C.R.I. Clayton

Carbon isotope fractionation during natural gas generation from kerogen

Abstract In petroleum exploration it is important to be able to determine the origin of any gas which is found. This paper describes a new method of estimating the source-type and maturity of a gas based on a Rayleigh fractionation model. Kerogen is divided conceptually into a labile (dominantly oil-generating) fraction and a refractory, gas-prone, component. δ 13 C of methane from either kerogen type, and ethane, propane and butane for gases from labile kerogen, can be defined as a function of δ 13 C of the gas precursor groups in kerogen, a kinetic isotope fractionation factor, k , and the extent of gas generation. The isotopic ratio of the methane precursors relative to bulk kerogen, determined from laboratory pyrolysis, are −17.5‰ for labile kerogen and −1.4‰ for refractory kerogen. Values for ethane, propane and butane from labile kerogen, based on field correlations, are −4.9‰, −2.2‰ and −1.6‰ respectively. The corresponding fractionation factors are 0.9892, 0.9919, 0.9947 and 0.9975 for methane, ethane, propane and butane respectively from labile kerogen, and 0.9984 for methane from refractory kerogen. Using these parameters, summary diagrams are constructed which allow differentiation of these sources from each other and from biogenic gases and cracked oil, and recognition of gases of mixed origin. If an independent estimate of δ 13 C for the source kerogen is possible, then δ 13 C of the gas components can be used to estimate maturity in terms of the Gas Generation Index, the fraction of gas potential which has been realized.

An assessment of the effectiveness of atmospheric correction algorithms through the remote sensing of some reservoirs

Although satellite remote sensing techniques have been widely implemented for a variety of applications, using either single or time-series images, few studies have explicitly considered atmospheric effects, and how they can most effectively be minimized. Despite the considerable number of available atmospheric correction algorithms, there is little literature concerning their relative merits. Over water bodies, atmospheric effects account for the majority of the at-satellite measured radiance in the visible bands, and therefore targets of this type provide an opportunity for assessing the effectiveness of the different methods available. This paper reports a study of atmospheric effects and their correction, using multi-spectral satellite remote sensing data for an area to the west of London that includes eight large water reservoirs and a major international airport (Heathrow). Through comparisons of reflectance within a time series of 12 Landsat-5 Thematic Mapper (TM) images, the overall impact of atmospheric contributions is shown. The available atmospheric correction literature is then reviewed, and the methods applied to the satellite imagery of the reservoirs. The results are compared with reflectances acquired using a field spectro-radiometer. A critical appraisal of the results, and of problems encountered in applying the various methods, leads to an evaluation of their value in practice.

A laboratory investigation into the seismic velocities of methane gas hydrate-bearing sand

Remote seismic methods, which measure the compressional wave (P wave) velocity (Vp) and shear wave (S wave) velocity (Vs), can be used to assess the distribution and concentration of marine gas hydrates in situ. However, interpreting seismic data requires an understanding of the seismic properties of hydrate-bearing sediments, which has proved problematic because of difficulties in recovering intact hydrate-bearing sediment samples and in performing valid laboratory tests. Therefore a dedicated gas hydrate resonant column (GHRC) was developed to allow pressure and temperature conditions suitable for hydrate formation to be applied to a specimen with subsequent measurement of both Vp and Vs made at frequencies and strains relevant to marine seismic investigations. Thirteen sand specimens containing differing amounts of evenly dispersed hydrate were tested. The results show a bipartite relationship between velocities and hydrate pore saturation, with a marked transition between 3 and 5% hydrate pore saturation for both Vp and Vs. This suggests that methane hydrate initially cements sand grain contacts then infills the pore space. These results show in detail for the first time, using a resonant column, how hydrate cementation affects elastic wave properties in quartz sand. This information is valuable for validating theoretical models relating seismic wave propagation in marine sediments to hydrate pore saturation.

Influence of gas hydrate morphology on the seismic velocities of sands

This paper reports the results of a series of resonant column tests on specimens where gas hydrate has been formed in sands using an “excess water” technique. In these specimens the amount of hydrate formed is restricted by the amount of gas in the specimen and with an excess of water being present in the pore space. Results of resonant column tests carried out to determine compressional and shear wave velocities suggest that gas hydrate formed in this way are frame supporting. In contrast, the behavior observed in sands where the hydrate is formed from finite water where the remaining pore space is saturated with methane gas, termed in this paper the “excess gas” method, exhibits a cementing behavior, while tetrahydrofuran-hydrate sands or where the hydrate is formed from dissolved methane within the pore water, exhibit a pore-filling behavior for hydrate saturations less than 40%. For sands where the hydrate is formed using the excess water method, much larger volumes of hydrate are required before a significant increase in shear wave velocity occurs, although increases in compressional wave velocity are seen at lower hydrate contents. These results suggest that hydrate interaction with the sediment is strongly dependent on morphology, and that natural hydrate may exhibit contrasting seismic signatures depending upon the geological environment in which it forms.

The use of hall effect semiconductors in geotechnical instrumentation

For the past five years or so Hall effect semiconductors have been increasingly used in the geotechnical engineering laboratories at the University of Surrey. They have been incorporated as sensing elements in local radial and axial strain measuring devices, for the small-strain instrumentation of triaxial specimens, and in small diameter boundary normal and shear stress cells. Triaxial internal load cells are currently being built incorporating Hall effect semiconductors. This paper describes the Hall effect principle and the methods of configuring magnet/sensor systems to achieve suitable measuring systems. Some geotechnical instruments built at the University of Surrey are detailed, and their characteristics discussed. The calibrations of the instruments described in the paper show a performance generally at least as good as might be expected from some commercially available instruments. Hall effect semiconductors are shown to be of use in a range of situations where displacement can form the basis of measurement. The displacements measured can vary from as little as 5 µm to as much as 10 mm, and the best repeatability so far obtained has been of the order of 1/100 of a micrometre.

Stress changes in the ground below ballasted railway track during train passage

Abstract The design of railway track formations has traditionally been empirically rather than analytically based, with ballast and sub-ballast layer thicknesses specified mainly on the basis of previous practice. Recent design methods are more scientifically based, and for the most advanced design methods currently in use, input parameters are typically determined from cyclic triaxial testing. The changes in stress experienced by an element of soil below a railway track as a train passes are complex, involving (for example) a cyclic rotation of the principal stress directions. In these conditions, soil element testing in uniaxial compression may lead to the underestimation of vertical strains. Testing in a hollow cylinder apparatus, which can impose the rotations in principal stress direction likely to be experienced by a soil element in the field, may therefore be preferable to triaxial testing. However, there are as yet no data to guide the designer to a rational specification of a testing programme in this more complex apparatus. This article reports the results of finite element analyses carried out to investigate the stress changes experienced by an element of soil beneath a ballasted railway track during train passage. The effects of element location, the initial in situ stress state of the soil, and the elastic parameters used to characterize its behaviour are investigated, and the modelling of the stress paths in a cyclic hollow cylinder apparatus is discussed.

The use of selected pseudo-invariant targets for the application of atmospheric correction in multi-temporal studies using satellite remotely sensed imagery

Because atmospheric effects can have a significant impact on the data obtained from multi-spectral satellite remote sensing, it is frequently necessary to make corrections before any other image processing can be started. This paper describes a robust and relatively simple atmospheric correction method that uses pseudo-invariant targets (PITs) in conjunction with the empirical line method. The method is based on the selection of a number of suitable generic PITs, on the basis that they are large, distinctive in shape, and occur in many geographical areas. Whereas the multi-temporal normalization method corrects all images to a selected reference image, in this method images are simultaneously corrected using targets with a range of estimated surface reflectance values. The paper describes some applications of the method for a range of environmental studies involving water quality and air pollution monitoring, and mapping land-cover changes.

Assessment of temporal variations of water quality in inland water bodies using atmospheric corrected satellite remotely sensed image data

Although there have been many studies conducted on the use of satellite remote sensing for water quality monitoring and assessment in inland water bodies, relatively few studies have considered the problem of atmospheric intervention of the satellite signal. The problem is especially significant when using time series multi-spectral satellite data to monitor water quality surveillance in inland waters such as reservoirs, lakes, and dams because atmospheric effects constitute the majority of the at-satellite reflectance over water. For the assessment of temporal variations of water quality, the use of multi-date satellite images is required so atmospheric corrected image data must be determined. The aim of this study is to provide a simple way of monitoring and assessing temporal variations of water quality in a set of inland water bodies using an earth observation- based approach. The proposed methodology is based on the development of an image-based algorithm which consists of a selection of sampling area on the image (outlet), application of masking and convolution image processing filter, and application of the darkest pixel atmospheric correction. The proposed method has been applied in two different geographical areas, in UK and Cyprus. Mainly, the method has been applied to a series of eight archived Landsat-5 TM images acquired from March 1985 up to November 1985 of the Lower Thames Valley area in the West London (UK) consisting of large water treatment reservoirs. Finally, the method is further tested to the Kourris Dam in Cyprus. It has been found that atmospheric correction is essential in water quality assessment studies using satellite remotely sensed imagery since it improves significantly the water reflectance enabling effective water quality assessment to be made.

The role of free gas in the activation of submarine slides in Finneidfjord

Free gas is evident on high resolution seismic profiles near the 1996 Finneidfjord submarine landslide, Norway. A bright reflector at about 6 m sub-seabed depth on intact terrain adjacent to the slide coincides with the initial failure surface. We hypothesise that the bright reflector comprises free gas collecting in relatively sandy layers, and that the free gas could have contributed to the generation of excess pore pressures and the initiation of the submarine landslide. Preliminary measurement of the bright layer reflection coefficient from seismic sections, and interpretation of the available geotechnical data, support this hypothesis.

The Assessment of Atmospheric Pollution using Satellite Remote Sensing Technology in Large Cities in the Vicinity of Airports

This paper investigates the potential of usingsatellite remotely sensed imagery for assessing atmosphericpollution. A novel approach, which comprised radiativetransfer calculations and pseudo-invariant targets fordetermining aerosol optical thickness has been developed.The key parameter for assessing atmospheric pollution inphotochemical air pollution studies is the aerosol opticalthickness. The need for identifying suitable pseudo-invariantobjects in satellite images of urban areas is of great interest for increasing the potential of earthobservation for monitoring air pollution in such areas. Theidentification of large water bodies and concrete apronsthat can serve as suitable dark and bright targetsrespectively in different geographical areas wasdemonstrated in this study. This study added evidence onthe correlation found between the visibility valuesmeasured at Heathrow Airport area during satellite overpassand aerosol optical thickness derived from Landsat-5 TMband 1 images.