show that, by listening to seismic signals at different distances and by analysing them with modern waveform based methods, we can provide a detailed picture of deep magmatic processes and enable future eruption early warning.</p>
Dike
Seafloor Spreading
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