Magnetic resonance sounding applied to aquifer characterization

Magnetic resonance sounding (MRS) is distinguished from other geophysical tools used for ground water investigation by the fact that it measures a magnetic resonance signal generated directly from subsurface water molecules. An alternating current pulse energizes a wire loop on the ground surface and the MRS signal is generated; subsurface water is indicated, with a high degree of reliability, by nonzero amplitude readings. Measurements with varied pulse magnitudes then reveal the depth and thickness of water saturated layers. The hydraulic conductivity of aquifers can also be estimated using boreholes for calibration. MRS can be used for both predicting the yield of water supply wells and for interpolation between boreholes, thereby reducing the number of holes required for hydrogeological modeling. An example of the practical application of MRS combined with two-dimensional electrical imaging, in the Kerbernez and Kerien catchments area of France, demonstrates the efficiency of the technique.     

Low-frequency anisotropy in fractured and layered media

Computing effective medium properties is very important when upscaling data measured at small scale. In the presence of stratigraphic layering, seismic velocities and anisotropy parameters are scale and frequency dependent. For a porous layer permeated by aligned fractures, wave-induced fluid flow between pores and fractures can also cause significant dispersion in velocities and anisotropy parameters. In this study, we compare the dispersion of anisotropy parameters due to fracturing and layering at low frequencies. We consider a two-layer model consisting of an elastic shale layer and an anelastic sand layer. Using Chapman's theory, we introduce anisotropy parameters dispersion due to fractures (meso-scale) in the sand layer. This intrinsic dispersion is added to anisotropy parameters dispersion induced by layering (macro-scale) at low frequencies. We derive the series coefficients that control the behaviour of anisotropy parameters at low frequencies. We investigate the influences of fracture length and fracture density on fracturing effect, layering effect and combined effect versus frequency and volume fraction of sand layer. Numerical modelling results indicate that the frequency dependence due to layering is not always the dominant effect of the effective properties of the medium. The intrinsic dispersion is not negligible compared with the layering effect while evaluating the frequency-dependent properties of the layered medium.     

Frequency-dependent PP and PS reflection coefficients in fractured media

When a porous layer is permeated by mesoscale fractures, wave-induced fluid flow between pores and fractures can cause significant attenuation and dispersion of velocities and anisotropy parameters in the seismic frequency band. This intrinsic dispersion due to fracturing can create frequency-dependent reflection coefficients in the layered medium. In this study, we derive the frequency-dependent PP and PS reflection coefficients versus incidence angle in the fractured medium. We consider a two-layer vertical transverse isotropy model constituted by an elastic shale layer and an anelastic sand layer. Using Chapman's theory, we introduce the intrinsic dispersion due to fracturing in the sand layer. Based on the series coefficients that control the behaviour of velocity and anisotropy parameters in the fractured medium at low frequencies, we extend the conventional amplitude-versus-offset equations into frequency domain and derive frequency-dependent amplitude-versus-offset equations at the elastic–anelastic surface. Increase in fracture length or fracture density can enlarge the frequency dependence of amplitude-versus-offset attributes of PP and PS waves. Also, the frequency dependence of magnitude and phase angle of PP and PS reflection coefficients increases as fracture length or fracture density increases. Amplitude-versus-offset type of PP and PS reflection varies with fracture parameters and frequency. What is more, fracture length shows little impact on the frequency-dependent critical phase angle, while the frequency dependence of the critical phase angle increases with fracture density.     

The kinematics of S waves in acoustic orthorhombic media

Orthorhombic models are often used in the seismic industry nowadays to describe azimuthal and polar anisotropy and reasonably realistic in capturing the features of the earth interior. It is challenging to handle so many model parameters in the seismic data processing. In order to reduce the number of the parameters for P wave, the acoustic orthorhombic medium is proposed by setting all on-axis S wave velocities to zero. However, due to the coupled behaviour for P and S waves in the orthorhombic model, the ‘S wave artefacts’ are still remained in the acoustic orthorhombic model, which kinematics needs to be defined and analysed. In this paper, we analyse the behaviour of S wave in acoustic orthorhombic media. By analysis of the slowness surface in acoustic orthorhombic media, we define the S waves (or S wave artefacts) that are more complicated in shape comparing to the one propagating in an acoustic transversely isotropic medium with a vertical symmetry axis. The kinematic properties of these waves are defined and analysed in both phase and group domain. The caustics, amplitude and the multi-layered case for S wave in acoustic orthorhombic model are also discussed. It is shown that there are two waves propagating in this acoustic orthorhombic medium. One of these waves is similar to the one propagating in acoustic vertical symmetry axis media, whereas another one has a very complicated shape consisting of two crossing surfaces.     

Water retention effects on elastic properties of Opalinus shale

Shales play an important role in many engineering applications such as nuclear waste, CO₂ storage and oil or gas production. Shales are often utilized as an impermeable seal or an unconventional reservoir. For both situations, shales are often studied using seismic waves. Elastic properties of shales strongly depend on their hydration, which can lead to substantial structural changes. Thus, in order to explore shaly formations with seismic methods, it is necessary to understand the dependency of shale elastic properties on variations in hydration. In this work, we investigate structural changes in Opalinus shale at different hydration states using laboratory measurements and X-ray micro-computed tomography. We show that the shale swells with hydration and shrinks with drying with no visible damage. The pore space of the shale deforms, exhibiting a reduction in the total porosity with drying and an increase in the total porosity with hydration. We study the elastic properties of the shale at different hydration states using ultrasonic velocities measurements. The elastic moduli of the shale show substantial changes with variations in hydration, which cannot be explained with a single driving mechanism. We suggest that changes of the elastic moduli with variations in hydration are driven by multiple competing factors: (1) variations in total porosity, (2) substitution of pore-filling fluid, (3) change in stiffness of contacts between clay particles and (4) chemical hardening/softening of clay particles. We qualitatively and quantitatively analyse and discuss the influence of each of these factors on the elastic moduli. We conclude that depending on the microstructure and composition of a particular shale, some of the factors dominate over the others, resulting in different dependencies of the elastic moduli on hydration.     

A new parameterization for generalized moveout approximation,based on three rays

A simple and accurate traveltime approximation is important in many applications in seismic data processing, inversion and modelling stages. Generalized moveout approximation is an explicit equation that approximates reflection traveltimes in general two-dimensional models. Definition of its five parameters can be done from properties of finite offset rays, for general models, or by explicit calculation from model properties, for specific models. Two versions of classical finite-offset parameterization for this approximation use traveltime and traveltime derivatives of two rays to define five parameters, which makes them asymmetrical. Using a third ray, we propose a balance between the number of rays and the order of traveltime derivatives. Our tests using different models also show the higher accuracy of the proposed method. For acoustic transversely isotropic media with a vertical symmetry axis, we calculate a new moveout approximation in the generalized moveout approximation functional form, which is explicitly defined by three independent parameters of zero-offset two-way time, normal moveout velocity and anellipticity parameter. Our test shows that the maximum error of the proposed transversely isotropic moveout approximation is about 1/6 to 1/8 of that of the moveout approximation that had been reported as the most accurate approximation in these media. The higher accuracy is the result of a novel parameterization that do not add any computational complexity. We show a simple example of its application on synthetic seismic data.     

Seismicityof the Novaya Zemlya archipelago: relocated event catalog from 1974 to 2014

We have relocated seismic events registered in the area of the Novaya Zemlya Archipelago, which are not identified as nuclear explosions but are probably of tectonic nature. For the relocation, we collected all available seismic bulletins and waveform data. The location was performed by applying a modified method of generalized beamforming. Verification of the modified method and selection of the travel time model were performed using data on two Novaya Zemlya nuclear explosions that occurred on 02.11.1974 and 24.10.1990. The modified method and the BARENTS travel time model provide sufficient accuracy for event location in the region. The relocation procedure was applied for 9 seismic events registered in the area of the Novaya Zemlya Archipelago. As a result, the new coordinates of the five events turned out to be significantly different from those that were defined previously.     

Isotopic composition of dissolved inorganic carbon in subsurface sediments of gas hydrate-bearing mud volcanoes, Lake Baikal: implications for methane and carbonate origin

We report on the isotopic composition of dissolved inorganic carbon (DIC) in pore-water samples recovered by gravity coring from near-bottom sediments at gas hydrate-bearing mud volcanoes/gas flares (Malenky, Peschanka, Peschanka 2, Goloustnoe, and Irkutsk) in the Southern Basin of Lake Baikal. The δ¹³C values of DIC become heavier with increasing subbottom depth, and vary between ?9.5 and +21.4‰ PDB. Enrichment of DIC in ¹³C indicates active methane generation in anaerobic environments near the lake bottom. These data confirm our previous assumption that crystallization of carbonates (siderites) in subsurface sediments is a result of methane generation. Types of methanogenesis (microbial methyl-type fermentation versus CO₂-reduction) were revealed by determining the offset of δ¹³C between dissolved CH₄ and CO₂, and also by using δ¹³C and δD values of dissolved methane present in the pore waters. Results show that both mechanisms are most likely responsible for methane generation at the investigated locations.