Observations of borehole-source amplitude reduction due to casing

Theoretical results show that the amplitude of a borehole source is reduced when the well in which it is operated is cased and cemented. This reduction is a strong function of the formation velocity and is more weakly dependent on the direction of propagation of the wave travelling from source to receiver and on the diameter of the borehole itself. We have tested these predictions with data gathered in a cross-hole seismic experiment conducted in two stages in 1990 and 1991. The source and receivers were located in the MIT/Stech 1-21A and MIT/Burch 1-20B wells at the Earth Resources Laboratory (ERL) test site in Michigan (USA). Though the source well (MIT/Stech 1-21A) was uncased in 1990, a steel casing and cement were added prior to the collection of data in 1991. Several receiver positions were reoccupied to compare data collected with the source in open and cased holes. Using a velocity model for this area and borehole diameter measurements from a calliper log, a compensation factor can be computed that will adjust the data collected in 1991 to have amplitudes comparable to that collected in the first stage of the experiment. The accuracy of the results demonstrates the validity of the theory, which can be very useful in better understanding seismic waveforms recorded in cross-hole experiments.     

ELF and VLF wave generation by HF heating: A comparison of AM and CW techniques

Antenna field 2 of the Tromsø Heating facility consists of six rows of six horizontal crossed full wave dipoles aligned with the rows running geographic east-west. In previous experiments on ELF/VLF wave generation it has been the practice to feed the rows in parallel, with HF radiation amplitude modulated at the ELF/VLF frequency it was desired to radiate (AM configuration). Here we describe how the antenna array was also configured so that it could be fed with a continuous wave (CW) input power but still carry information at an ELF/VLF frequency. To effect this the three southern most rows of the antenna array were driven with a CW signal at 4.04 MHz and the three northernmost rows with a CW signal at frequency greater than 4.04 MHz by the ELF/VLF frequency it was desired to radiate (CW configuration). Experiments were performed with modulation/difference frequencies of 565 and 2005 Hz and the signals were received at the Lycksele Geophysical Observatory, 500 km south of the heating facility. The signals were typically 11 dB greater in the AM than the CW configuration, despite the fact that the average power delivered to the ionosphere in the CW configuration was four times that in the AM configuration. Significant harmonic radiation (both odd and even) was produced in the AM configuration but no harmonic radiation was detected in the CW configuration. A simple theory has been developed to model the fields produced by HF heating using the two techniques (AM,CW). A good agreement has been obtained between the experimental observations and model computations. The model has been extended to show how the relative efficiency of generation, AM/CW, varies with the frequency of the ELF/VLF radiation and HF antenna element spacing. Radiation patterns for the ‘ionospheric ELF/VLF antenna’ have also been derived for the two generation techniques.     

Mitigating the Effects of Climate Change on the Water Resources of the Columbia River Basin

The potential effects of climate change on the hydrology and water resources of the Columbia River Basin (CRB) were evaluated using simulations from the U.S. Department of Energy and National Center for Atmospheric Research Parallel Climate Model (DOE/NCAR PCM). This study focuses on three climate projections for the 21st century based on a `business as usual' (BAU) global emissions scenario, evaluated with respect to a control climate scenario based on static 1995 emissions. Time-varying monthly PCM temperature and precipitation changes were statistically downscaled and temporally disaggregated to produce daily forcings that drove a macro-scale hydrologic simulation model of the Columbia River basin at 1/4-degree spatial resolution. For comparison with the direct statistical downscaling approach, a dynamical downscaling approach using a regional climate model (RCM) was also used to derive hydrologic model forcings for 20-year subsets from the PCM control climate (1995–2015) scenario and from the three BAU climate(2040–2060) projections. The statistically downscaled PCM scenario results were assessed for three analysis periods (denoted Periods 1–3: 2010–2039,2040–2069, 2070–2098) in which changes in annual average temperature were +0.5,+1.3 and +2.1 °C, respectively, while critical winter season precipitation changes were –3, +5 and +1 percent. For RCM, the predicted temperature change for the 2040–2060 period was +1.2 °C and the average winter precipitation change was –3 percent, relative to the RCM controlclimate. Due to the modest changes in winter precipitation, temperature changes dominated the simulated hydrologic effects by reducing winter snow accumulation, thus shifting summer streamflow to the winter. The hydrologic changes caused increased competition for reservoir storage between firm hydropower and instream flow targets developed pursuant to the Endangered Species Act listing of Columbia River salmonids. We examined several alternative reservoir operating policies designed to mitigate reservoir system performance losses. In general, the combination of earlier reservoir refill with greater storage allocations for instream flow targets mitigated some of the negative impacts to flow, but only with significant losses in firm hydropower production (ranging from –9 percent in Period1 to –35 percent for RCM). Simulated hydropower revenue changes were lessthan 5 percent for all scenarios, however, primarily due to small changes inannual runoff.     

An accurate and robust multigrid algorithm for 2D forward resistivity modelling

We present an adaptation of the full multigrid algorithm in DC resistivity modelling in an effort to increase its accuracy. There is a great difficulty with conventional multigrid solvers in representing the physics of an arbitrary distribution of electrical conductivity on a very coarse grid. In general, conventional rectangular finite‐difference or 5‐point approximations of Poisson's equation cannot represent, at a coarse grid level, the effective anisotropy on a coarse scale which results from fine structure in the model. An exception to this generalization occurs when the principal axes of structural anisotropy are aligned with the coordinate axis. Additional and similarly generated problems arise when a coarse cell is obliged to represent fine structure containing very high conductivity contrasts. We have developed an adaptation of the usual resistive‐network representation of the continuum, which avoids some of these problems, and have compared it with the traditional resistive network currently used. The network adaptation consists of replacing the usual 5‐point Laplacian operator stencil used on the finite‐difference grid with a 9‐point stencil, and the conductivity scalar with a 6‐parameter conductivity parametrization. This parametrization permits representation of arbitrarily orientated anisotropy as well as more complex behaviour related to high conductivity contrasts. The importance of multigrid solvers does not lie in their speed at forward modelling (which is comparable with other methods), but rather in their potential for inverse modelling. Inverse solvers which proceed by refinement of an initially very coarse solution can, in principle, take time only linearly proportional to the number of gridpoints in the final desired model.     

The solar wind and the Sun–Earth link

Plasma streams out from the Sun in the form of the solar wind. Shadia Rifai Habbal and Richard Woo examine the workings of this enigmatic link between the Earth and the Sun.     

Calcium carbonate dissolution in deep sea sediments: reconciling microelectrode, pore water and benthic flux chamber results

We report the benthic fluxes of O₂, titration alkalinity (TA), Ca²⁺, NO₃⁻, PO₄³⁻, and Si(OH)₄ from in situ benthic flux chamber incubations on the Ceara Rise and Cape Verde Plateau and compare them to previously published results. We find within analytical uncertainty that the TA flux is twice the calcium flux, suggesting that dissolution/precipitation of CaCO₃ is the principal mechanism controlling benthic TA and Ca²⁺ fluxes. At sites where the sediments contain significant (>35%) CaCO₃ and the overlying waters are supersaturated with respect to CaCO₃, the ratios of the total dissolution rate to the remineralization rate are significantly less than at all other study sites. We propose that these observations can be explained by precipitation of fresh CaCO₃ at the supersaturated sediment surface followed by redissolution deeper in the sediments because of metabolically-produced CO₂. A numerical simulation is presented to demonstrate the feasibility of this explanation. In addition, surface exchange reactions in high-CaCO₃ sediments coupled with high rates of particle mixing may also impact rates of metabolic dissolution and depress chamber-derived estimates of carbonate alkalinity and calcium benthic fluxes. These results suggest that at supersaturated, high CaCO₃ locations, previous models of sediment diagenesis may have overestimated the impact of metabolic dissolution on the preservation of CaCO₃ deposited on the sea floor.     

Geochemical evidence for abrupt changes in relative strength of the Arabian monsoons during a stadial/interstadial climate transition

The occurrence and propagation of abrupt climate change between the high and low-latitudes has become an important focus of paleoclimatic and paleoceanographic research. The causes of abrupt change have significant implications for understanding future manifestations of similar forcings under late Holocene (‘Anthropocene’) boundary conditions. Of particular interest are signals indicative of sub-millennial scale climate change in the sub-tropics of similar magnitude and frequency to those recorded in Greenland ice cores. Earlier research in the Arabian Sea has highlighted the sensitivity of sedimentary organic carbon and nitrogen isotope measurements for recording the state of the SW monsoon and associated Arabian Sea Oxygen Minimum Zone. In this study, we exploit the unprecedented fidelity of the sedimentary δ¹⁵N record to identify a 20 cm interval at ODP Site 723 containing a stadial/inter-stadial interval between 43-42 Kyr BP. We employ sedimentary nitrogen isotopes, chlorin pigment and alkenone abundances, major and minor element analyses of highly-resolved (2 mm ≈ 10 yr) samples across this interval to compare a comprehensive, multi-proxy data set to understand (a) the processes contributing to the δ¹⁵N signal in the longer records of denitrification; and (b) the associated climatic events, especially the relative intensity of summer and winter monsoons at these times. A lack of evidence for bioturbation in excess of our 2 mm sampling resolution facilitates decadal-scale oceanographic and climatic reconstructions. Using a four-component flux-dilution model, we show that the deposition of carbonate decreased in parallel with an increase in Total Organic Matter flux from stadial to inter-stadial time. This interval is also marked by a significant drop in lithogenic (dust) accumulation, analogous to a similar decrease noted during deglaciation in the Western Arabian Sea. Combined with alkenone U₃₇^K′-derived estimates for sea surface temperature (SST), we conclude that the climatological shift from stadial to inter-stadial conditions at low latitudes was characterized by repeated switches in mean monsoon state approximately every 200 yr. The winter monsoon was the dominant mode during maximum stadial conditions; conversely the summer monsoon was dominant during maximum interstadial-like conditions. However, each interval was separated by a distinct ‘inter-monsoon’ mode, indicated by a higher continental dust flux but warmer SST. Proxy records for changing bottom-water oxygenation show near-identical results down to the mm-scale, but hint at increased export production leading the onset of anoxia during the stadial/inter-stadial transition. The coherence of all sedimentary signals depicts a wholesale reorganization of the Arabian Sea climate and marine ecosystem over approximately 200 years, a period that may be associated with monsoon modulation by small oscillations in solar irradiance.     

Dislocating modernity: Identity, space and representations of street trade in Durban, South Africa

South Africa’s cities have experienced dramatic changes over the past decade. Cities are now home to a multiracial population, and have been transformed by new forms of economic and social interaction. For some, these changes have become a significant source of fear and anxiety. In this paper, we examine reactions to urban spatial change in the city of Durban, as expressed in local newspapers and interviews with suburban residents. We describe how the discourses of urban change in Durban have centred on the increased presence of street traders within the city’s public spaces, and the various ways in which the activity of street trade has disrupted long-established modernist norms governing the occupation and use of the urban space. Specifically, we offer a detailed reading of three prominent narratives within the discussion of street traders in Durban--chaos, congestion and pollution. We argue that street traders have come to embody a wide range of more deeply seated cultural anxieties, which have been brought to the fore in the context of South Africa’s transition. These anxieties arise from the ways in which modern understandings of order, agency and subjectivity have been called into question by material changes in the city, and have implications for the nature of citizenship and civic engagement in post-apartheid South Africa.     

Middle Campanian–lowermost Maastrichtian nannofossil and foraminiferal biostratigraphy of the northwestern Australian margin

The middle–late Campanian was marked by an increase in the bioprovinciality of calcareous microfossil assemblages into distinct Tethyan, Transitional, and Austral Provinces that persisted to the end of the Maastrichtian. The northwestern Australian margin belonged to the Transitional Province and the absence of key Tethyan marker species such as Radotruncana calcarata and Gansserina gansseri has led petroleum companies operating in the area to use the locally developed KCCM integrated calcareous microfossil zonation scheme. The KCCM zonation is a composite scheme comprising calcareous nannofossil (KCN), planktonic foraminiferal (KPF) and benthonic foraminiferal (KBF) zones. This paper presents the definitions and revisions of Zones KCCM8–19, from the highest occurrence (HO) of Aspidolithus parcus constrictus to the lowest occurrence (LO) of Ceratolithoides aculeus, and builds on our previous early–late Maastrichtian study. The presence of a middle–upper Campanian disconformity is confirmed by microfossil evidence from the Vulcan Sub-basin, Exmouth and Wombat plateaus, and the Southern Carnarvon Platform. In the Vulcan Sub-basin and on the Exmouth Plateau (ODP Hole 762C) the hiatus extends from slightly above the LO of common Rugoglobigerina rugosa to above the LO of Quadrum gothicum. On the Wombat Plateau (ODP Hole 761B) it spans from above the LO of Heterohelix semicostata to above the LO of Quadrum gothicum; and in the Southern Carnarvon Platform the disconformity has its longest duration from above the HO of Heterohelix semicostata to above the LO of Quadrum sissinghii. A significant revision of the events which define Zones KCCM18 and 19 was necessary owing to the observation that the LO of Ceratolithoides aculeus occurs below the HOs of Archaeoglobigerina cretacea and Stensioeina granulata incondita and the LO of common Rugoglobigerina rugosa. In the original zonation these events were considered to be coincident.