Sequence stratigraphy, chemostratigraphy and facies analysis of Cambrian Series 2 – Series 3 boundary strata in northwestern Scotland

Globally, the Series 2 – Series 3 boundary of the Cambrian System coincides with a major carbon isotope excursion, sea-level changes and trilobite extinctions. Here we examine the sedimentology, sequence stratigraphy and carbon isotope record of this interval in the Cambrian strata (Durness Group) of NW Scotland. Carbonate carbon isotope data from the lower part of the Durness Group (Ghrudaidh Formation) show that the shallow-marine, Laurentian margin carbonates record two linked sea-level and carbon isotopic events. Whilst the carbon isotope excursions are not as pronounced as those expressed elsewhere, correlation with global records (Sauk I – Sauk II boundary and Olenellus biostratigraphic constraint) identifies them as representing the local expression of the ROECE and DICE. The upper part of the ROECE is recorded in the basal Ghrudaidh Formation whilst the DICE is seen around 30m above the base of this unit. Both carbon isotope excursions co-occur with surfaces interpreted to record regressive–transgressive events that produced amalgamated sequence boundaries and ravinement/flooding surfaces overlain by conglomerates of reworked intraclasts. The ROECE has been linked with redlichiid and olenellid trilobite extinctions, but in NW Scotland, Olenellus is found after the negative peak of the carbon isotope excursion but before sequence boundary formation

Sequence Stratigraphy, Chemostratigraphy and Facies Analysis of Cambrian Series 2 - Series 3 Boundary Strata in Northwestern Scotland

Globally, the Series 2 - Series 3 boundary of the Cambrian System coincides with a major carbon isotope excursion, sea-level changes and trilobite extinctions. Here we examine the sedimentology, sequence stratigraphy and carbon isotope record of this interval in the Cambrian strata (Durness Group) of NW Scotland. Carbonate carbon isotope data from the lower part of the Durness Group (Ghrudaidh Formation) show that the shallow-marine, Laurentian margin carbonates record two linked sea-level and carbon isotopic events. Whilst the carbon isotope excursions are not as pronounced as those expressed elsewhere, correlation with global records (Sauk I - Sauk II boundary and Olenellus biostratigraphic constraint) identifies them as representing the local expression of the ROECE and DICE. The upper part of the ROECE is recorded in the basal Ghrudaidh Formation whilst the DICE is seen around 30m above the base of this unit. Both carbon isotope excursions co-occur with surfaces interpreted to record regressive-transgressive events that produced amalgamated sequence boundaries and ravinement/flooding surfaces overlain by conglomerates of reworked intraclasts. The ROECE has been linked with redlichiid and olenellid trilobite extinctions, but in NW Scotland, Olenellus is found after the negative peak of the carbon isotope excursion but before sequence boundary formation

Time-lapse imaging of CO2 migration within near-surface sediments during a controlled sub-seabed release experiment

The ability to detect and monitor any escape of carbon dioxide (CO2) from sub-seafloor CO2 storage reservoirs is essential for public acceptance of carbon capture and storage (CCS) as a climate change mitigation strategy. Here, we use repeated high-resolution seismic reflection surveys acquired using a chirp profiler mounted on an autonomous underwater vehicle (AUV), to image CO2 gas released into shallow sub-surface sediments above a potential CCS storage site at 120 m water depth in the North Sea. Observations of temporal changes in seismic reflectivity, attenuation, unit thickness and the bulk permeability of sediment were used to develop a four-stage model of the evolution of gas migration in shallow marine sediments: Proto-migration, Immature Migration, Mature Migration, and Pathway Closure. Bubble flow was initially enabled through the propagation of stable fractures but, over time, transitioned to dynamic fractures with an associated step change in permeability. Once the gas injection rate exceeded the rate at which gas could escape the coarser sediments overlying the injection point, gas began to pool along a grain size boundary. This enhanced understanding of the migration of free gas in near-surface sediments will help improve methods of detection and quantification of gas in subsurface marine sediments

Towards improved monitoring of offshore carbon storage: A real-world field experiment detecting a controlled sub-seafloor CO2 release

Carbon capture and storage (CCS) is a key technology to reduce carbon dioxide (CO2) emissions from industrial processes in a feasible, substantial, and timely manner. For geological CO2 storage to be safe, reliable, and accepted by society, robust strategies for CO2 leakage detection, quantification and management are crucial. The STEMM-CCS (Strategies for Environmental Monitoring of Marine Carbon Capture and Storage) project aimed to provide techniques and understanding to enable and inform cost-effective monitoring of CCS sites in the marine environment. A controlled CO2 release experiment was carried out in the central North Sea, designed to mimic an unintended emission of CO2 from a subsurface CO2 storage site to the seafloor. A total of 675 kg of CO2 were released into the shallow sediments (∼3 m below seafloor), at flow rates between 6 and 143 kg/d. A combination of novel techniques, adapted versions of existing techniques, and well-proven standard techniques were used to detect, characterise and quantify gaseous and dissolved CO2 in the sediments and the overlying seawater. This paper provides an overview of this ambitious field experiment. We describe the preparatory work prior to the release experiment, the experimental layout and procedures, the methods tested, and summarise the main results and the lessons learnt

4D Chirp – high resolution time-lapse imaging of the marine subsurface

Time-lapse seismic imaging refers to the comparison of 3D seismic reflection volumes acquired at different times. This approach has greatly improved our capability to measure and understand dynamic processes in the subsurface. However, there are very few examples using ultra-high frequency(kHz-range) seismic data, especially in marine environments. Exacting requirements for navigation can be prohibitive for acquiring coherent, true-3D volumes and residual errors manifest as noise in time-lapse differences. This is compounded with time-varying amplitudes caused by noise from swell and wash, making it difficult to interpret real subsurface changes. Over coming these challenges opens up a range of applications for monitoring the subsurface at very high resolution. In order to satisfy the requirements for time-lapse imaging, improvements were made to the processing workflow of the 3D Chirp, an ultra-high-frequency sub bottom profiler developed at the University of Southampton. Specifically, methods for post-processing inertially-aided GPS data were incorporated to improve navigation accuracy and stability, minimising the impact of multipath and signal dropouts when surveying in challenging, shallow water environments. This approach provides the ability to acquire repeatable trace data with centimetric-accuracy, absolute positioning, binned onto 25 cm or smaller common-midpoint grids. Following these improvements, two 4D Chirp case studies are presented. In the first, changes to a shallow gas blanket in the Southampton estuary are investigated. Differences are quantified between stacked data acquired at different tidal states. Reflections from the top and bottom of a gas pocket are imaged at low tide, whereas at high tide only the upper reflection is imaged, attributed to hydrostatic pressure-related changes in the free gas. Changes in environmental noise between the volumes were minimized by matching amplitudes at the seabed resulting in a mean repeatability of approximately 40% NRMS, slightly greater than conventional seismic reflection data acquired with towed streamers. The second case study compares two 3D Chirp volumes acquired within a dry dock in Blyth. An artificial sedimentary sequence was disturbed by excavating trenches, burying targets and traversing the site on foot and with machinery. Differences in the acoustic data were used to quantitatively map the anthropogenic disturbance of the dock bed and the shallow subsurface sediments. A comparison of diffractions from an unchanging concrete structure revealed high waveform similarity and amplitude repeatability equal to 5.1% of the stacked volumes. Elsewhere, windowed dock bed RMS amplitudes increased by a mean factor of 3.72, most likely due reduced sediment porosity through compaction. Amplitude differences within the dock were spatially variable and well correlated with centimetric bed-level changes and the location excavated trenches. This relationship was refined by reducing coherent 4D noise: a non-repeatable acquisition footprint resulting primarily from residual navigation errors. This was achieved by applying carefully parameterized trim statics to both 3D volumes, increasing amplitude fidelity and interpretability of the seismic data. These two case studies demonstrate the viability of time-lapse UHF 3D seismic reflection for mapping real, decimetre-scale changes within the shallow marine subsurface, and how a quantitative interpretation of time-varying processes can improve our understanding of complex and heterogenous near-surface sediments and lead to the development of better physical models

Time-lapse imaging using 3D ultra-high-frequency marine seismic reflection data

Time-lapse (4D) seismic imaging is now widely used as a tool to map and interpret changes in deep reservoirs as well as investigate dynamic, shallow hydrological processes in the near surface. However, there are very few examples of time-lapse analysis using ultra-high-frequency (UHF; kHz range) marine seismic reflection data. Exacting requirements for navigation can be prohibitive for acquiring coherent, true-3D volumes. Variable environmental noise can also lead to poor amplitude repeatability and make it difficult to identify differences that are related to real physical changes. Overcoming these challenges opens up a range of potential applications for monitoring the subsurface at decimetric resolution, including geohazards, geologic structures, as well as the bed-level and subsurface response to anthropogenic activities. Navigation postprocessing was incorporated to improve the acquisition and processing workflow for the 3D Chirp subbottom profiler and provide stable, centimeter-level absolute positioning, resulting in well-matched 3D data and mitigating 4D noise for data stacked into 25×25 cm common-midpoint bins. Within an example 4D data set acquired on the south coast of the UK, interpretable differences are recorded within a shallow gas blanket. Reflections from the top and bottom of a gas pocket are imaged at low tide, whereas at high tide only the upper reflection is imaged. This case study demonstrates the viability of time-lapse UHF 3D seismic reflection for quantitative mapping of decimeter-scale changes within the shallow marine subsurface.</p