Effect of autocorrelation on temporal trends in rainfall in a valley region at the foothills of Indian Himalayas     

Atul?Kant?PiyooshEmail author  Sanjay?Kumar?Ghosh 《Stochastic Environmental Research and Risk Assessment (SERRA)》2017,31(8):2075-2096

This study examines the effect of autocorrelation on step and monotonic trends in seasonal and annual rainfall. Initially, for step change, modified-Pettitt test is applied in two ways. First, using the corrected and unbiased trend-free-pre-whitening (TFPWcu) approach. Second, using a new approach in which time series is modelled by intervention analysis for modified Pettitt test. Subsequently, for monotonic trends, Mann–Kendall (MK) and six approaches of modified Mann–Kendall (MMK) test are applied to NCDC data for period 1901–2012 and its sub-periods. Approaches of MMK include pre-whitening (PW), trend-free-pre-whitening (TFPW), TFPWcu, two Variance Correction Approaches (VCAs) based on empirical formula (VCA:CF1) and Monte-Carlo-Simulations (VCA:CF2) and long term persistence (MK-LTP). A single change point is identified in 1970 for annual and monsoon rainfall from original and modified-Pettitt’s test using TFPWcu, while time series modelling approach has not exhibited any change point. Process shift in rainfall series is also studied using CUSUM and multiple change points are identified using Segment-Neighbourhood method. Outcomes of MMK show that TFPWcu is able to efficiently limit the effect of autocorrelation and may be preferred over PW and TFPW. The VCA:CF2 is not dependent on whole autocorrelation structure and corrects variance of all data series using lag-1 autocorrelation and may be preferred over VCA:CF1. MK-LTP considers long term persistence and it has exhibited presence of weaker trends than exhibited by other approaches. VCA:CF2 and MK-LTP are used to study trends of rainfall in Dehradun.  相似文献   

Monitoring field scale CO<Subscript>2</Subscript> injection from time-lapse seismic and well log,integrating with advanced rock physics model at Cranfield EOR site     

Ranjana Ghosh 《Acta Geophysica》2017,65(6):1207-1218

Causes and effects of global warming have been highly debated in recent years. Nonetheless, injection and storage of CO₂ (CO₂ sequestration) in the subsurface is becoming increasingly accepted as a viable tool to reduce the amount of CO₂ from the atmosphere, which is a primary contributor to global warming. Monitoring of CO₂ movement with time is essential to ascertain that sequestration is not hazardous. A method is proposed here to appraise CO₂ saturation from seismic attributes using differential effective medium theory modified for pressure (PDEM). The PDEM theory accounts pressure-induced fluid flow between cavities, which is a very important investigation in the CO₂-sequestered regime of heterogeneous microstructure. The study area is the lower Tuscaloosa formation at Cranfield in Mississippi, USA, which is one of the active enhanced oil recovery (EOR), and CO₂ capture and storage (CCS) fields. Injection well (F1) and two observation wells (F2 and F3) are present close (within 112 m) to the detailed area of study for this region. Since the three wells are closely situated, two wells, namely injection well F1 and the furthest observation well F3, have been focused on to monitor CO₂ movement. Time-lapse (pre- and post-injection) log, core and surface seismic data are used in the quantitative assessment of CO₂ saturation from the PDEM theory. It has been found that after approximately 9 months of injection, average CO₂ saturations in F1 and F3 are estimated as 50% in a zone of thickness ~ 25 m at a depth of ~ 3 km.  相似文献