Near Surface 2011 - 17th EAGE European Meeting of Environmental and Engineering Geophysics

Airborne electromagnetic methods are practical tools for large scale geological and hydrogeophysical mapping. In this abstract we show results from the SkyTEM survey on the volcanic islands of Mayotte. The SkyTEM survey is an essential part of a project aiming on establishing a basic geological model of Mayotte. Mayotte is a small group of islands between Africa and Madagascar. The results will for instance be used for future urban planning with respect to the risk of landslides and ground stability. Also ground water resources are of high interest and hereby the need for detailed knowledge about the hydrological system and especially the risk of saltwater intrusion to the aquifers. The results from the SkyTEM survey are very promising. The results reveal geological structures both at large scale as well as at the small scale. Though, there is still a large and challenging task at hand in the geological interpretation of the geophysical result and linking resistivity to lithology. Project partners: BRGM, France, Conseil Général de Mayotte, France, and Aarhus University, Denmark.

In anthropized areas, airborne Time-Domain ElectroMagnetic (TDEM) data may be affected by spikes, oscillations, shifts and/or bumps. Since EM noise spectrum is complex, thresholding and stacking standards techniques are hardly efficient in such environment. Time-consuming and subjective manual cleaning is therefore required. We propose an alternative fast and efficient user-assisted filtering approach. We adapted the Singular Value Decomposition (SVD) to denoise TDEM data. The SVD uses principal component analysis to separate noise and geological components of the signal; the latters are used to reconstruct the EM decays with the geological signal only. The SVD procedure was applied on an anthropized area, in the center of France. The comparison between each reconstructed decay and its corresponding measured decay allowed rejecting efficiently mainly spikes and oscillations. In a second phase, an ad hoc analysis of the map of weights of the “noisy components” showed high correlation with man-made installations and provided a tool to reject mainly most likely soundings biased by shifts. Bumped decays were also localized based on the analysis of specific SVD components. The established denoising procedure provides accurate denoising tools and makes, at least, the manual cleaning much less time consuming and less subjective.

A methodology for classifying automatically metallic, plastic and concrete targets using pattern recognition techniques on GPR data under controlled field conditions was developed. The method consists to develop an Artificial Neural Network (ANN) classifier, using the multilayer perceptron (MLP), with features extracted from GPR profiles over targets in subsoil, and then using it to classify diffraction hyperbolas indicating their position and depth. The classification allows a high resolution reconstruction of the subsurface with reduced computing time. The system was developed in MATLAB and applied to data obtained from the IAG-USP test site, located in the city of São Paulo, Brazil, containing metallic and plastic drums and pipes and concrete tubes under controlled field conditions. The results using real data indicate that the automatic classification of the targets in the subsoil is efficient, contributing for ambiguities reduction in the near surface geophysical data interpretation, besides having application on mapping of targets in subsoil.

This paper presents a practical approach for the application modeling seismic inversion of real seismic amplitude data. The estimation of recorded seismic amplitudes from reflection seismic gathers is performed with the aid of post-stack time migration, which enhances continuity and reflection strength. This approach is applied to new acquisition 3D seismic of West Area Coastal Plain Pekanbaru,Indonesia. The application example represents overlaying structure map with reservoir properties and facies. This will help us to develop and manage reservoir better in mature oil field within the 3D seismic area.

Classical full waveform inversion is a powerful tool to retrieve the Earth properties from seismic measurements. However, the associated objective function that measures the misfit between observed and computed data, suffers from many local minima. We propose an alternative method referred as the Normalized Integration Method, where the objective function measures the misfit between the integral of the absolute value, or of the square, or of the envelope of the signal. Because we only compare functions increasing with time, the objective function has a more convex shape. We first present the different formulations and show how to efficiently compute the gradient of the misfit functions. We then compare the new approach to the classical full waveform inversion and to the Laplace transform through a simple application on a 2D synthetic data set. This example shows that the new approach can be useful for the determination of the long wavelengths of the velocity model.

Deep argillaceous formations are considered in many countries as potential host media for high-level long-lived radioactive waste due to their physical properties. In the experimental station of Tournemire, strike-slip faults with small vertical offsets were intercepted from underground galleries in the clay layer. Our aim is to assess the capacities and limits of high resolution seismic methods to detect and characterise the fault zones from underground works. Elastic Full Waveform Inversion (FWI) and Reverse Time Migration (RTM) are tested using original measurement configurations involving one, two or three perpendicular galleries. In the frame of numerical simulations, we show how RTM can detect only a part of the fault, and how FWI can provide a full or partial quantitative image of the fault zone depending on the measurement configuration.

The aim of this paper was to study effect of an opening to GPR signal, measured with 800 MHz and 1600 MHz transmitter/antenna. Tests were carried out in laboratory scale using two sawn rock blocks. The opening between them varied from 0 to 10 cm. First GPR signal’s velocity was defined in the blocks by calculating two way travel time, as the thickness of the blocks was known. Next the top and bottom of the opening were interpreted from the GPR profile by utilising theoretical change of the reflected signal’s phase in the opening. Openings apertures were calculated from the interpretations and compared to the real ones. Good results were achieved for the greatest apertures, but apertures smaller than 1/6 of the signals wavelength could not be interpreted due to signal summing. Summing could be less of a problem when using other filling materials than air in the opening. These are the next research subjects for the authors.

The passive surface wave method has the potential to reach investigation depths significantly greater than active sources and can be effectively used in very noisy settings. In this study the optimal spread layout and orientation is investigated and determined that would allow surface wave penetration deep enough into the bedrock to permit studies of stress and strain conditions. An array of 336 receivers was deployed in a 2-D spread. The 4.5 Hz vertical geophones were orientated at parallel and orthogonal alignments to nearby railroad tracks. The dominant energy source were trains passing both along the north and west sides of the site. By selecting the appropriate recording times it was possible to uniquely capture trains in a wide range of azimuthal orientations. The relatively large overall spread made it possible to compare various sub-arrays by processing only selected receivers. Arrays orientated parallel to the direction of incident low frequency waves, resolve frequencies as low as 4 Hz even using small number of receivers. These lower frequencies allow a marked increase in the maximum depth of investigation, thereby, helping to solve engineering problems at greater depths than possible with active source investigations.

Two geoelectrical methods were used to characterize the state of an old French landfill cover. The objectives were to locate different materials used for the covering such as clay material or geomembrane and to identify heterogeneities that could be linked to possible defects in the cover due to fractures or cracks. These damages can induce preferential water pathways and unusual increase of leachate within the waste mass. The geoelectrical methods used were the electrical resistivity cartography with an Automatic Resistivity Profiling (ARP©) and the Self Potential method (SP). Results have put in evidence three distinct zones with different geoelectrical signatures that are correlated with three different phases of landfill covering. ARP also seems to have detected the presence of geomembrane installed all around the old landfill to improve its stability and geodrains over two closed alveoli. Local differences in apparent resistivity and self-potential signals have also been identified and could result in default in the clay cover due to thickness variations but also in different lithology, compaction and water content.

Geophysical methods are prone to fail in an urban environment as all sources of noise are present in cities. The Fælledparken Park, a large open space area in Copenhagen, Denmark is not an exception. However, an extension of the metro along sections of high variability on the surface of a limestone surface motivated the use of geoelectrics as complement to seismic methods. The site conditions have implications for hydrogeological understanding and groundwater modelling validation needed for dewatering design during construction. The geo-electrical survey aim was the mapping of sediment layers in open areas where the recharge through Quaternary layers to the limestone aquifer is important for reliable groundwater modelling of dewatering scenarios. The main objectives of the surveys were to; a) Produce information that can be used to refine or update the site specific hydrogeological model for Cityringen tunnel and station locations. b) Map the sediment layers in the area, including an assessment of their areal distribution. c) Map the top of the limestone. The main outcome was that the area had heterogeneous conditions, where the west part was comprised of both sands and tills and a buried valley structure, and in the east a continuous sequence of sands.