Application of diffracted wave analysis to time‐lapse seismic data for CO2 leakage detection

Time‐lapse seismic analysis is utilized in CO₂ geosequestration to verify the CO₂ containment within a reservoir. A major risk associated with geosequestration is a possible leakage of CO₂ from the storage formation into overlaying formations. To mitigate this risk, the deployment of carbon capture and storage projects requires fast and reliable detection of relatively small volumes of CO₂ outside the storage formation. To do this, it is necessary to predict typical seepage scenarios and improve subsurface seepage detection methods. In this work we present a technique for CO₂ monitoring based on the detection of diffracted waves in time‐lapse seismic data. In the case of CO₂ seepage, the migrating plume might form small secondary accumulations that would produce diffracted, rather than reflected waves. From time‐lapse data analysis, we are able to separate the diffracted waves from the predominant reflections in order to image the small CO₂ plumes. To explore possibilities to detect relatively small amounts of CO₂, we performed synthetic time‐lapse seismic modelling based on the Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) Otway project data. The detection method is based on defining the CO₂ location by measuring the coherency of the signal along diffraction offset‐traveltime curves. The technique is applied to a time‐lapse stacked section using a stacking velocity to construct offset‐traveltime curves. Given the amount of noise found in the surface seismic data, the predicted minimum detectable amount of CO₂ is 1000–2000 tonnes. This method was also applied to real data obtained from a time‐lapse seismic physical model. The use of diffractions rather than reflections for monitoring small amounts of CO₂ can enhance the capability of subsurface monitoring in CO₂ geosequestration projects.     

Forest cover change assessment in conflict-affected areas of northwest Pakistan: the case of swat and shangla districts

This study examines the spatial and temporal forest cover changes in Swat and Shangla districts to understand the deforestation pattern in context of the recent security conflict in these districts. We used multi-resolution satellite images to assess the long term deforestation from 2001 to 2009 and also to identify episodic forest cutting areas appeared during the conflict period of Oct. 2007 - Oct. 2008. There are only 58 ha of deforestation identified during the conflict period while 1268 ha of gross annual deforestation were assessed during last eight years. Most of the deforestation patches persist around the administrative boundaries at sub-district levels (tehsils) which can be attributed to ambiguity in unclear jurisdiction between the forest official. The results highlight that the forest cutting appeared in Swat and Shangla during the conflict period is not as significant when compared with the long term deforestation pattern in the area. On the one side the results of the study are supportive to the picture that emerges from international studies which report high rate of deforestation in the country and on the other side it negates any relation between the security situation and the increasing deforestation in the north western Pakistan. The study concludes that deforestation assessments require verification by independent sources of data, such as satellite imagery to improve our understanding of deforestation processes.     

The practice of Integrated Water Resources Management in South Africa: challenges of women in water user associations

South Africa’s National Water Act and National Water Resource Strategy set out an ambitious societal vision with a strong focus on the redistribution of water resources towards the marginalised and on empowering historically disadvantaged communities including women. This vision is reflected in the framework for Integrated Water Resources Management (IWRM) that acknowledges women as pivotal in water management practices. Based on this premise, this paper examines the challenges women face in performing their roles in IWRM in rural South Africa. It draws on a study of a water user association that operates in the Limpopo Province of South Africa. The design of the study was qualitative in nature with a semi-structured interview as the main data collection tool. The interview involved 14 respondents from Limpopo. The results showed marked gender difference in terms of roles performed. Based on the study, three kinds of roles were revealed: domestic, productive and decision-making roles. Men were significantly involved in productive roles, giving low priority to domestic roles. The key factors found to affect the role of women in decision-making in IWRM were cultural practices, low self-confidence, low levels of capacity, and high workloads. These factors were identified as key institutional inherent within the specific society. As such, these findings have significant implications for the efforts aimed at promoting gender equality. Particularly, the impact of culture on women in water management raises concerns regarding gender issues in rural and remote areas where people are poorer and more culturally conservative.     

Use of electrical resistivity tomography in delineating zones of groundwater potential in arid regions: a case study from Diriyah region of Saudi Arabia

A detailed geophysical investigation in the form of electrical resistivity tomography (ERT) was carried out using the SYSCAL Pro Unit at two locations with a dipole?Cdipole configuration in one of the private farms in Diriyah area of Riyadh, Saudi Arabia with the purpose of delineating the approximate depth of the water-bearing formations. The survey helped in delineating a superficial moist zone at the first site which was mainly due to the seepage from surface water pipes used for irrigating the date palms. At the second site, a potential water-bearing zone was detected starting from a depth of 23 to 46?m. However, an already existing bore well in close vicinity of the site of the second survey showed a water table depth of 85?m, indicating the presence of a layered aquifer. Finally, it was concluded that a shallow bore well drilled up to a depth of 50?C60?meters would be able to tap the water resources of shallow water-bearing horizon discovered at the second site during the survey; however, the yield of the bore well might not be sustainable over a longer period of time. The survey at the first site did not show any promising groundwater potential up to the investigated depth which was around 70?m. The study confirms the usage of ERT surveys for exploring sources of freshwater supplies in arid regions.     

Groundwater vulnerability to pesticides in Northwest Bangladesh

The transport and leaching potential hazards of various pesticides were studied in a shallow unconfined aquifer located in Northwest Bangladesh. Pesticide leaching potential was quantified using a one-dimensional advective–dispersive transport equation for a non-conservative chemical that follows first-order decay and linear adsorption in soils. Leaching potential index (LPI) was calculated for 69 sites in the study area to evaluate the relative vulnerability to pesticide leaching and to prioritize sites for model study and soil sampling. The numerical ranks of computed LPI were grouped by quantiles into very high, high, moderate, low and very low categories; and based on these rankings, the most vulnerable site was selected. The fate and transport of pesticides in this most vulnerable site was modeled using MT3D. The model results indicate that pesticides with high sorptivity and moderate to high persistence have low potential impact on groundwater. Top soils are found to be particularly vulnerable to the accumulation of organochlorine pesticides. Results also revealed that decreasing the soil organic matter and increasing the half-life of the pesticides at deeper depths did not make any significant change. Finally, six soil samples were collected from the same site at depths of 0.0, 1.5, 3.0, 4.5, 6.0, and 7.5 m for the analysis of pesticide residues. The soil–water was extracted from the samples following standard extraction technique and tested using gas chromatography (GC) and high-performance liquid chromatography (HPLC) for pesticide residues. Results showed no trace of pesticide residues in the soil–water; however, a few unknown peaks were detected indicating the use of some unknown brand of chemicals in the study area.     

Fatigue reliability analysis of deep water rigid marine risers associated with Morison-type wave loading

This work presents a simple and efficient framework for the fatigue reliability assessment of a vertical top-tensioned rigid riser. The fatigue damage response is considered as a narrow-band Gaussian stationary random process with a zero mean for the short-term behavior of a riser. Non-linearity in a response associated with Morison-type wave loading is accounted for by using a factor, which is the ratio of expected damage according to a non-linear probability distribution to the expected damage according to a linear method of analysis. Long-term non-stationary response is obtained by summing up a large number of short-term stationary responses. Uncertainties associated with both the strength and stress parts of the limit state function are quantified by a lognormal distribution. A closed form reliability analysis is carried out, which is based on the limit state function formulated in terms of Miner’s cumulative damage rule. The results thus obtained are compared with the well-documented lognormal format of reliability analysis based on time to fatigue failure. The validity of using the lognormal hazard rate function in predicting the fatigue life is discussed. A Monte Carlo simulation technique is also used as a reliability assessment method. A simple algorithm is used to reduce the uncertainty associated with direct sampling at small probability of failure values and a small number of simulations. Simulation results are compared with closed form solutions. A worked example is included to show the practical riser design problem based on reliability analysis.     

Estimation of dielectric permittivity,water content,and porosity for environmental engineering and hydrogeological studies using ground penetrating radar,a short review

Ground penetrating radar is a noninvasive geophysical technique that can be used to infer subsurface geological, geotechnical, and petrophysical attributes. For water contamination studies, ultimate target, i.e., contaminant flow, is demarcated with the help of dielectric permittivity, water content, and porosity. The dielectric permittivity facilitates to evaluate water content; whereas the dielectric permittivity can be estimated by using the interval velocity, and porosity can be estimated by using the information of water content and dielectric permittivity. All these parameters are interlinked. This paper describes different empirical and mixing relationships to calculate approximately these parameters and their utilization in predicting the hydrogeological model of the area.     

Facies Analysis, Sequence Stratigraphy and Diagenetic Studies of the Jurassic Carbonates of the Kohat Basin, Northwest Pakistan: Reservoir Implications

The present study deals with the depositional facies, diagenetic processes and sequence stratigraphy of the shallow marine carbonates of the Samana Suk Formation, Kohat Basin, in order to elucidate its reservoir quality. The Samana Suk Formation consists of thin to thick-bedded, oolitic, bioclastic, dolomitic and fractured limestone. Based on the integration of outcrop, petrographic and biofacies analyses, the unit is thought to have been deposited on a gentle homoclinal ramp in peritidal, lagoonal and carbonate shoal settings. Frequent variations in microfacies based sea-level curve have revealed seven Transgressive Systems Tracts (TSTs) and six Regressive Systems Tracts (RSTs). The unit has undergone various stages of diagenetic processes, including mechanical and chemical compaction, cementation, micritization, dissolution and dolomitization. The petrographic analyses show the evolution of porosity in various depositional and diagenetic phases. The fenestral porosity was mainly developed in peritidal carbonates during deposition, while the burial dissolution and diagenetic dolomitization have greatly enhanced the reservoir potential of the rock unit, as is further confirmed by the plug porosity and permeability analyses. The porosities and permeabilities were higher in shoal facies deposited in TSTs, as compared to lagoonal and peritidal facies, except for the dolomite in mudstone, deposited during RSTs. Hence good, moderate and poor reservoir potential is suggested for shoal, lagoonal and peritidal facies, respectively.