Si VII/Mg VII and Si IX/Mg IX line diagnostics for solar plasma     

Anita Mohan  B. N. Dwivedi 《Solar physics》1996,167(1-2):145-151

New calculations for Si vii/Mg vii and Si ix/Mg ix theoretical line ratios as a tool for plasma density diagnostics and for the estimation of relative element abundance in the solar atmosphere are presented, making use of the EUV spectrum of an active region obtained by the Solar EUV Rocket Telescope and Spectrograph (SERTS). Theoretical line ratios, hitherto not observed, are discussed for diagnosing solar plasma from the observations obtainable with the Coronal Diagnostic Spectrometer (CDS) instrument on board the SOHO mission.In the honour of Dr P. K. Raju, Indian Institute of Astrophysics, Bangalore, on the completion of his 60 years.  相似文献   

Comparison of ocean color chlorophyll algorithms for IRS-P4 OCM sensor usingin-situ data     

Prakash Chauhan  M Mohan  S R Nayak  R R Navalgund 《Journal of the Indian Society of Remote Sensing》2002,30(1-2):87-94

In-situ chlorophyll concentration data and remote sensing reflectance (R_rs) measurements collected in six different ship campaigns in the Arabian Sea were used to evaluate the accuracy, precision, and suitability of different ocean color chlorophyll algorithms for the Arabian Sea. The bio-optical data sets represent the typical range of biooptical conditions expected in this region and are composed of 47 stations encompassing chlorophyll concentration, between 0.072 and 5.90 mg m^-3, with 43 observations in case I water and 4 observations in case II water. Six empirical chlorophyll algorithms [i.e. Aiken-C, POLDER-C, OCTS-C, Morel-3, Ocean Chlorophyll-2 (OC2) and Ocean Chlorophyll-4 (OC4)] were selected for analysis on the Arabian Sea data set. Numerous statistical and graphical criterions were used to evaluate the performance of these algorithms. Among these six chlorophyll algorithms two chlorophyll algorithms (i.e. OC2 and OC4) performed well in the case I waters of the Arabian Sea. The OC2 algorithm, a modified cubic polynomial function which uses ratio of R_rs490 nm and R_rs555 nm (where, R_rs is remote sensing reflectance), performed well with r²=0.85; rms =0.15. The OC4 algorithm, a four-band (443, 490, 510, 555 nm), maximum band ratio formulation was found best on the basis of statistical analysis results with r²=0.85 and rms=0.14. Both OC2 and OC4 algorithms failed to estimate chlorophyll inTrichodesmium dominated waters. The OC2 algorithm was preferred over OC4 algorithm for routine processing of the OCM data to generate chlorophyll-a images, as it uses a band ratio of 490/555 nm and atmospheric correction is more accurate in 490 nm compared to 443 nm band, which is used by OC4 algorithm.  相似文献   

Site response studies in Mumbai using (H/V) Nakamura technique     

G. Surve  G. Mohan 《Natural Hazards》2010,54(3):783-795

The importance of characterizing the site effects in urban areas, especially Mumbai, the commercial capital of India, with a quarter of land below sea level, is well realized. Mumbai is built on a cluster of seven basaltic islands that were merged together through reclamation of land from the Arabian Sea. Due to rapid urbanization, the demarcation between reclaimed areas and original islands is blurred. A pilot study is undertaken to investigate and characterize the local site effects at 27 locations in Mumbai. The Nakamura technique is used to estimate the fundamental frequency of soft soils at each site, characterized by the ratio (H/V) of the Fourier spectra of the horizontal and vertical components of ambient noise measurements made with a 3-component short period (1 Hz) seismograph. Validation of peak frequency was done using both pre-event and event data. The peak amplification was also validated through measurements at a 10-m exposed soil section and over a soil dump. Overall, the site responses correlate well with the local geology and the lithologs obtained at 40 boreholes at 8 locations The reclaimed areas are characterized by resonance frequencies ranging from 3.3 to 4.6 Hz with significant peak amplification (>4) in contrast to hard rock sites that do not exhibit peak amplification. The hard rock sites with soil cover exhibit peak amplification in the frequency range 3.3–10.5 Hz indicating large variations in soil thickness. The H/V curves at most sites exhibit clear single peaks with large amplitude, which could be associated with sharp discontinuities corresponding to a uni-model of a single layer with large velocity contrast overlying the basement. The low resonance frequencies together with large amplification of site responses enable identifying and demarcating reclaimed areas that form important inputs in the seismic microzonation of Mumbai.  相似文献   

Structural information derived from ambient noise tomography over a hydrocarbon-producing region in the Cachar fold belt,lower Assam,northeast India     

Bharath Shekar  Gollapally Mohan  Sunil Kumar Singh 《Geophysical Prospecting》2023,71(1):140-162

Ambient noise tomography is a powerful tool that has found increasing application in reservoir analysis and imaging. The Cachar fold belt in lower Assam, northeast India encompasses several wells under active hydrocarbon production, along with several dry wells. To overcome the lack of active seismic data over the entire fold belt, a passive seismic study was carried out to image the concealed three-dimensional sub-surface structures. The data were recorded from February to November 2011 by a network of 65 wideband seismometers spanning an area of about 40 × 60 km². The data are crosscorrelated in the 2–5 s band, followed by phase-weighted stacking to estimate noise correlation functions with surface wave signatures. The traveltimes picked from the frequency-time analysis are utilized in a tomographic inversion for Rayleigh wave group velocities. The group velocity anomalies have a lateral resolution of ~ 3.5 × 5.5 km² and variations of up to $\pm 20\%$ for each period. The group velocities are in turn inverted for S-wave velocity distribution as a function of depth. The three-dimensional S-wave velocity tomograms reveal the tight anticlines and broad synclines, with high- and low-velocity zones corresponding to structural highs and lows, respectively. The structural interpretation is supported for the part of the region with producing wells and covered by active seismic data, wherein the post-stack time migrated seismic section shows anticlinal and synclinal features similar to those obtained from ambient noise tomography. The structures revealed by ambient noise tomography can help identify zones of interest to be targeted by active seismic surveys in the Cachar fold belt.  相似文献