André Pugin

High-resolution seismic stratigraphy of an Holocene lacustrine delta in western Lake Geneva (Switzerland)

A high-resolution seismic survey was conducted in western Lake Geneva on a small delta formed by the Promenthouse, the Asse and the Boiron rivers. This dataset provides information on changes in the geometry and sedimentation patterns of this delta from Late-glacial to Present. The geometry of the deposits of the lacustrine delta has been mapped using 300-m spaced grid lines acquired with a 12 kHz Echosounder subbottom profiler. A complete three dimensional image of the sediment architecture was reconstructed through seismic stratigraphie analysis. Six different delta lobes have been recognized in the prodelta area. Depositional centers and lateral extension of the delta have changed through time, indicating migration and fluctuation of river input as well as changes in lake currents and wind regime from the time of glacier retreat to the Present. The delta slope is characterized by a high instability causing slumps developing and by the accumulation of biogenic gas that prevents seismic penetration.

Downhole seismic logging for high‐resolution reflection surveying in unconsolidated overburden

Downhole seismic velocity logging techniques have been developed and applied in support of high‐resolution reflection seismic surveys. For shallow high‐resolution reflection surveying within unconsolidated overburden, velocity‐depth control can sometimes be difficult to achieve; as well, unambiguous correlation of reflections with overburden stratigraphy is often problematic. Data obtained from downhole seismic logging can provide accurate velocity‐depth functions and directly correlate seismic reflections to depth. The methodologies described in this paper are designed for slimhole applications in plastic‐cased boreholes (minimum ID of 50 mm) and with source and detector arrays that yield similar frequency ranges and vertical depth resolutions as the surface reflection surveys. Compressional- (P-) wave logging uses a multichannel hydrophone array with 0.5-m detector spacings in a fluid‐filled borehole and a high‐frequency, in‐hole shotgun source at the surface. Overlapping array positions downhole result...

First-Arrival Alignment Static Corrections Applied to Shallow Seismic Reflection Data1

The proper handling of static corrections is an issue that is of critical importance to shallow seismic reflection surveys because of the high frequencies used, and the significant velocity and thickness variations that frequently exist in the very-near surface. The application of standard conventional methods of determining static corrections must be very carefully considered, as these are sometimes inadequate for shallow seismic reflection data. This paper addresses the problem of static corrections for shallow reflection data in terms of long-, medium-, and short-wavelength statics related to topography and variations in the very-low-velocity, near-surface layer, and presents a first-arrival alignment method of static corrections which is an adaptation of refraction and common offset methods. First-break picking is completed on the entire data set, and a refraction analysis of first-arrival data at selected intervals along the survey line is used to estimate a laterally-interpolated, layered, near-surf...

High-resolution seismic imaging of outcrop-scale channels and an incised-valley system within the fluvial-dominated Lower Freshwater Molasse (Aquitanian, western Swiss Molasse Basin)

Abstract High-resolution seismic reflection surveys on Lake Geneva near Lausanne, Switzerland, allow the interpretation of outcrop-scale sedimentary features within the fluvial-dominated Lower Miocene, Lower Freshwater Molasse bedrock. Seismic modelling of an outcrop is used to calibrate the seismic data and assist with interpretation. Seismic resolution is sufficient to image sandstone channel-fill complexes 4–5 m thick. Five seismic facies are recognized and interpreted as fluvial architectural elements of the Lower Freshwater Molasse. Two of these facies are characterized by fairly continuous reflections of variable frequency and low-to-moderate amplitude, interpreted as metre-thick channel-fill complexes. The lateral extent of such channels ranges from 100 to 170 m. The other three seismic facies are characterized by moderate-to-high amplitude, sub-parallel continuous reflections. The three facies are mainly distinguished using reflection frequency and continuity. They correspond to distinct floodplain settings, i.e. proximal, intermediate and distal, with respect to the active channel belt. On a larger scale, a portion of a ca. 50-m deep incised-valley system, probably caused by a relative base-level fall, was identified. Two distinct valley-fill units are recognized. The lower unit was interpreted as comprising valley-edge slump units by comparison with outcrop data. The upper unit is interpreted as the fluvial fill deposited in the most landward portion of the valley.

Bottom-current and wind-pattern changes as indicated by Late Glacial and Holocene sediments from western Lake Geneva (Switzerland)

The Late-Glacial and Holocene sedimentary history of the Hauts-Monts area (western Lake Geneva, Switzerland) is reconstructed combining high resolution seismic stratigraphy and well-dated sedimentary cores. Six reflections and seismic units are defined and represented by individual isopach maps, which are further combined to obtain a three-dimensional age-depth model. Slumps, blank areas and various geometries are identified using these seismic data.

Bedrock, Quaternary sediments and recent fault activity in central Lake Neuchâtel, as derived from high-resolution reflection seismics

A high-resolution seismic reflection survey (ca. 65 km) was conducted in the central part of Lake Neuchâtel in order to trace the continuation of the major strike-slip fault of La Lance across the lake. On seismic data, this dextral fault appears at the top Molasse/Mesozoic bedrock as a ca. 1 km wide zone affecting the top Molasse and offsetting the Mesozoic. These seismic observations can be correlated with onshore data. Four seismic facies are distinguished and interpreted from bottom to top within the Quaternary: glacial, subglacio-lacustrine, glacio-lacustrine and lacustrine. They show similarities with other perialpine lakes (Lake Geneva, Lake Annecy, Lake Le Bourget). Slumps and faults affecting the upper part of the Quaternary may be associated with the recent activity of the La Lance fault zone. Moreover, glacial erosion of the Molasse bedrock seems to have been locally guided by active tectonics.

Facies model for deglaciation in an overdeepened alpine valley (Bulle area, Western Switzerland)

Abstract This study is concerned with glaciolacustrine sedimentation at the front of the Western Swiss Alps, at the former meeting point of the subsidiary Sarine glacier and the ice stream of the Rhone glacier. During the last glaciation (Late Wurmian), these two glaciers eroded into pre-existing glacial sediments and excavated two overdeepened basins in the bedrock. The infill of these basins by glaciolacustrine sediments occurred in two phases. First, diamicts and turbidites were deposited in stagnant or flowing water conditions under the glacier. This episode was followed by deposition of eskers, glaciolacustrine and glaciofluvial outwash, deltaic deposits, alluvial fans and intraglacial moraines in a proglacial lake basin. The glacial sediments are overlain by clay, marl and chalk deposited in a temperate climate. The Quaternary sedimentary succession is a record of rapid climatic change: in 10,000 years, the climate evolved from polar to temperate.

BURIED VALLEY IMAGING USING 3-C SEISMIC REFLECTION, ELECTRICAL RESISTIVITY AND AEM SURVEYS

In the Canadian Prairies buried valleys are important sources of groundwater. Hydrological methods such as pumping tests provide very limited spatial information to efficiently predict the sustainability of these aquifers. To obtain a full assessment in three dimensions of such complex reservoir geometry, geophysical tools are an absolute necessity. The Spiritwood valley in southwestern Manitoba, is a Canada-USA transborder buried-valley aquifer. In March 2010, the Geological Survey of Canada conducted an airborne electromagnetic (AEM) survey (AeroTEM III) over a 1062 km2 area along the buried valley north of the US border. The results show multiple resistive elongated features which have been interpreted as coarse sediment filled channels inside a 15 km wide more conductive valley filled with finer sediments such as diamictons. The spatial distribution, directionality, and size of the channels are complex. Follow up ground surveys were carried out during the summer and included a ground based, multi-electrode electrical resistivity survey to calibrate the resistivity of the various units seen in the AEM data, as well as a high-resolution seismic survey to obtain detailed architectural and depth information. The seismic data were collected using a Minivib I in inline horizontal vibrating mode (20240 Hz sweep) at a shot spacing of 6 m and a 3-component (3-C) landstreamer receiver array with 48 sleds spaced at 1.5 m. These data allow us to obtain both shear wave and compressive wave profiles. The younger, less compacted channels were better imaged with P-wave data, while some areas with shallow gas or organic peats were better imaged with S-wave data. The seismic images show detailed sedimentary sequences and permit some inferences on the relative ages of channels formed during multiple ice advances. The sections also showed the presence of other channels, which are interpreted to be infilled with finer sediments based on the seismic facies, and which are not associated with resistive features in the AEM data. This combination of AEM, electric sounding and 3-C seismic profiling provides exceptional 3-D coverage which has highlighted key hydrological features such as buried channel aquifers and potential sub-surface hydraulic pathways or connections. Such information is critical to groundwater prospecting and to the accurate assessment of recharge and discharge potentials associated with buried valley aquifers.

AIRBORNE TIME-DOMAIN ELECTROMAGNETICS FOR THREE- DIMENSIONAL MAPPING AND CHARACTERIZATION OF THE SPIRITWOOD VALLEY AQUIFER

The Geological Survey of Canada commissioned a helicopter-borne time-domain electromagnetic (HTEM) survey over a 1062 km 2 area of the Spiritwood Valley in southern Manitoba to test the effectiveness of airborne time-domain electromagnetics for mapping and characterizing buried valley aquifers in the Canadian Prairies. The HTEM data exhibit rich information content; apparent conductivity maps clearly image the Spiritwood Valley in addition to a continuous incised valley along the broader valley bottom. We detect complex valley morphology with nested scales of valleys including at least three distinct valley features and multiple possible tributaries. Conductivity-depth images (CDI) derived from the TEM decays indicate that the fill materials within the incised valleys are more resistive than the broader valley fill, consistent with an interpretation of sand and gravel. Comparison of ground-based electrical resistivity and seismic reflection data allow for calibration of CDI models. Lateral spatial information is in excellent agreement between data sets. The seismic data reveal the presence of additional valley features that are not imaged by the HTEM data as having a distinct electrical signature, possibly due to diamicton fill. The CDI model underestimates the dynamic range of electrical conductivity while overestimating depths to valley bottoms; these issues may be associated with system limitations, system bandwidth, algorithm limitations and penetration depth. The integrated data sets illustrate that HTEM surveys have the potential to map complicated buried valley aquifers at a level of detail required for groundwater prospecting and management.

Multicomponent vibroseismic profiling over high velocity glacial ground: an example from Southern Ontario

A 3-D Quaternary mapping project conducted by the Ontario Geological Survey (OGS) in the southern part of Simcoe County involves borehole drilling, airborne geophysics, such as TDEM and magnetics and ground gravity surveys. Geophysical surveys are necessary to define the top of bedrock, including buried bedrock valleys and the architecture of overlying sediments for evaluating groundwater resources. In support of this project, the Geological Survey of Canada (GSC) carried out a three-line 21.2 km seismic reflection survey. Geophysical logging in two deep boreholes was undertaken to assist with the calibration of the seismic sections. The seismic survey was performed using an IVI “Minivib 1” source with a “landstreamer” three-component geophone array built by the GSC. The landstreamer consists of 72 - 3 kg metal sleds, spaced at 1.5 m, towed using low-stretch belts. Data were acquired with shot points every 4.5 m. The source vibrates a 140 kg mass in in-line (H1) horizontal mode, using a 7 second nonlinear logarithmic sweep of -2 DB/Oct from 20 to 300 Hz. This type of sweep increases the time spent in the low end of the sweep which has the effect to increase the low frequency energy to enhance shear body wave energy. Data were recorded using seven 24- channel Geometrics Geode engineering seismographs operated in the cab of the Minivib. Only the vertical component of the 24 geophones, furthest from the source, was recorded in order to obtain a better coverage of the P-wave data acquisition window. Uncorrelated records were collected to allow pre-whitening of the data and careful choice of the correlating function was the first step in the data processing sequence. P-wave sections were derived from processing the first 0.5 sec. (after correlation) of data acquired on the vertical geophones, while S-wave sections were produced using the in-line, H1, component over a correlated window of 2 seconds. Seismic sections were then correlated with borehole geophysical data. Interpretation of the equivalent compressional (P-) wave section permits delineation of seismic facies sequences. The P-wave velocity is an order of magnitude higher than the shearwave velocity and as a result, the vertical resolution of the section is lower. However, the acoustic impedance contrast with underlying materials (coarser sediments, tills or bedrock) is lower than in the case of shear-wave. The shear-wave data produce remarkably detailed sections over buried valleys down to 150 m.