We present a development of capacitively coupled EM sensors integrated in non-corrosive casings for permanent CSEM monitoring in boreholes. Capacitive sensors are required to detect low-frequency (diffusive-field) signals where voltage measurements fail and ammeters need to be used. The permanent installation in boreholes necessitates surface placement of the electronic components to ensure their longevity and accessibility. An issue is that small current signals need to be transferred over a large distance via cables whose capacitances are larger than the ones from the sensors, so a circuit of a Zero-Resistance Ammeter with Integrator (ZRA-I) was developed for annihilating the cable-capacitance effect. Via modelling, lab and small-scale field testing, we were able to show that capacitive sensors with ZRA-I electronics worked well: although the desired signal is slightly decreased compared to the one from galvanically coupled sensors, the signal-to-noise ratios are comparable, for the frequencies used. So we show that capacitive sensors can successfully be integrated in composite casings and, with the proper sensor electronics, can well be used for permanent CSEM monitoring in boreholes.@en

Fluid mud plays an important role in navigability in ports and waterways. Characterizing and monitoring the seismic properties of the fluid mud can help understand its geotechnical behavior. Estimation of the wave velocities in fluid mud with high accuracy and repeatability enables investigating the behavior of parameters like the yield stress in a nonintrusive and reliable way. We perform ultrasonic reflection measurements in a laboratory to investigate the wave propagation in a water/fluid-mud layered system. The component of wave propagation in the water layer inevitably brings kinematic dependence on the characteristics of that layer, making the estimation of exact velocities in the fluid mud more challenging. In order to extract the wave velocities only in the fluid-mud layer, we use a reflection geometry imitating field measurement to record the ultrasonic data from sources and receivers in the water layer. We then use seismic interferometry to retrieve ghost reflections from virtual sources and receivers placed directly at the water-mud interface. Using velocity analysis applied to the ghost reflections, we successfully obtain the P-wave and S-wave velocities only inside the fluid-mud layer, and investigate the velocity change during the self-weight consolidation of the fluid mud. Our results indicate that the S-wave velocities of the fluid mud increase with consolidation time, and show that reflection measurements and ghost reflections can be used to monitor the geotechnical behavior of fluid mud.@en

Knowledge about the characteristics of fluid mud in ports and waterways would allow safer navigating through fluid mud. The properties of the fluid mud determine the feasibility of navigating vessels through the fluid mud. Seismic waves have the potential to help characterize the fluid-mud layers, especially when both P- and S-waves are used. To investigate the possibility of using reflections measurements for more accurate fluid-mud characterization, we perform ultrasonic reflection experiment on fluid mud from Port of Rotterdam. We apply seismic interferometry to the measurements to retrieve non-physical (ghost) arrivals from inside the fluid mud layer and to eliminate the kinematic influence of the water layer above it. We show how we retrieve P-wave ghost reflections and analogously how we can retrieve S-wave and P-to-S-converted ghost reflections.

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In ports and waterways, the bathymetry is regularly surveyed for updating navigation charts ensuring safe transport. In port areas with fluid-mud layers, most traditional surveying techniques are accurate but are intrusive and provide one-dimensional measurements limiting their application. Current non-intrusive surveying techniques are less accurate in detecting and monitoring muddy consolidated or sandy bed below fluid-mud layers. Furthermore, their application is restricted by surveying-vessels availability limiting temporary storm- or dredging-related bathymetrical changes capture. In this chapter, we first review existing non-intrusive techniques, with emphasis on sound techniques. Then, we give a short review of several seismic-exploration techniques applicable to non-intrusive fluid-mud characterization and monitoring with high spatial and temporal resolution. Based on the latter, we present recent advances in non-intrusive fluid-mud monitoring using ultrasonic transmission and reflection measurements. We show laboratory results for monitoring velocity changes of longitudinal and transverse waves propagating through fluid mud while it is consolidating. We correlate the velocity changes with shear-strength changes while the fluid mud is consolidating and show a positive correlation with the yield stress. We show ultrasonic laboratory results using reflection and transmission techniques for estimating the fluid-mud longitudinal- and transverse-wave velocities. For water/mud interface detection, we also use distributed acoustic sensing (DAS) and distributed temperature sensing (DTS)@en

Ultrasound measurements are routinely used to evaluate the safe depth for ships navigation-nautical depth-at waterways and ports using single-beam dual-frequency echo-sounders. The nautical depth is routinely defined by suspension density in the range of 1100-1300 kg/m3 in the mud layer. While ultrasound measurements have a weak sensitivity to density variations, calibration is always needed to convert ultrasound measurements into reliable indicators for nautical depth levels in the mud layers using densely distributed density rheological in-situ measurements. We present a laboratory ultrasonic transmission experiment to monitor the fluid mud's settling and consolidation processes using a sample from the Port of Rotterdam. We use P-and S-wave ultrasonic transducers in the frequency range between 200 to 1000 kHz. Our results show that the P-wave velocities slightly increase during the consolidation and settling process while the P-wave amplitudes decrease. On the other hand, we observe a high S-wave velocity that increases together with amplitudes over time. The P-and S-wave amplitude and S-wave velocity variation over time correlate well with the mud average density variation. The presented results can be very useful for fluid-mud monitoring at a lab scale, besides possible utilization for large-scale monitoring field campaigns.@en

The velocities of the seismic waves propagating in the fluid-mud layer are governed by the rheological properties and density of the fluid mud. Performing seismic transmission measurements inside the fluid mud can give good estimates of the seismic velocities and, thus, of the rheological properties and density. Laboratory ultrasonic transmission measurements of the wave velocities in the fluid-mud layer and their temporal evolution are shown. It is found that the shear-wave velocity and yield stress are positively correlated. Performing a seismic reflection survey for characterization of the fluid-mud layers could be more practical because it allows towing the sources and receivers above the top of fluid-mud layer. Interpretation of the results from a reflection survey, though, is influenced by the water layer above the fluid mud. Applying seismic interferometry to reflection measurements can eliminate the influence of the water layer and retrieve a reflection response from inside the fluid-mud layer. This eliminates the influence of the temperature and salinity of the water layer to obtain information about the seismic properties of the fluid-mud layer. To introduce the approach of retrieving and extracting the reflection response from inside the fluid-mud layer, data from laboratory measurements are used. The obtained compressional- and shear-wave velocities are validated by comparing them with values from current transmission measurements.@en

Artworks are an inseparable part of the cultural heritage of societies and provide us with a unique look at cultural developments through time and space. For the best possible conservation, it is paramount to know the constituent materials, condition, and construction techniques of the objects (e.g. painting on wood, fresco, sculpture). Such information is required not only for the surfaces of the objects, but also for the interiors; in the imaging discipline, this is known as depth imaging. Here, we introduce a new method for non-invasive depth imaging as an alternative to traditional non-invasive methods when the latter cannot be used to obtain the required information. We use ultrasonic transverse-wave transmission measurements and turn them into virtual reflection measurements. We achieve this by applying seismic interferometry with active sources. Obtaining reflection measurements by seismic interferometry allows us to apply an advanced imaging technique – prestack depth migration, as used in seismic exploration – to produce a high-resolution depth image of an object. We apply our method to ultrasonic data recorded on a mockup of a painting on a wooden support. We validate our method by comparing our results with an image from X-ray computed tomography.

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Monitoring of seismic changes inside the reservoir layer during CO2 sequestration can be valuable for extraction of reservoir quantities like saturation and pore pressure. The accuracy of the monitoring could be deteriorated due to nonrepeatability errors in the source and receiver geometry. Applying seismic interferometry (SI) to permanent networks of seismic stations to retrieve virtual sources at the positions of the stations eliminates the non-repeatability in the source positioning. SI is traditionally applied using crosscorrelation. We show results from application of SI to ultrasonic data for layer-specific monitoring of sequestration of supercritical CO2. The data are recorded on a two-layer sample consisting epoxy (caprock) and Bentheimer sandstone (reservoir). We apply SI by crosscoherence, which has the potential to retrieve results with higher temporal resolution that SI by crosscorrelation. Using SI, we retrieve non-physical reflections from the bottom of the sandstone as if source and receiver were placed at the top of the sandstone. The velocities we estimate from the non-physical reflections during injection of brine aiming to displace supercritical CO2 and during injection of supercritical CO2 aiming to displace brine indicate rather similar saturation for both injection cases. We confirm the latter by transmission measurements, but with lower resolution.@en

The seismic method with active sources has proven to be a very valuable tool for CO2 sequestration monitoring. The seismic method can be used for extraction of reservoir quantities like saturation and pore pressure. But nonrepeatability in the positioning of the source and receiver during base and monitoring surveys can deteriorate the accuracy of the estimated changes in the reservoir parameters. Application of seismic interferometry (SI) to reflection recordings on permanent networks of seismic stations could help eliminate the monitoring errors due to the nonrepeatability errors. Retrieving virtual sources at the positions of the stations eliminates the non-repeatability in the source positioning. SI is traditionally applied using crosscorrelation. We show results from application of SI to ultrasonic data of sequestration of supercritical CO2. The data are recorded on a two-layer sample consisting of epoxy (caprock) and Bentheimer sandstone (reservoir). We apply SI by crosscoherence, which has the potential to retrieve results with higher temporal resolution than SI by crosscorrelation. Our aim is to monitor layer-specific changes inside the reservoir during the displacement of brine by supercritical CO2 and during the displacement of supercritical CO2 by brine. To achieve layer-specific monitoring, we retrieve with SI non-physical reflections from the bottom of the sandstone as if source and receiver were placed at the top of the sandstone. The velocities we estimate from the non-physical reflections during injection of brine aiming to displace supercritical CO2 and during injection of supercritical CO2 aiming to displace brine indicate rather similar saturation for both injection cases. We confirm the latter by transmission measurements, but with lower resolution. @en

Purpose: Time-lapse seismic monitoring constitutes the foundation for most monitoring programmes involving CO2 storage. When using time-lapse seismics, two major sources of uncertainty in the estimation of changes in the reservoir properties, like saturation and pressure, are the non-repeatability of the source positions and the difficulty to separate the effect of the overburden from that of the changes taking place in a CO2 reservoir. The goal of this research is to propose a new concept of using non-physical (ghost) reflection events retrieved by seismic interferometry and test this concept through ultrasonic laboratory experiments that mimic realistic CO2 sequestration in a porous reservoir rock. Experimental description: Results from two laboratory experiments will be presented. In both experiments, a two-layer sample consisting of a top layer of epoxy, representing the impervious cap rock, and a lower layer of Bentheimer sandstone (porosity ~ 22%, permeability 1.34 Darcy, density 2080 kg/m3), representing the reservoir rock, is used. In the first experiment, ultrasonic tests using piezoelectric transducers were performed under ambient (room) conditions of temperature and pressure, and water was displaced by ethanol. In the second experiment, elaborate ultrasonic experiments were carried out under controlled (elevated) pressure and temperature conditions mimicking a true CO2 reservoir where supercritical CO2 displaced brine. An array of seismic receiver was used to record the ultrasonic reflections from the top and the bottom of the porous layer. Results and conclusions: Using non-physical (or ghost) reflections retrieved by seismic interferometry, we could successfully estimate the acoustic wave velocity in the porous reservoir and its temporal change associated with changes in pressure and fluid-content in the pores. The estimation of layer-specific wave-velocity, eliminating effectively the effect of the changes occurring in the overburden and that of source irreproducibility, has been possible for the first time. The advantage of using cross-coherence over cross-correlation in the application of seismic interferometry, in order to address velocity changes in a thin reservoir layer, has been established. It was possible to obtain reliable values of the rock-physical properties from the estimated layer-specific acoustic wave velocity obtained by the proposed approach.@en

Electromagnetic waves are generated by acoustic waves at interfaces between fluids and fluid-saturated porous media, and at porous medium interfaces between different fluids. These signals can be measured in the laboratory as predicted by theory. A potential application is oil spill remediation monotoring in soils. We designed and developed an experimental setup in which acoustic to electromagnetic (EM) wave conversions at interfaces can be measured. Theoretical results are obtained with an electrokinetic full-waveform theoretical model, where use was made of the Sommerfeld approach. Using bimodal samples, different fluid–solid interface effects and saturating fluids were investigated. The contrast between water and water-saturated porous glass samples is larger than the contrast between water and oil-saturated porous glass samples. The contrast between water and water-saturated Fontainebleau sandstone is larger than the contrast between oil and water-saturated Fontainebleau sandstone.@en