In recent years there have been significant advances in the acquisition and processing of crosshole seismic reflection data, and the method has been shown to be a high resolution imaging technique. However, the fidelity of the final images produced by this technique needs to be considered carefully to avoid incorrect interpretation. This thesis concerns the imaging capability of crosshole surveys, as well as advances made in processing techniques for application to crosshole seismic reflection surveys. In a migrated seismic section, a meaningful image is only obtained if a range of dips around the local structural dip is sampled at each image point. For crosshole seismic reflection surveys, the distribution of dips sampled at an image point is controlled principally by the survey geometry, including source and receiver array lengths and their element spacings. By considering the dips sampled, the imaging capability of crosshole reflection surveying is discussed, with suggestions as to how to ensure optimal imaging of the target zone. To overcome problems encountered in applying standard processing procedures, two new processing techniques are presented which enhance the imaging potential of crosshole reflection seismics. Generalised Berryhill migration has been developed as a full generalised Kirchhoff migration to include the near-field term, with the aim of improving image accuracy close to the source and receiver arrays. 3-D f-k-k filtering is an improved method of wavefield separation for crosshole seismic data. Finally, the results of processing three types of dataset are presented. One is from a site in the Groningen gas field, another was acquired through a model interrogated at ultrasonic frequencies in a water tank, and the third type was acquired using coal exploration boreholes in Yorkshire. The results demonstrate the imaging capability of the crosshole reflection method, and the success of the two new processing schemes.

A network of four independently-recording seismic stations was operated by the University of Durham in South-Central Afar during 1973 and 1974. Each station consisted of a three-component set of seismometers, whose signals were recorded on to magnetic tape. This study concerns local earthquakes recorded from February to September, 1974.250 earthquakes were located from relative arrival times of P and S phases using an optimized, laterally homogeneous, 4-layer structural model. Upper crustal P-wave velocities are found to be 4.4±0.2 km s(^-1) (0 to 4.5 km depth) and 6.2± 0.1 km s(^-1) (4.5 to 11 km). Deeper structure is poorly constrained. Anomalous upper mantle exists, with low seismic velocity (Vp about 7.4 km s(^-1)) and raised Poisson's ratio (0.31). S(_n) is transmitted, 8.0 km s(^-1) upper mantle cannot exist above about 43 km depth. Earthquake focal depths within Afar do not exceed 5 km. Epicentres correlate well with Recent axial volcanism. Spatial epicentral patterns reflect intense regional NW-SE extensional faulting. One line of epicentres shows the NNE-SSW trend of the Main Ethiopian rift. Focal mechanisms are very poorly constrained, but are consistent with NW-SE strike-slip or normal faulting, or with NE-SW dextral transcurrent faulting. Signal duration magnitude and Richter local magnitude scales are defined for Afar, Frequency-magnitude b-coefficient values are 0.87+0.05, The three-component records are polarization filtered, a technique previously applied only to teleseisms. The performance of the filters is discussed. Azimuths and apparent angles of incidence of events are determined from their first arrivals at a single recording station. Hypocentres are then obtained by ray tracing. Earthquake frequency spectra are computed through the fast Fourier transform. The spectra are dominated by the effects of the superficial crust below the receivers. Crustal transfer ratios are discussed. Increased attenuation is demonstrated below the Tendaho graben. Seismic source parameters are calculated using BRUNE's (1970) method. All results are consistent with diffuse NE-SW crustal extension. It is concluded that well-defined spreading axes do not yet exist.

There have been relatively few investigations into the attenuation properties of unconsolidated sediments using marine surface seismic data. Several methods of measuring attenuation were assessed for reliability in a noise-free case and with the addition of noise using a set of synthetically absorbed and dispersed wavelets. Wavelet modelling proved to be superior to the other techniques, followed by spectrum modelling and the spectral ratios method. Complex trace analysis using the analytical signal proved to be unreliable for non-sinusoidal wavelets, whilst the risetime method was found to be very susceptible to noise for practical purposes. Numerical modelling was carried out to assess the spectral effects of layering on a propagating pulse. The thin layer / peg-leg phenomenon has varying filtering effects on the propagating pulse. In particular, layers which are less than the "tuning thickness" of the propagating pulse have a low-pass effect. The quality factor, Q, was measured in two case studies. In the first, the mean Q was determined from wavelet and spectrum modelling and found to be 60 for fine sands and 47 for coarse sands in the 1 kHz to 3 kHz frequency band. In the second, Q was determined as 59 for poorly sorted sandy diamicts in the 100 Hz to 240 Hz frequency band. The close fit between synthesised spectra and wavelets and observed data showed that a constant- Q mechanism would account for the spectral changes between the seabed and the deeper target reflection events in the two case studies. The spectra of the target reflection events in both case studies were lacking in low frequencies which is likely to be due to low-pass filtering from composite reflection events due to thin bed layering. For practical purposes, the determination of Q from a mean normalised seismic trace yielded the same result as measuring a mean Q from individual traces. In a third case study, the seabed multiple was compared to the seabed reflection using wavelet and spectrum modelling. A lack of low frequencies in the seabed multiple showed that the seabed can act as a low-pass filter to an incident pulse. As the numerical methods rely on the seabed as having a white reflection and transmission response, the low-pass effect will result in erroneous estimates of the quality factor, Q.

The intrinsic high accuracy of modern HF direction finders cannot be realized in practice due to the perturbations and tilts which exist in the ionosphere. Particular attention is given in this dissertation to the effect of Travelling Ionospheric Disturbances (TIDs) since their occurrence is difficult to predict and the magnitude of the induced bearing error extremely variable. TID activity is quantified by monitoring simultaneously the bearings of a number of transmitters whose locations and frequencies are well known. This information is then employed to determine the expected bearing error on a target transmission reflected in the same area of the ionosphere. The major limitations of this correction scheme are investigated. The large-scale temporal and spatial variations of the quiet ionosphere (Systematic Ionospheric Tilts, SITs) produce bearing errors of similar magnitude to those due to TIDs. Rapid calculation of these errors for various path geometries and ionospheric conditions is possible by raytracing through 3-D ionospheric models. The limitations of a correction scheme which employs a 3-D model based on ionospheric predictions are examined. Variance is a particularly useful statistic in assessing the 'reliability' of bearings made on a particular HF circuit. A scheme is proposed in which the bearings of a network of reference transmitters are monitored continuously allowing a variance to be assigned to any target transmission, even if it is only present for a very short time. These variance values are of considerable value in practical DF applications. Whenever possible the experimental observations are related to the underlying physical processes in the ionosphere which generate these DF errors.

As part of the 1989 BABEL project, Durham University recorded large quantities of high resolution wide-angle data from six deep seismic lines shot in the Gulf of Bothnia. Fifteen analogue and three digital seismic recording stations were used, located in Sweden around the Sea of Bothnia. The wide-angle data is of very high quality due to the low noise and good transmission of seismic energy through the cratonic crust of the Baltic Shield. BABEL line 6 is a 240 km long, north-south profile in the western half of the Sea of Bothnia that runs almost parallel to several deep seismic refraction profiles previously acquired in Sweden and Finland. A crustal model has been developed for this line from P- and S-wave wide-angle data recorded at five of the online recording stations and from the normal-incidence data. This model has been generated by raytracing methods using a modified version of the BEAMS? gaussian beam package. Wide-angle arrivals are interpreted as diving rays and wide-angle reflections and as 'diffraction-type' arrivals from offsets on reflecting boundaries. The wide-angle reflections appear to correspond to the boundaries between high and low reflectivity zones in the normal-incidence data. The 'diffraction-type' arrivals appear to be related to surface features such as the Ljusnan tectonic zone and the Aranda rift. There is no evidence for a Moho trench similar to those seen on neighbouring profiles. Several high velocity layers are required in the model. These may be the related to the numerous post-orogenic intrusions that have occurred in the Svecofennian region of the Baltic Shield. Traditional approaches to modelling wide-angle data assume that the crust consists of a series of extensive, quasi-horizontal, layers. There are several problems with this approach. In particular it does not explain the patchy reflectivity seen in the normal-incidence data or the rapid variation in amplitude seen along wide-angle arrivals in high resolution data sets. Apparently continuous wide-angle arrivals may be generated from a series of short length reflectors. These show an amplitude variation similar to that seen in the experimental data. This suggests that wide-angle arrivals may arise from the assemblages of reflectors that are often imaged on normal-incidence data.

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Local, regional and teleseismic earthquakes recorded at seismic stations in northern Kenya have been analysed to determine the seismicity in this region and the crustal and upper mantle structure beneath an array on the north-eastern flank of the Kenya dome. A total of 389 microearthquakes, occurring during the period 17th Jan. to 26th Aug., 1981, have been located using a simple half space velocity model. The seismic activity is mainly confined to the Kenya rift and a zone approximately 150km to the east of the main rift. The northward extent of the activity suggests that the active section of the rift continues beneath Lake Turkana. Apparent velocities of local and regional earthquakes recorded at the Ngurunit array have been interpreted in terms of crustal velocity structure. A two layered crustal velocity model has been derived. The velocity of the upper crustal layer is considered to increase linearly with depth, according to the function V = 5.8 + 0.0245z. The velocity of the lower crustal layer is 6.5 0.2 km/s and the intermediate crustal boundary is placed at 24km. The variation of apparent velocity with azimuth of Moho arrivals suggests an eastwardly dipping Moho of 7 beneath the array. The depth of the Moho directly beneath the array is derived to be 46km and the velocity of sub-Moho material is 8.3 0.2 km/s. Delay times and slowness measurements of teleseismic P wave arrivals indicate the presence of anomalously low P wave velocity material in the upper mantle. Three-dimensional ray tracing modeling suggests a thickening of the anomalous body to the north and east of the array, such that the top surface of the body reaches to within 60-80km of the ground surface. The thickening may be associated with rift structures beneath Lake Turkana and Quaternary volcanic activity observed on the eastern flank of the Kenya dome.

A seismic refraction profile was undertaken to investigate the source of an aeromagnetic anomaly located above the Widmerpool Gulf, East Midlands. Ten shots were fired into 51 stations at c. 1.5km spacing in a 70km profile during 41 days recording. The refraction data were processed using standard techniques to improve the data quality. A new filtering technique, known as Correlated Adaptive Noise Cancellation was tested on synthetic data and successfully applied to controlled source and quarry blast data. Study of strong motion data reveals that the previous method of site calibration is invalid. A new calibration technique, known as the Scaled Amplitude method is presented to provide safer charge size estimation. Raytrace modelling of the refraction data and two dimensional gravity interpretation confirms the presence of the Widmerpool Gulf but no support is found for the postulated intrusion. Two dimensional magnetic interpretation revealed that the aeromagnetic anomaly could be modelled with a Carboniferous igneous source. A Lower Palaeozoic refractor with a velocity of 6.0 km/s is identified at a maximum depth of c. 2.85km beneath the Widmerpool Gulf. Carboniferous and post-Carboniferous sediments within the gulf have velocities between 2.6-5.5 km/s with a strong vertical gradient. At the gulf margins, a refractor with a constant velocity of 5.2 km/s is identified as Dinantian limestone. A low velocity layer of proposed unaltered Lower Palaeozoics is identified beneath the limestone at the eastern edge of the Derbyshire Dome. The existence and areal extent of this layer are also determined from seismic reflection data. Image analysis of potential field data, presents a model identifying 3 structural provinces, the Midlands Microcraton, the Welsh and English Caledonides and a central region of complex linears. This model is used to explain the distribution of basement rocks determined from seismic and gravity profiles.

The Derbyshire Dome is thought to have been a stable uplifted area since at least Lower Carboniferous times. This project is principally concerned with four 30km seismic refraction lines which crossed the limestone outcrop of Derbyshire and N. Staffordshire in order to investigate the Dome's upper crustal structure, using quarry blasts as seismic sources. A time-term analysis of refracted arrival data defined basement structure more complicated than implied by the surface geology. The interpretation of these data was complicated by high (5.6-5.8km/s) velocity refractions from dolomitic horizons within the limestone sequence; the mean overburden velocity was determined to be about 5.2 km/s. The Dome could be divided into two pre-Carboniferous geological units separated approximately by the line of the NNW trending Bonsall Fault. To the north a broadly domal refractor of velocity 5.5-5.55km/s was mapped, and thought to correlate with both the shallow pre-Carboniferous volcanics encountered by the Woo Dale borehole and"the Ordovician shales encountered by the Eyam borehole below 1.8km of limestone. This refractor accordingly deepens beneath the Carboniferous sedimentary basins flanking the Dome. To the south of the Bonsall Fault zone, the Carboniferous was found to be underlain by a refractor of velocity 5.63-5.7km/s, thought to be of Precambrian material similar to the rocks of Charnwood Forest, Leicestershire, some 40km south. By analysing later arrivals, this refractor has been mapped to the north of the Bonsall Fault at a depth of 2.5-3.5km. The shallower Lower Palaeozoic refractor is thought to be no more than 500m thick, and underlain by lower velocity, possibly Cambrian, material. This interpretation is consistent with the Bouguer anomaly map of the region, and sheds light on the structural control of Carboniferous sedimentation. The basement fault dividing the two pre-Carboniferous units is thought to have been active during the Dinantian as the northern unit tilted eastwards.