Said Attia al Hagrey

2D Optimized Electrode Arrays for Borehole Resistivity Tomography and CO2 Sequestration Modelling

Modern optimization approaches for electrode configurations can significantly improve the resolution of 2.5D resistivity imaging surveys. This study presents a brief review of the 2.5D optimization approach, particularly for borehole–borehole surveys with applications for mapping virtual CO2 plumes sequestrated in deep saline reservoir formations. The applied algorithm searches for arrays that maximize the spatial resolution of the survey among the comprehensive dataset of best possible spatial resolution (i.e. least temporal resolution). A main goal of this study is to increase the temporal resolution of ERT borehole–borehole surveys by selecting optimized electrode configurations in order to minimise the required data acquisition time while sustaining a high spatial resolution. The optimized dataset starts with a base set and is iteratively increased based on the model resolution matrix (R) until the required number of data points is achieved. Among four different optimization methods, the compare R (CR) method of the best resolution is applied to directly calculate R for each new array added to the optimized dataset. Small optimized datasets generated by this technique are only <5% of their comprehensive sets but of an average resolution ratio (Rr) of >0.95 (i.e. almost the same resolution). With increasing the size of the optimized dataset (during its generation), the algorithm progressively enhances Rr values in the central interwell region (of low sensitivities and low resolution) far higher than in the near borehole region (of high sensitivities). Also the inverted tomogram reliability increases by increasing the optimized data size. Briefly, the optimized arrays improve the resolution in the interwell region which is commonly low in borehole–borehole ERT studies. The inverted output model is evaluated quantitatively using the model difference relative to the input model. The results reflect the common smearing effects and artefacts of varying degrees that overpredict volumes, underpredict magnitudes and blur boundaries of the target anomalies. This input model is a synthetic resistivity model that was used to generate synthetic (forward solution) data used during the inversion. Applications on synthetic CO2 models show that the mapping resolution for optimized datasets is better than that for other highly resolving arrays of the same number of data points. Problems of smeared boundaries and thin layers are less visible in the optimized array than in the other highly resolving arrays.

Electric study of fracture anisotropy at Falkenberg, Germany

Electric and electromagnetic methods have been applied for mapping subsurface fractures and the directional dependence of in‐situ electric parameters at the hot dry rock site at Falkenberg, Germany. This study includes the determination of several anisotropy parameters like the mean, longitudinal and transverse resistivity components (ρm, ρ𝓁 and ρt, respectively), the anisotropy coefficient λ, and the strike angle Θ. Terrain conductivity measurements using the technique of frequency‐domain electromagnetic induction reveal a dominant anomaly strike of east‐south‐east— west‐north‐west, nearly parallel to the fracturing strike of N110°. With increasing distance from the central borehole, the mise a la masse potential differences exhibit a transition from a direct to a paradoxical relationship to the resistivity anisotropy induced by the fracturing. These observations are explained using a model for an ellipsoidal fracture. The qualitative interpretation of the sounding data of Schlumberger and crossed‐square...

Numerical and experimental mapping of small root zones using optimized surface and borehole resistivity tomography

An exact mapping of root zones is essential to understand plant growth, root biomass, and soil functions important for environmental and climatic management and protection. Numerical and experimental techniques of the electrical resistivity tomography were applied in 2D and 3D to resolve small root zones in the centimeter range. Numerically, we studied two scenarios of conductive and resistive root zones as a function of (1) eight different quadripole electrode configurations (standard, nonstandard, and optimized), (2) four different survey designs with electrode arrays at the soil surface and in boreholes, and (3) eight different inversion constraints. The best resolved output tomogram was evaluated semiquantitatively using the criteria of visual similarity to the input model, least data set, rms error, and iteration number and quantitatively by the model difference relative to the input model. The results showed that the surface-borehole configurations have the best resolution for the whole root zone. The single-surface and borehole surveys resolve only the respective upper and middle-lower root parts. The results reflect the potential of the optimization approach to generate small data sets of far higher resolution than the standard sets. Based on these results, we used the surface-borehole survey around a young hibiscus planted in a sandy soil in a laboratory experiment. The surface-borehole surveys using small, optimized configurations result in an optimum spatiotemporal resolution for simultaneous applications for 3D mapping of targets (root zones and water and soil heterogeneities) and 4D monitoring of their processes.

Hydro-, bio-geophysics

The global increase in demand for water, the dominant agent in all biological processes, calls for sustainable management of water catchments and better understanding of water and solute movement. At Kiel, we have started joint studies with eight European partners in such disciplines as botany, agronomy, and hydrology (projects “WATERUSE” and “GeoModel”) to develop integrated techniques to quantify water flow through the soil-plant-atmosphere-continuum that will be adequate for use in heterogeneous stands in dry regions. To develop novel integrated hydro-/biogeophysical 3D techniques with high resolution, we erected a unique, full-scale tank analog (3 m × 5 m × 2 m and named GeoModel) at the Kiel campus for controlled experiments on simulated soil models in their natural scale. It forms a bridge between scaled laboratory models (typically 1 m3) and field surveys (several km3) that takes advantage of both. GeoModel is used to study the static water content and dynamic flow in soils and plants using infiltr...

CO2 plume modeling in deep saline reservoirs by 2D ERT in boreholes

The need to manage CO2 emissions for mitigating the greenhouse effect has led to worldwide research to reduce atmospheric CO2. Techniques of carbon capture and storage (CCS) must (1) be effective and cost-competitive, (2) provide stable, long-term storage, and (3) be environmentally benign. Potential terrestrial media for CO2 storage include depleted oil and gas reservoirs, coal seams that cannot be mined, and deep saline water reservoirs capped by impermeable rock to prevent upward leakage.

Mapping root zones of small plants using surface and borehole resistivity tomography

The main function of roots is absorbing water and nutrients (sap) required for the development of plants and trees and anchoring them to the ground. The increasing water shortage in many countries calls for a better understanding of root structure, root volume, water uptake by roots, and water redistribution in the soil-plant zone to serve as the base for a well-directed and sustainable supply of water. Root zones mainly consist of a mixture of root branches and soil material. The root branches within the root envelope are of two kinds: fine, soft roots that absorb the sap from the surrounding soils and thick, wooden roots that transport the sap to the trunk and leaves. The sap is used mainly for transpiration processes and slightly for growth processes (photosynthesis and formation of carbon).

Geophysical assessments of renewable gas energy compressed in geologic pore storage reservoirs

Renewable energy resources can indisputably minimize the threat of global warming and climate change. However, they are intermittent and need buffer storage to bridge the time-gap between production (off peak) and demand peaks. Based on geologic and geochemical reasons, the North German Basin has a very large capacity for compressed air/gas energy storage CAES in porous saltwater aquifers and salt cavities. Replacing pore reservoir brine with CAES causes changes in physical properties (elastic moduli, density and electrical properties) and justify applications of integrative geophysical methods for monitoring this energy storage. Here we apply techniques of the elastic full waveform inversion FWI, electric resistivity tomography ERT and gravity to map and quantify a gradually saturated gas plume injected in a thin deep saline aquifer within the North German Basin.For this subsurface model scenario we generated different synthetic data sets without and with adding random noise in order to robust the applied techniques for the real field applications. Datasets are inverted by posing different constraints on the initial model. Results reveal principally the capability of the applied integrative geophysical approach to resolve the CAES targets (plume, host reservoir, and cap rock). Constrained inversion models of elastic FWI and ERT are even able to recover well the gradual gas desaturation with depth. The spatial parameters accurately recovered from each technique are applied in the adequate petrophysical equations to yield precise quantifications of gas saturations. Resulting models of gas saturations independently determined from elastic FWI and ERT techniques are in accordance with each other and with the input (true) saturation model. Moreover, the gravity technique show high sensitivity to the mass deficit resulting from the gas storage and can resolve saturations and temporal saturation changes down to ±3% after reducing any shallow fluctuation such as that of groundwater table.

Environmental and Process Monitoring

For enhanced gas recovery (EGR) using CO2 as well as for CO2 storage in depleted gas fields it needs to be shown that injection and storage is save and neither population nor environment is exposed to risks during operation or afterwards. This requires the development and application of methods to monitor groundwater, vadose zone and atmosphere. Therefore, extensive investigations of the near-surface aquifers were performed to characterize the geological structure and the geochemical and hydraulic conditions as part of a baseline-monitoring and to specify input parameters for model simulations. If CO2 leakage should occur and CO2 migrates upwards from the storage complex, shallow freshwater aquifers are the first protected good that might be affected. Based on the model simulations, parameters that would be affected by leakages were specified and parameter changes as well as spatial extension of the expected changes quantified. A comparison of the model results with measured natural variabilities show that especially pH and TIC (total inorganic carbon), but under certain conditions also electric conductivity and aqueous calcium concentration (Ca) are most suited parameters for the detection of CO2 leakages based on observation wells in shallow aquifers. It was an important result that the temporal fluctuations of groundwater composition are generally small but spatial variations are large.

Geophysical Imaging Techniques

Soils are the biggest terrestrial carbon store. The contribution of plant roots to soil and atmospheric carbon is significant and difficult to survey accurately. Worldwide interest in reducing greenhouse gases has led to research activities for quantifying the root biomass and evaluating their critical role in space and time. Geophysical methods image the medium under study in 2D and 3D, and monitor changes and processes in 4D. They offer good parametrical and spatiotemporal resolution combined with a minimum-invasive character. The high-resolution techniques of electric resistivity imaging (ERI) and ground penetrating radar (GPR) have extended newly their applications into the less known in vivo investigation of biogeophysical targets of living plants and trees. This includes mapping roots and trunk structures, diagnosing wood decay, analysing fluid content and physiological processes of water redistribution, sap uptake, etc. (e.g., Hagrey and Michaelsen, Eur J Environ Eng Geophys 7:75–93, 2002; Hagrey and Michaelsen, Geophysics 64:746–753, 1999; Hagrey et al., Geophys J Int 138:643–654, 1999; Hagrey et al. Proceedings, Meeting of Engineering Geology, Kiel, Germany, 2003; Hanafy and Hagrey, Geophysics 71:k9–k18, 2006; Hubbard et al., The Leading Edge 21:552–559, 2002). Applications of ERI and GPR techniques to image roots and root-zones are based on the electrical and electromagnetic contrast of these targets relative to surrounding soils (e.g., Hruska et al. Tree Physiol 19:125–130, 1999; Butnor et al., Tree Physiol 21:1269–1278, 2001; Butnor et al., Am J Soil Sci Soc 67:1607–1615, 2003; Hagrey et al. Geophys J Int 138:643–654, 2004, Hagrey 2007; Werban et al. J Plant Nutr Soil Sci 171:927–935, 2007; Amato et al. Tree Physiol 28:1441–1448, 2008; Amato et al. Eur J Agron 31:213–222, 2009; Petersen and Hagrey, The Leading Edge 28(10):1220–1224, 2009; Doolittle and Butnor, Soils, peatlands and biomonitoring. In: Jol HM (Ed) Ground penetrating radar: theory and applications. Elsevier, Amsterdam, 177–192, 2009; Hagrey and Petersen, Geophysics 76(2), G25–G35, 2011). In this chapter, we present principles of ERI and GPR techniques for root studies.