Mathieu J. Duchesne

Exposure to seismic survey alters blue whale acoustic communication

The ability to perceive biologically important sounds is critical to marine mammals, and acoustic disturbance through human-generated noise can interfere with their natural functions. Sounds from seismic surveys are intense and have peak frequency bands overlapping those used by baleen whales, but evidence of interference with baleen whale acoustic communication is sparse. Here we investigated whether blue whales (Balaenoptera musculus) changed their vocal behaviour during a seismic survey that deployed a low-medium power technology (sparker). We found that blue whales called consistently more on seismic exploration days than on non-exploration days as well as during periods within a seismic survey day when the sparker was operating. This increase was observed for the discrete, audible calls that are emitted during social encounters and feeding. This response presumably represents a compensatory behaviour to the elevated ambient noise from seismic survey operations.

The St Lawrence Platform and Appalachian deformation front in the St Lawrence Estuary and adjacent areas (Quebec, Canada): structural complexity revealed by magnetic and seismic imaging

New magnetic and seismic datasets offer a unique opportunity to document the geometry of the Appalachian front in a poorly studied segment of the orogen. Interpretation of high-resolution magnetic data allows, for the first time, the documentation of the contact between the autochthonous St Lawrence Platform and the Appalachians and highlights the regional significance of previously poorly documented ENE faults that experienced post-Ordovician strike-slip motion. Seismic data reveal tectonic slices in the foreland domain underlying the Appalachians and show that the depth of the decollement at the base of the Appalachian tectonic wedge varies significantly. Taken together, geological, magnetic and seismic data suggest that the geometry of the Appalachian front exhibits significant variations in map and cross-section views and recorded a polyphased structural history.

Did you smooth your well logs the right way for seismic interpretation

Correlations between physical properties and seismic reflection data are useful to determine the geological nature of seismic reflections and the lateral extent of geological strata. The difference in resolution between well logs and seismic data is a major hurdle faced by seismic interpreters when tying both data sets. In general, log data have a resolution of at least two orders of magnitude greater than seismic data. Smoothing physical property logs improves correlation at the seismic scale. Three different approaches were used and compared to smooth a density log: binomial filtering, seismic wavelet filtering and discrete wavelet transform (DWT) filtering. Regression plots between the density logs and the acoustic impedance show that the data smoothed with the DWT is the only method that preserves the original relationship between the raw density data and the acoustic impedance. Smoothed logs were then used to generate synthetic seismograms that were tied to seismic data at the borehole site. Best ties were achieved using the synthetic seismogram computed with the density log processed with the DWT. The good performance of the DWT is explained by its adaptive multi-scale characteristic which preserved significant local changes of density on the high-resolution data series that were also pictured at the seismic scale. Since synthetic seismograms are generated using smoothed logs, the choice of the smoothing method impacts on the quality of seismic-to-well ties. This ultimately can have economical implications during hydrocarbon exploration or exploitation phases.

Marine geology of the St. Lawrence Estuary

The St. Lawrence Estuary, Eastern Canada, contains a very thick (>450 m) Quaternary sedimentary sequence. The results from recently conducted geophysical surveys in conjunction with piston coring indicate that these sediments were deposited under very high sedimentation rates, sometimes as high as ~30 m/ka during the last deglaciation. Results also reveal evidence of large submarine landslides during the Holocene, changes in sedimentation rates and the significant role of submarine canyons and channels to transfer sediments from the coast to the deeper marine environment. Finally, this paper highlights the presence of more than 1900 pockmarks on the seafloor of the St. Lawrence Estuary and discusses their possible origins: active hydrocarbon seeps in the Laurentian Channel and biogenic gas seepage on the northwestern shoulder of the Laurentian Channel.

Timing and controls on the delivery of coarse sediment to deltas and submarine fans on a formerly glaciated coast and shelf

The evolution of deltas and submarine fans is often envisioned as largely controlled by relative sea-level variations. However, in some cases, relative sea level can have less effect on delta and submarine fan activity than sediment supply and shelf geomorphology. In order to document the relative importance of these three factors on deltaic and submarine fan evolution in a formerly glaciated environment, this paper documents the delivery of coarse sediment to the Laurentian Channel (eastern Canada). The well-constrained stratigraphic and geomorphologic framework of both the glacio-isostatically uplifted deltas and the modern Laurentian Channel fans allows us to document and contrast the evolution of river-fed deltas, river-fed canyon/fan systems, and longshore drift–fed fans during deglacial and postglacial times. The evolution of these different types of fans can be divided into three phases. The first phase was characterized by delta progradation on the shelf while relative sea level was at its maximum, although already falling, and the ice margin gradually retreated inland. The second phase was characterized by the delivery of deltaic sediment in the deep realm of the Laurentian Channel, permitted by the supply of large amounts of glaciogenic

Detection of near-surface hydrocarbon seeps using P- and S-wave reflections

The combined use of P- and S-wave seismic reflection data is appealing for providing insights into active petroleum systems because P-waves are sensitive to fluids and S-waves are not. The method presented herein relies on the simultaneous acquisition of P- and S-wave data using a vibratory source operated in the inline horizontal mode. The combined analysis of P- and S-wave reflections is tested on two potential hydrocarbon seeps located in a prospective area of the St. Lawrence Lowlands in Eastern Canada. For both sites, P-wave data indicate local changes in the reflection amplitude and slow velocities, whereas S-wave data present an anomalous amplitude at one site. Differences between P- and S-wave reflection morphology and amplitude and the abrupt decrease in P-velocity are indirect lines of evidence for hydrocarbon migration toward the surface through unconsolidated sediments. Surface-gas analysis made on samples taken at one potential seeping site reveals the occurrence of thermogenic gas that presumably vents from the underlying fractured Utica Shale forming the top of the bedrock. The 3C shear data suggest that fluid migration locally disturbs the elastic properties of the matrix. The comparative analysis of P- and S-wave data along with 3C recordings makes this method not only attractive for the remote detection of shallow hydrocarbons but also for the exploration of how fluid migration impacts unconsolidated geologic media.

The Corossol Structure: a glaciated crater of possible impact origin in the northwestern Gulf of St Lawrence, eastern Canada

The majority of confirmed impact craters on Earth have been discovered on land. However, the increased use of high-resolution, full-bottom-coverage seafloor mapping methods such as multibeam bathymetry have enabled the discovery of previously unknown subaqueous impact craters on continental margins and in large lakes. High-resolution bathymetric and sub-bottom profiler data collected south of the city of Sept-Iles in the northwestern Gulf of St Lawrence, eastern Canada (Fig. 1a), revealed a previously unreported circular structure of possible impact origin located 20 km off the mainland in water depths of 40–208 m (Fig. 1b, c) that had been overrun by the Laurentide Ice Sheet (Lajeunesse et al. 2013). Fig. 1. The Corossol Structure, northwestern Gulf of St Lawrence, eastern Canada. ( a ) Location of study area (red box; map from GEBCO_08). ( b ) The Corossol Structure from an oblique view. ( c ) The structure is delimited to the north by series of cuesta ridges. Streamlined glacial lineations and till on its surface indicate …

Partly-filled U-shaped morphology of the Laurentian Channel, St Lawrence Estuary, Canada

The geomorphology of the St Lawrence Estuary seafloor is dominated by post-glacial landforms and processes. However, some segments of the estuary are still characterized by a U-shaped morphology, inherited from one or more glacial episodes, that is only partially modified by postglacial sedimentation. Marine-geophysical data on the morphology and stratigraphy of the estuary are used to describe and interpret this partly sediment-filled submarine environment. The Laurentian Channel is a long and continuous trough >300 m deep that extends 1500 km from the mouth of the Saguenay River in the St Lawrence Estuary to the edge of the continental shelf in the Atlantic Ocean (Fig. 1). It is a clear physiographic feature, whose U-shaped morphology in cross-section, including steep bounding escarpments, is particularly well developed at its upstream (western) extent (Fig. 1a). Fig. 1. Multibeam bathymetry, high-resolution seismic and schematic geometry of the U-shaped segment of the Laurentian Channel in the St Lawrence Estuary. ( a ) Sun-illuminated multibeam-bathymetric image showing the U-shaped portion of the Laurentian Channel (LC). Acquisition system Kongsberg EM1000 (before 2005) and Kongsberg EM1002 (2005 and 2006 surveys). Frequency 95 kHz. Grid-cell size 10 m. ( b ) Perspective view from the NE. VE ×15. ( c ) Perspective view from the east …