Recent technological advances have led to dramatic improvements in seismic resolution which now enable seismic attenuation to be measured on a routine basis. The usefulness of seismic attenuation as an interpretation parameter is restricted at present by our incomplete knowledge of the relationships between attenuation and important reservoir parameters such as porosity, permeability and the nature of the pore fluid. In order to redress this imbalance, P-wave and S-wave velocities and attenuations were measured in the laboratory at a frequency of about 0.8 MHz and at an effective pressure of 60 MPa on a set of reservoir rocks comprising twenty-seven shaly sandstones and two sandy shales. It was discovered that both P-wave and Swave attenuation increase with increasing percentage of intrapore minerals (especially clay, but also micrite) from the composition of a clean sandstone to a value of 50%, and then they decrease with increasing clay content as the rock approaches the composition of a pure shale. Porosity plays a subsidiary role to pore fill. More work is needed to predict permeability from its complex relationships with porosity and pore filling minerals. Ultrasonic P-wave and S-wave velocities (Vp and Vs) were shown to be strongly dependent on the amount of pore fill: velocity is reduced by increasing amounts of clays and micrite, but it is increased by increasing amounts of sparry calcite cement. Contrary to popular understanding, the effect on velocity of the percentage of pore filling minerals is much stronger than that of porosity on velocity, although porosity is still an important parameter. There is a strong correlation between Vp and Vs, and between P-wave quality factor (Qp) and S-wave quality factor (Qs). Interesting relationships were also discovered between Qp and Vp, and especially between Qs and Vs. However, high frequency laboratory measurements may not be representative of the low frequencies used in seismic exploration. The frequency dependence of velocity and attenuation in reservoir sandstones was investigated by exploiting the inverse relationship between frequency and pore fluid viscosity predicted by the Biot theory. P-wave and S-wave velocities and attenuations were measured at a frequency of about 0.8 MHz and at an effective pressure of 50 MPa on shaly sandstones saturated with pore fluids of viscosities 0.3 cP to 1000 cP (equivalent frequency range 2.6 MHz to 780 Hz). The Biot theory accounts for the very high Q values encountered in clean sandstones, but not the very low Q values observed in clay-rich sandstones. The observed velocity dispersion in both clean and clay-rich sandstonesim plies a local fluid flow mechanism which predicts the opposite frequency-viscosity dependence of the Biot theory. Qp and Qs in clay-rich sandstones remain constant over nearly four decades in equivalent frequency. This implies a range of relaxation times which may be attributed to the wide distribution of pore sizes of clays and other pore filling minerals. Moreover, this constant Q behaviour and the magnitude of the laboratory Qp and Qs values tie in well with those obtained from field studies. This suggests that Qp and Qs are broadly independent of frequency from seismic frequencies to ultrasonic frequencies in clay-rich sandstones.

Extracting coal from underground mineworkings causes the overlying rocks to subside with associated changes in the stress regime. The aim of the study reported here was to apply the surface seismic reflection method to study the effect of subsidence on seismic velocity. Two sets of time-lapse surveys were carried out over two longwall mining panels in the Selby Coalfield. Seismic lines were profiled parallel and perpendicular to adjacent panels H45 and H46, respectively. A total of twenty-one repeated surveys were carried out along the two lines over a period of three years. The effect monitored was due to mining in the Bamsley Seam, at 550 m depth. As mining progressed, the traveltime of a strong reflection event from an anhydrite bed at 150 m depth was measured after processing the data with standard techniques. An overall increase in traveltime of about 4 % was observed. The progressive increase in traveltime over panel H45 correlated well with empirical calculations of differential subsidence between the surface and the anhydrite. However, the magnitude of the change must principally be accounted for by a decrease in seismic velocity, associated with a reduction in the vertical effective stress. Although the traveltime over panel H46 was also found to increase, and to correlate quite well with die expected differential subsidence, the agreement was less good along this transverse profile. This is attributed to asymmetric subsidence effects because the ground on the SW side of the panel had already been worked by panel H45, but the ground on the NE side was unworked. At the time of each seismic survey across panel H46, the profile was also levelled, and it was found that surface subsidence values along the profile increased towards panel H45. As most of the subsidence caused by mining panel H45 would have been completed by the time the H46 profile was surveyed, the effect must be at least partly attributed to asymmetric subsidence due to panel H46. Where the ground had been weakened by subsidence due to mining H45, near-total subsidence from mining H46 took place rapidly; but in the previously unworked ground on the NE side of panel H46, the residual subsidence was presumably delayed by competent strata in the overburden. Further work is needed to confirm whether this explanation is correct.

No description available.

The research project aimed to establish the seismic characteristics of the Upper Cretaceous and Precambrian basement reservoirs in the area of the Nafoora-Augila field. The characteristics studied include acoustic impedance, amplitude, phase, and reflection strength, derived from seismic profiles closely tied to well data. The project demonstrates to a certain degree the successful use of combined techniques to assist in the interpretation of the seismic data by improving its quality. The techniques included: one-dimensional and two-dimensional modelling, seismic attributes and seismic inversion techniques. The generated synthetic seismograms show a reasonably good match with observed seismic data at the well locations. The mismatch between reflection interfaces at some intervals indicated that it could be the result of inaccurate sonic measurements. Investigation on the sonic readings was conducted in the laboratory by measuring the transit time for core samples obtained from these intervals. The laboratory measurements generally supported the sonic. Two-dimensional modelling was carried out in order to compare synthetic sections with the observed seismic sections. The comparison indicated that model sections can be used for the recognition and confirmation of the reflection events of interest in the observed seismic sections and especially the seismic expression of the thin layers such as the Rachmat shale and the Bahi sandstone. The presence of multiples on the final seismic sections has obscured the primary reflection events of interest. Velocity analysis using the velocity spectra technique is conducted to improve the stack of the seismic data and minimize the effect of multiples. The newly picked stacking velocities show better results than those picked by the contractor by using the velocity function technique. The post-stack deconvolution was applied to attenutate the existing multiples by testing different deconvolution parameters. The tested parameters gave very encouraging results by bringing out distinctive reflectors and suppressing multiples. Seismic attributes and inverse modelling were used to enhance the interpretation of the seismic data. The use of the attributes on the observed seismic data are generally of little use due to the poor quality of the data. However, the amplitude and the phase displays show a reasonably good response even with the effect of noise and multiples. Comparison of pseudo impedance logs and sections with well impedance logs and seismic sections shows fair to reasonably good comparison.

Intensive exploitation of the reservoir at The Geysers geothermal area, California, induces myriads of small-magnitude earthquakes that are monitored by a dense, permanent seismometer network that covers most of the reservoir. However, majority of the seismic stations, which belong to the UNOCAL network are poorly calibrated. Station polarities, and sensor orientations for the 8 three-component stations of this network were determined by using accurate focal mechanism solutions from a temporary network and using a simple method of observing the waveform from known earthquake locations. Using data from the UNOCAL network, tomographic inversions were performed for the three-dimensional Vp and Vp/Vs-ratio structure of the reservoir for February 1993, October 1996 and August 1998, adding to the inversions for April 1991 and December 1994 that had already been performed by other investigators. The extensive low -Vp/Vs anomaly known to characterise the reservoir grew progressively in strength from a maximum of 9% to a maximum of 12.4% at sea level during the seven-year study period. The anomaly growth is attributed to the depletion of pore liquid water in the reservoir and its replacement with steam. This causes Vp to decrease by increasing compressibility, and Vs to increase because of the reduction in pore pressure and the drying of argillaceous minerals, e.g., illite, which increases the shear modulus. All these effects serendipitously combine to lower the Vp/Vs ratio, resulting in an exceptionally strong overall effect that provides a convenient tool for monitoring reservoir depletion in the seismogenic zone. Variations in the separate Vp and Vs fields indicate that water depletion was the most important process in the central part of the exploited reservoir, and that pressure reduction and mineral drying were the dominant effects more northwesterly and southeasterly. Relative relocation of micro earthquakes was also performed using the same network. Four regions were studied. Although most multiplets relocated into tighter clusters and the reduction in the RMS of the relative relocations was good, further work is needed to substantiate these initial findings.

The Geysers geothermal area, California is the world's largest and most intensively exploited steam field, providing about 6% of California's electrical power. The geothermal area is very active seismically, generating about 140 earthquakes per month with M(_D)≥l.2. Non-DC earthquakes have been routinely detected in other geothermal and volcanic areas such as the Hengill-Grensdalur volcanic complex, Iceland but previously went undetected at The Geysers. The steam field is, however, a likely source of non-DC earthquakes because large volumes of steam are extracted and condensate injected during the course of commercial exploitation which might cause cracks or fractures to open and close. Maps of seismic activity through time show conclusively that earthquakes initiate at the onset of production, continue through it and stop when production ceases. Furthermore the volume of steam extracted and/or condensate injected may directly control the rate of seismicity within the geothermal area. A temporary field experiment in April, 1991 recorded about 4000 high-quality earthquakes on three-component digital sensors. Three-dimensional tomographic models of V(_p) and, for the first time at The Geysers, V(_p)/v(_s) were determined using 3906 P-wave and 944 5-wave arrival times from 185 earthquakes. Variations m lithology, temperature and the pore-fluid phase probably produce the variations in V(_p). A strong low in the V(_p)/v(_s) model defines fluid-deficient areas in the steam reservoir and is surrounded by a "halo" of high-V(_p)/v(_s) anomalies. V(_s)/v(_s) can remotely monitor temporal depletion of liquid reserves in the steam reservoir. Well-constrained moment tensor solutions for 30 earthquakes were determined by inverting the polar' ices and amplitude ratios of P- and 5-wave arrivals. Strong evidence for the existence of non-DC earthquakes at The Geysers geothermal area was found. Explosive and implosive events occur in equal numbers and probably reflect source processes involving opening and closing cracks or cavities. The events form a symmetric pattern on source-type plots extending from the postive dipole-to-negative dipole loci, passing through the DC locus. The association with dipole loci rather than crack loci suggests the source process must also involve a compensating flow of fluids, liquid or steam.

This thesis describes the interpretation of the deep seismic structure from the wide-angle reflection/refraction Caledonian Suture Seismic experiment which extends from the Mid North Sea High across Northern England, the Irish Sea to southwest Ireland. A new method of displaying the large number of travel times in the form of contoured composite plots for a particular phase has been developed. The interpretation methods used include analysis of the wide-angle reflection travel times, time-term analysis, gravity modelling, and, ray tracing and synthetic seismogram modelling. A mid-crustal gradient which returns PcP occurs between depths of 15.0-18.5 km beneath the line and is overlain by an upper crust with an average velocity of 6.16-6.20 km/s excluding sediments. This mid-crustal gradient was not recognised previously in the Irish Sea. A lower-crustal boundary recognised from the PmP phase occurs at 25.0 km depth beneath the Irish Sea and at 30 km beneath the North Sea. The average crustal velocity is 6.49-6.54 km/s excluding sediments and for the lower crust is 6.75-6.77 km/s. The sub-Moho Pn velocity is estimated to be 8.19 km/s beneath the Irish Sea, 8.32 km/s beneath Northern England and 8.02 km/s beneath the North Sea. The PmP and Pn boundaries diverge beneath the Irish Sea and define a transitional Moho over a 5 km depth range with an average velocity of 7.8-7.9 km/s. The PcP and Pn boundaries correspond approximately with the top and base respectively of the reflective deep crust observed beneath the Irish Sea on BIRPS WINCH. The transitional Moho beneath the Irish Sea has a reflective character similar to the lower crust. The coincident PmP and Pn boundaries beneath the North Sea define an abrupt increase in velocity from 6.75 to 8.0 km/s which approximately corresponds to the prominent reflections beneath the non reflective lower crust observed on the BIRPS NEC line at 10.5 to 11.0 s .The upper crust of velocity 6.15-6.20 km/s appears to lie below the suture beneath Northern England and the North Sea along the line but may also occur above it further west. The lower crust and deeper structures lie below the suture and represent crust of the Southern Caledonides. The anomalous structure of the Moho and upper mantle beneath the Irish Sea found from CSSP and BIRPS may be related to the post-Caledonian formation of the Irish Sea basins. The reflective lower crust and transitional Moho may result from crustal thinning produced by ductile stretching.

The structure of oceanic crust is surprisingly uniform, which suggests that crustal accretionary processes at mid-ocean ridges must be broadly similar, despite their different spreading rates and seafloor morphologies. Seismic studies have revealed the presence of sub-axial magma chambers at fast, slow and intermediate spreading ridges, but constraints on their shape and size are generally restricted to the fast spreading East Pacific Rise. The aim of this study is to compare the processes of crustal accretion at fast, slow and intermediate ridges by investigating the detailed crustal structure and magma chamber geometry of a magmatically active intermediate spreading ridge, the Valu Fa Ridge. A multidisciplinary geophysical experiment was conducted over the Central Valu Fa Ridge and its overlap with the Northern Valu Fa Ridge during R/V Maurice Swing Cruise EW9512, and wide-angle seismic data, recorded on a set of digital ocean bottom seismometers, were used to generate velocity-depth models on two across-axis, two along-axis and two axis-parallel profiles. These models were further constrained by modelling of the normal incidence seismic and gravity data and the resulting combined models of crustal structure were interpreted to reveal that a composite magma chamber exists beneath the Valu Fa Ridge crest. The magma chamber consists of a thin, narrow (1-1.5 km) melt lens, with an interconnected melt fraction, overlying a wider (-4 km) region of hot rock or low melt fraction. A reflection from the top of the melt lens is identified on both the normal incidence seismic and wide-angle seismic data and delay- time modelling indicates that velocities as high as 5.5 km s(^-1) are achieved -250 m below the top of the melt lens. The main body of the magma chamber corresponds to the region of hot rock below the melt lens and is delineated by anomalously low velocities, extending down through seismic layer 3 to within 1.5-2 km of the Moho. Moho reflections from beneath the overlapping spreading centre and a low on the mantle Bouguer anomaly map implies that this region is currently, or has recently been, the site of enhanced magmatism. This observation is contrary to popular models of ridge segmentation and melt delivery. The transition from pre-rift crust (both island arc and back-arc crust) to post-rift material, marked by considerable thinning of seismic layer 2, has also been uniquely identified in this study and describes the limit of VFR-generated crust. The size and temporal stability of magma chambers are largely dependent on their magma budget and the Valu Fa Ridge magma chamber model, developed as part of this study, may bridge the gap between the large, long-lived magma chambers identified at the East Pacific Rise and the more transitory magma chambers proposed at slow spreading ridges. Melt ascends as small isolated pockets through the main body of the magma chamber at the Valu Fa Ridge and resides in the melt lens until eruption. Seismic layer 2 is constructed solely from material 'erupted' from the melt lens, with the main body of the magma chamber cooling to form seismic layer 3. Convection currents, induced by large thermal gradients at the sides of the magma chamber, both accelerate the cooling process, thus limiting its size, and helps to generate the thick layered sequences as observed in ophiolite studies. The entire crust is emplaced within the axial region and a distinct Moho is formed at -0 Ma.

In November 1979, the Department of Geological Sciences, University of Durham, in conjunction with the Department of Geology, University of Glasgow, carried out a marine seismic refraction survey between Barra In the Outer Hebrides and Girvan on the Ayrshire coast. In August/September 1981, a shorter profile, between Mull and Kintyre, was undertaken. Temporary recording stations were set up on land and explosives and alrguns used as sources at sea. In addition, data were obtained from the permanent recording networks in Scotland. The application of digital filtering techniques to the alrgun lines Is presented and reviewed. The explosive shot data were interpreted using time-term analysis, the plus- minus method and ray-tracing. Gravity and magnetic studies were used as a control on the interpretation. The depth to the basin has been examined, indicating sedimentary cover of between 1 and 3 km along the line. The variation in basement velocity has been determined. A velocity model for the upper crust has been developed indicating large lateral changes. A steep gradient Is thought to exist between Golonsay and Jura and a more gentle gradient beneath Mull. There does not seem to be evidence for a mid-crustal refractor with a sharp velocity transition across the boundary. Large changes In Pn time-terms across the Minch and Inner Hebrides basin are thought to be largely due to the varying velocity structure of the basement. The basin formation and deformation of the lower crust are thought to Involve movement within the mantle.

In June 1987, during the BIRPS MOBIL normal-incidence seismic profiling programme, off the East coast of England, the University of Durham recorded simultaneously at several land based seismic stations in Northern England. The resulting wide-angle data, particularly from Line 1, have excellent resolution in both space and time due to the airgun source and 50 m shot spacing. The interpretation of the Line 1 wide-angle data at Durham used BEAM87, Cerveny's Gaussian beam modelling package. The main arrivals interpreted include the upper crustal refraction (Pg), the Moho wide-angle reflection (PmP), the upper mantle refraction (Pn), and a very high amplitude arrival (D) which merges into PmP. Modelling gave a crust about 30 km thick with a change in velocity gradient and a slight velocity contrast at about 20 km depth. There are several wide-angle reflections from interfaces at mid-crustal depths, between 10 and 20 km depth, and the bottom 2 km of the crust has a high velocity of about 7 kms(^-1). Two interesting results are that a lateral velocity change about 40 km offshore is required to fit the Pg travel times; also that arrival D is modelled best as the remnant of a step on the Moho at the same location. These appear to be borne out by the normal incidence data for line 1, which show a lateral decrease in the mid-crustal reflectivity above a set of strong, westerly-dipping reflections at Moho depths. These results suggest the presence of a major crustal fault about 40 km offshore. It is suggested that this fault may be the northward continuation of the Dowsing Fault Zone.