Geophysical studies were an integral part of a multidisciplinary investigation
of the basement aquifer in Zimbabwe, carried out by British Geological Survey
(BGS) and the Ministry of Energy and Water Resources and Development (MEWRD)
of Zimbabwe, and funded by the Overseas Development Administration (ODA) of
the British Government. The study was centred on Masvingo Province, in
conjunction with the Provincial Water Engineer based in Masvingo as well as
the Hydrogeology Department in Harare.
A geophysical study had been made in 1986 (Smith and Raines, 1987) of a number
of key areas, which represented the range of major rock types, in a variety of
different hydrological and hydrogeological settings. It was intended as an
orientation study, to obtain additional data on a number of previous drilling
exercises which had fully reported, but which had a proportion of unsuccessful
boreholes. On the basis of this experience, it was hoped to apply methods
which might enhance the success rate.
This second season's work represents the application of this experience to the
identification of suitable drilling targets in conjunction with parallel
hydrogeological, structural and geomorphological assessments. It was hoped
that the drilling would improve our understanding of the structure and
hydrogeological behavior of major linear features identifiable from aerial
photography, as well as provide suitable sites for the installation of pumps
for water supply.
When creating an electrical earth for a transformer with vertically driven earthing rods, problems can arise either because the ground is too hard or because the ground is too resistive to achieve the required earthing resistance. To assist in the planning of earthing installations a geographic information system (GIS) layer has been created. In its simplest form it consists of a colour coded map that indicates the most likely earthing installation: a single vertically driven rod (indicated by dark green); multiple vertically driven rods (indicated by light green); a horizontal trench, where a rod installation is unlikely (indicated by yellow); for difficult ground, a specialist installation (i.e. drilling; indicated by red). However, the GIS can be interrogated to provide site-specific information such as site conditions, likely depth of installation and quantity of earthing materials required. The GIS was created from a spatial model constructed from soil, superficial and bedrock geology that has been attributed with engineering strength and resistivity values. Calculations of expected earthing rod resistance, rod or trench length, and all possible combinations of ground conditions have been compared with the ‘likely’ conditions required for each of the four proposed installation scenarios to generate the GIS layer. The analysis has been applied to the electrical network distribution regions of Western Power Distribution, in the English Midlands, and UK Power Networks, which covers East Anglia, London and the SE of England. Because the spatial model that underlies the GIS has been constructed from national databases the analyses can be extended to other regions of the UK.
Hard rock cliffs erode through an initial catastrophic collapse along pre-existing discontinuities
in the rock mass. These may be ancient faults or fractures, orientated at a variety of angles to the
cliff face, or relatively new tension fractures formed during cycles of cliff recession, sub-parallel
to the cliff face. It is likely that an approaching cliff fall will be associated with increasing
fracture dilatancy within the fracture network. Hence if the change of dilatancy can be measured
then it may be possible to generate alerts of impending cliff collapse.
Since fractures often occur in sets with a preferred orientation they impose anisotropic physical
properties on the rock mass. Hence, the apparent resistivity of the rock will vary with azimuth
reflecting the dominant fracture orientation. Measures of anisotropy can be calculated from the
measurements and would be expected to vary with time if the fractures are dilating.
Work package one of the 5th Framework co-funded project ‘PROTECT’ (PRediction Of The
Erosion of Cliffed Terrains) was to detect fracture dilatancy. Azimuthal apparent resistivity data
were collected at five research sites in the UK, France and Denmark, all situated on outcropping
chalk. At each research site, data were collected with the Square array at three locations near the
cliff edge and at a Control site set back from the cliff edge by about 50 m. Data were collected
approximately every two months for two years to create a temporal data set. After processing the
data to remove the effect of the infinite resistance afforded by the cliff face, the data were fitted
to an ellipse in order to test for anisotropy. Measures of anisotropy were then calculated from
these data.
The anisotropy has been interpreted as fracturing and indicates a number of tectonic fracture
orientations that agree with geological mapping. At several of the research sites a cliff-parallel
fracture set was identified in a zone 10 to 20 m wide adjacent to the cliff edge. It is assumed that
this fracture set develops in response to the stress relief at the cliff face. At the Birling Gap
research site a cliff collapse within the zone of resistivity measurements produced a dramatic
drop in the magnitude of the post-collapse calculated measures of anisotropy. However, other
cliff falls that occurred outside of the immediate zone of resistivity measurements did not
generate appreciable changes in the calculated measures of anisotropy. It appears that the
tectonic fractures that limit the lateral extent of the cliff fall may also limit the fracture dilatancy
within the cliff parallel fracture set. At some sites there was a seasonal variation in the measures
of anisotropy with peaks in the summer and troughs in the winter. It appears that the most likely
driver for these variations is rock temperature that is itself controlled by the external air
temperature.
Overall, the research has been successful in establishing that there are measurable changes in the
rock mass prior to a collapse. However, the methodology is not yet advanced enough to be able
to develop technology for the reliable early warning of a cliff fall. The next stage of any research
would be to install a system for continuous monitoring in order to establish the magnitude of the
changes in the measures of anisotropy immediately prior to a cliff collapse.
Three multicomponent (9‐C) seismic surveys were conducted at Postle Field, Oklahoma. Interpretation of the surveys illustrates that the Morrow A sandstone can be detected. The sandstone was previously considered acoustically invisible yet the combination of multicomponent and time‐lapse seismic data has enabled us to detect the reservoir with average thickness of 28 ft (8.5 m) buried beneath a complex overburden at 6100 ft (1850m) depth. Even though the sandstone is thin it has a greater elastic impedance contrast than acoustic impedance contrast. We have found that shear wave data enables reservoir mapping of at least half the minimum thickness seen on P. This is because the shear wave reflectivity contrast between the sandstone and adjacent shale is three times that of P‐wave thus enabling higher definition of the thin sandstone reservoir with shear wave data. Dynamic changes introduced by water and carbon dioxide flooding enables further delineation of the sandstones in the shale dominated interval.
Multicomponent, time-lapse seismology has great potential for monitoring production processes in reservoirs. The main reason is simply the presence of fluid-filled fractures. Shear waves (s-waves) are much more sensitive than compressional waves (p-waves) to the presence of fractures or microfractures and the fluid content within the fracture network. Fractures introduce seismic anisotropy into a reservoir, causing two shear modes (S1 and S2) to propagate with different velocities and therefore different arrival times. This phenomenon is referred to as s-wave splitting or birefringence, and is critical for estimating fracture density (see Martin and Davis, 1987).At Central Vacuum Unit (CVU), s-wave splitting is developing as an important key to monitoring production processes associated with carbon dioxide (CO2) flooding. Fluid property changes associated with CO2 flooding produce changes in the velocities of the split s-waves passing through the reservoir interval. Fluid properties change in response to CO2 and oil becoming a miscible phase in the presence of in-situ fluids. S-wave splitting can also be used to identify areas of anomalous reservoir pressure. S-wave splitting and velocities are extremely sensitive to the local stress field because all rocks, especially carbonates, contain incipient networks of microfractures at a state of near-criticality (Zatsepin and Crampin, 1997).S-wave splitting can assist in separating effective stress changes associated with abnormal fluid pressures from fluid property change. This conclusion is inferred by results of the CVU study. During the first phase, Phase-I of this study, a prominent s-wave splitting anomaly was detected to the south of a cyclic CO2 injection well (CVU 97). It is believed that this anomaly corresponded to the tertiary flood bank that developed south of this temporary injection well (Figure 1a). Noticeable in the periphery to this anomaly are anisotropy anomalies of opposite sign related to offset wells that were used to contain the CO2 bank through water injection. The sign change of s-wave anisotropy occurs because the relative velocities of the split s-waves reverse. In the case of the miscible CO2-oil bank, the S2 velocity increased and S1 decreased, whereas, in the case of water injection, the effective stress causes S2 to decrease and S1 to increase. Similar effects were observed during the second phase, Phase-II of the monitoring study (Figure 1b). These results imply that s-wave anisotropy can be used to monitor secondary (water flooding) as well as tertiary (CO2) methods in a spatial context beyond the wellbore. This dynamic reservoir characterization could provide the industry with the ability to be more proactive than reactive in the management of reservoirs.
Passive refraction microtremor (ReMi) surveys utilise standard field seismic-refraction recording equipment and linear geophone arrays to record ambient background noise due to microtremors caused by natural and anthropogenic activities.The technique relies upon the detection of coherent phases of Rayleigh waves that have propagated along the axis of the geophone array, which is the -2 -14/07/2011
The aims of this study were to evaluate some surface wave based methods and their limitations with regard to aggregate variability and thickness determinations. We compared the results of field assessments of sand and gravel sequences using two different surface wave survey approaches. The first, followed a seismic refraction approach, and the second, a CSW survey methodology. Further probing using an ultra-lightweight cone penetrometer provided verification of results, and also, an active extraction programme at the field site provided the opportunity to directly observe the subsurface geology post-survey.
A combination of conventional surveying and non-invasive techniques have been applied to characterising the geomorphology, soils and shallow substrates of a typical small catchment in the Southern Uplands in Scotland, in three dimensions. Integration of geospatial, geophysical and geotechnical data, in the resulting digital 3D model, enable the nature and extent of individual components of the landscape to be measured and their relationships at depth to interpreted and visualised. This type of baseline data is fundamental to understanding past, and monitoring and measuring the impacts of future environmental changes in environmentally sensitive areas.
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