Bernard Long

CAT-scan in marine stratigraphy: a quantitative approach

Abstract Computed axial tomography (CAT-scan) gives a pseudo-3D representation of sedimentary cores. Sediments may be described and numerically processed from digitized high resolution images. A 150 m long borehole was drilled and sampled at Ile-aux-Coudres, middle estuary of the St. Lawrence, Quebec, and analysed by a tomodensitometer (CAT-scanner). This borehole represents mainly the Illinoian-Sangamonian transition series (isotopic stage 5e, 130-80 kyr B.P.) of a post-glacial glacioisostatic marine invasion and the subsequent regressive prodelta. Four representative samples, 900 mm long, were chosen for analysis from a transition zone between glacial and prodeltaic sediments. They have been described from tomographic longitudinal and transversal sections, from CT number series, and also from classical analysis such as grain size, organic matter and carbonates. CT numbers reflect sedimentary characteristics and are closely linked to grain size and to organic matter. Carbonates are only weakly correlated, probably because of its low concentration. Bed thickness and numerical processing of CT number series are used to characterize lamina, and allow for depicting sub-annual, annual and longer periodicities. Clay facies, fine and thick rhythmites, and isolated diamicton beds were identified. From bottom to top, the first sample contains homogeneous clay rhythmites, typical of a distal environment; the second is composed of diamictons and thin, alternating light-dark annual lamina sets; in the third sample, annual light-dark rhythmites are slightly thicker and display subtle semi-annual cyclicities; in the fourth sample, annual rhythmites are several centimetres in thickness and contain preserved sub-annual events, of possible seasonal cyclicity. This succession of facies and associated stratal periodicities may be typical of transgressive sedimentation in a basin such as the St. Lawrence estuary during the transition from glacial to non-glacial conditions.

Assessment of the spatial variability of intertidal benthic communities by axial tomodensitometry: importance of fine-scale heterogeneity

At the water–sediment interface of aquatic ecosystems, the presence of biogenic structures produced by benthic invertebrates strongly affects biogeochemical processes. The quantification of these structures and the assessment of the vertical distribution of fauna are essential for determining the impact of communities in sediments. In the present study, computer axial tomodensitometry (CAT-scan) was used to measure the space occupied by an intertidal community of the St. Lawrence estuary. Three cores were sampled at a site that was considered homogeneous according to surface sediments. The vertical distribution of biogenic structures and gravel were measured in the three cores using CAT-scan; the vertical distribution of fauna was also analysed for each core. The biogenic structures were highest at the water–sediment interface and decreased with depth in the three cores. The number of invertebrates also decreased with depth. We observed similar distributions of biogenic structures in cores 1 and 2. However, fewer biogenic structures were observed below 90 mm in core 3. This result was correlated with a high quantity of gravel from 90 to 140 mm in core 3 whereas the other cores had lower quantities of coarse material. We found relationships among the distributions of biogenic structures, fauna, and sediment characteristics (gravel quantity) that can affect species distribution. The vertical distributions of Macoma balthica, Mya arenaria, Nereis virens, and small-sized gallery-producing species (nematodes and oligochaetes) could also be recorded with the CAT-scan method. Thus, CAT-scan is an excellent tool to determine the fine-scale heterogeneity in the space occupied by benthic invertebrates in sediments.

Mapping the Shallow Water Seabed Habitat With the SHOALS

The scanning hydrographic operational airborne light detection and ranging (LiDAR) Survey (SHOALS) consists of a bathymetric LiDAR system that provides high-precision measurements of water depth. Although the acquisition is focused on depth accuracy, the return signal, i.e., waveform, contains other relevant information because of integration signatures from the water surface, the water column, and the seabed. This paper highlights the benthic characterization in extracting statistical parameters derived from the bottom backscatter and classifying them. In implementing a specific unsupervised classification, it is significantly proven that the signals derived from habitat, described as statistically homogeneous throughout ground-truth analysis, are similar within an intrahabitat view, whereas they are different between themselves.

Supracrustal faults of the St. Lawrence rift system, Québec: kinematics and geometry as revealed by field mapping and marine seismic reflection data

Abstract The St. Lawrence rift system from the Laurentian craton core to the offshore St. Lawrence River system is a seismically active zone in which fault reactivation is believed to occur along late Proterozoic to early Paleozoic normal faults related to the opening of the Iapetus ocean. The rift-related faults fringe the contact between the Grenvillian basement to the NW and Cambrian–Ordovician rocks of the St. Lawrence Lowlands to the SE and occur also within the Grenvillian basement. The St. Lawrence rift system trends NE–SW and represents a SE-dipping half-graben that links the NW–SE-trending Ottawa–Bonnechere and Saguenay River grabens, both interpreted as Iapetan failed arms. Coastal sections of the St. Lawrence River that expose fault rocks related to the St. Lawrence rift system have been studied between Quebec city and the Saguenay River. Brittle faults marking the St. Lawrence rift system consist of NE- and NW-trending structures that show mutual crosscutting relationships. Fault rocks consist of fault breccias, cataclasites and pseudotachylytes. Field relationships suggest that the various types of fault rocks are associated with the same tectonic event. High-resolution marine seismic reflection data acquired in the St. Lawrence River estuary, between Rimouski, the Saguenay River and Forestville, identify submarine topographic relief attributed to the St. Lawrence rift system. Northeast-trending seismic reflection profiles show a basement geometry that agrees with onshore structural features. Northwest-trending seismic profiles suggest that normal faults fringing the St. Lawrence River are associated with a major topographic depression in the estuary, the Laurentian Channel trough, with up to 700 m of basement relief. A two-way travel-time to bedrock map, based on seismic data from the St. Lawrence estuary, and comparison with the onshore rift segment suggest that the Laurentian Channel trough varies from a half-graben to a graben structure from SW to NE. It is speculated that natural gas occurrences within both the onshore and offshore sequences of unconsolidated Quaternary deposits are possibly related to degassing processes of basement rocks, and that hydrocarbons were drained upward by the rift faults.

Predicting Species Diversity of Benthic Communities within Turbid Nearshore Using Full-Waveform Bathymetric LiDAR and Machine Learners

Epi-macrobenthic species richness, abundance and composition are linked with type, assemblage and structural complexity of seabed habitat within coastal ecosystems. However, the evaluation of these habitats is highly hindered by limitations related to both waterborne surveys (slow acquisition, shallow water and low reactivity) and water clarity (turbid for most coastal areas). Substratum type/diversity and bathymetric features were elucidated using a supervised method applied to airborne bathymetric LiDAR waveforms over Saint-Siméon–Bonaventures nearshore area (Gulf of Saint-Lawrence, Québec, Canada). High-resolution underwater photographs were taken at three hundred stations across an 8-km2 study area. Seven models based upon state-of-the-art machine learning techniques such as Naïve Bayes, Regression Tree, Classification Tree, C 4.5, Random Forest, Support Vector Machine, and CN2 learners were tested for predicting eight epi-macrobenthic species diversity metrics as a function of the class number. The Random Forest outperformed other models with a three-discretized Simpson index applied to epi-macrobenthic communities, explaining 69% (Classification Accuracy) of its variability by mean bathymetry, time range and skewness derived from the LiDAR waveform. Corroborating marine ecological theory, areas with low Simpson epi-macrobenthic diversity responded to low water depths, high skewness and time range, whereas higher Simpson diversity relied upon deeper bottoms (correlated with stronger hydrodynamics) and low skewness and time range. The degree of species heterogeneity was therefore positively linked with the degree of the structural complexity of the benthic cover. This work underpins that fully exploited bathymetric LiDAR (not only bathymetrically derived by-products), coupled with proficient machine learner, is able to rapidly predict habitat characteristics at a spatial resolution relevant to epi-macrobenthos diversity, ranging from clear to turbid waters. This method might serve both to nurture marine ecological theory and to manage areas with high species heterogeneity where navigation is hazardous and water clarity opaque to passive optical sensors.

Chapter Two Continuous Physical Properties of Cored Marine Sediments

Publisher Summary This chapter focuses on the continuous physical properties of cored marine sediments that provide the basis for stratigraphy and core correlation, the first insight into core lithology, continuous data for time series analyses and a decision tool for determining the best subsampling strategy. Physical properties that are now routinely measured continuously, both onboard and onshore, include: natural gamma radiation, gamma density, p -wave velocity, magnetic susceptibility, electrical resistivity, and color reflectance. Most of these properties can now be measured continuously and automatically, on whole or split cores placed horizontally or, when the sediment water interface must remain undisturbed, vertically. Emerging line scan systems and medical techniques may now also be used to continuously image the sediment surface on split cores or the internal structure of whole cores by high-resolution imaging, digital X-ray imaging and computerized coaxial tomography (CAT-scan). The chapter reviews continuous, cm-scale, nondestructive methods generally used to determine the physical properties of sediments, and describes emerging methods used both to image and/or determine in 2D or 3D the physical properties of long sedimentary sequences at the millimeter to micrometer-scale, allowing the reconstruction of paleoceanographic or paleoclimatological processes at temporal resolutions on the millennial- to seasonal-scale.

LIDAR Technology Applied in Coastal Studies and Management

Abstract The FUDOTERAM is a national Canadian light detection and ranging (LIDAR) project founded by the Canadian Network of Excellence GEOmatics for Informed DEcision (GEOIDE) that investigates data fusion from airborne, marine, and terrestrial mapping sensors. In March 2009, the second Fusion des Données TERrestres, Aériennes et Marines (FUDOTERAM) workshop was held in Quebec City, Quebec, Canada. The focus of the workshop was on international collaboration: Workshops can provide an international platform for sharing ideas and study results among academy, industry, mapping and charting organizations, and service providers. LIDAR work and research included data collected from seven different coastal areas in four nations. This special issue contains selected studies from the second FUDOTERAM workshop on LIDAR technology applied in coastal studies and management. Current studies in this special issue explore LIDAR processing in charting and mapping organizations, shoreline mapping, data integration, coastal processes and coastal management, and seafloor characterization.

What optech’s bathymetric LiDAR sees underwater

This article presents early results of the FUDOTERAM project using bathymetric LiDAR data acquired with the SHOALS-3000, the latest bathymetric LiDAR system from Optech. The survey area is in the coastal zone along the northern shore of Chaleurs Bay, in the western Gulf of St. Lawrence, Canada. The project aimed to apply the SHOALS- 3000 to geological mapping, sedimentary process monitoring and marine habitat mapping. This paper focuses on the sedimentological part of the study and presents the early raw data obtained to produce a bottom type classification based on some simple parameters, roughness, slope angle and direction. Two methods are evaluated for analysis of the SHOALS-3000 waveforms, the Moment Method and the Gaussian Mixture Model, and the latter is used as an approach to model the bottom type signal.

Accuracy and Limitations of Airborne LiDAR Surveys in Coastal Environments

The results of this work enable specification of the vertical and horizontal accuracy of LiDAR systems. The horizontal error is mainly the result of aircraft positioning and LiDAR spatial resolution limitations and is assumed to be smaller than 30 cm. LiDAR vertical errors were found to be slope-dependent, the greatest errors being observed on cliffs. Sea cliffs are in any case the most challenging features to survey and wave-cut notches constitute a major limitation to airborne surveys. The portion of the cliff that can be surveyed with an airborne LiDAR has been determined according to notch dimensions and laser beam incidence angle. Keywords-LiDAR; remote sensing; accuracy; coastal zone; erosion monitoring; sea cliffs

Shallow seabed mapping and classification using waveform analysis and bathymetry from SHOALS lidar data

This paper demonstrates a methodology for efficient mapping of seabed composition in shallow coastal areas using airborne light detection and ranging (lidar). Employing the latest Scanning Hydrographic Operational Airborne Lidar Survey (SHOALS) bathymetric survey system, we show that bottom sediment textures and algal cover types can be discriminated and classified using shape parameters of the lidar bottom-return signal, supplemented with bathymetric data, notably small-scale roughness. The study area is a region of moderate wave energy with siliciclastic sediment along the north shore of the Baie des Chaleurs in Quebec. Environmental factors affecting the accuracy of the method include sea state, turbidity, benthic cover, patchiness, and fuzzy class boundaries. Technical factors contributing to classification errors include signal definition, water column attenuation, band selection issues, class definition, and possible errors in interpretation of validation data. This study achieved an accuracy of 67% in a complex and challenging setting. Further development of the methodology is expected to improve the quality and expand the applicability of this approach.