Master of Science / Department of Geology / Abdelmoneam Raef / Recently, South Rub’ al-Khali Company Limited (SRAK) acquired a preliminary survey in the Saudi’s oil producing area to develop a feasible new hydrocarbon indication and monitoring (I & M) device using low frequency seismic signals. Based on broadband seismometer data, the new Hydrocarbon I & M might predict the possibility of a hydrocarbon basin underneath by way of evaluating the received spectra for an additional energy shell between 2.0-6.0 Hz. Such a study is also referred to as hydrocarbon microtremor analysis and recently some contracting geophysical service companies offer such studies. This report will concentrate on the hydrocarbon microtremor analysis of synchronized signal of one frequency and an extra re-determination possibly at a separate location. The paper reports on several critical likely misconceptions and examines repeatability of hydrocarbon microtremors. This work indicates that signal generated by manmade operations can yield same tremor as that assumed for hydrocarbon reservoirs. Equally important, the presence of surface waves generated by anthropogenic signal indicates frequency limits ranging from 1 to 10 Hertz as a result of isolated surface waves. The difficulty of isolating any presumed hydrocarbon related tremors from ambient noise hamper efforts of understanding and applying microseism signals to hydrocarbon exploration and monitoring. Repeatability study by Peter, H. & Sascha, B. (2008) raised questions regarding the source of hydrocarbon microtremors. For improved chances of isolating the implied hydrocarbon microtremors from manmade tremors and near-surface impacts, the data require precise recording based on three metrics, frequencies above 3 Hz should be conserved, highly sensitive seismometers should be engaged, and the data registering time should be enough to register ‘tremor-free’ readings.

A great number of the urban areas in Ethiopia are situated within the Great Rift Valley of Ethiopia, a system consisting of depressions and large faults. As a region with significant seismic activities, it is vital that careful planning is implemented to avoid constructing buildings on flat surfaces as they can amplify ground motion in the case of an earthquake.   This study was conducted in Adama, a city located within the rift system, to map and characterize the subsurface of a construction site with seismic refraction and investigate whether this is an optimal area to construct a building should an earthquake occur. Seismic refraction is based on Snell’s law, specifically the case of the critical angle, which is when the refracted angle is at 90 degrees and a number of the energy from the wave is rebounded back to the surface in accordance with Huygen’s Principle.       Seismic waves were generated with a sledgehammer and recorded with 24 vertical geophones. The acquired data was then analysed with SeisImager and produced a 2D-tomography of the site with the corresponding velocity layers for a P-wave. Comparing the P-wave velocities to a table of Seismic Velocities of Rocks and Various Materials, one could determine that the subsurface layers consisted of rock soils, sand and silt. These are incredibly loose materials that will amplify ground motion during earthquake crisis and are therefore not optimal or ideal for constructing buildings.

Seismic reflection surveying is used to generate acoustic images of the water column. This technique employs conventional multi-channel equipment which is used to image the solid Earth. In the water column, acoustic impedance contrasts are produced by variations in temperature and, to some extent, salinity. Acoustic impulses generated by an array of airguns suspended behind a vessel are reflected from these contrasts and recorded on long cables of hydrophones that are towed below the sea-surface. In this way, two- and three-dimensional images of thermohaline circulation can be generated. Critically, these images have equal vertical and horizontal resolutions of \textit{O}(10)~m. Here, I describe, process, and analyse a calibrated two-dimensional seismic survey from the Bellingshausen Sea of the Southern Ocean and a three-dimensional seismic survey from the Brazil-Falkland Confluence located offshore Uruguay. First, the Bellingshausen survey was designed to image the thermohaline structure across the west Antarctic shelf where warm-core eddies are reported. Processed and calibrated seismic images reveal the detailed thermohaline structure of Circumpolar Deep Water. Many warm-core eddies are observed, which have diameters of 1--12~km and thicknesses of 100--200~m. Pre-stack analysis demonstrates that this eddy field is being advected onto the shelf at speeds of \textit{O}(0.1)~m~s$^{-1}$. An iterative inverse modelling procedure is used to convert reflectivity into temperature and salinity, which confirms that the eddies have anomalously warm centres (i.e. $\sim$1$^{\circ}$C). These results have significant implications for ice shelf melting. Secondly, the Uruguay survey is used to investigate a large-scale frontal system. Although this system has been studied using hydrographic methods, these studies either have limited spatial resolution or have restricted depth penetration. The three-dimensional seismic survey, which was acquired in a `racetrack' pattern, permits the volume to be interrogated. Since the frontal system migrates southwestwards at a speed of \textit{O}(10)~km~day$^{-1}$, this survey is time-lapse in nature. Processed images reveal a band of dipping reflections that extend to depths of $\sim$2000~m. These reflections represent the frontal interface between the Brazil and Falkland currents. Physical oceanographic properties are calculated for images that cross this front. On the warm side of the front, the water mass is characterised by flat and continuous reflectivity. On the cold side of the front, the water mass is characterised by deformed reflectivity on all scales. Pre-stack analysis suggests that near-surface flow at the frontal interface is convergent. Between 0.5 and 1~km depth, a substantial eddy that is 30~km long and 250~m thick is visible on the cold side of the front. Detailed mapping suggests that this eddy grew and decayed over a period of 6~days. Its observed scale and duration are inconsistent with analytical and numerical studies of intra-thermocline eddies. Nevertheless, its duration is consistent with scaling arguments of frictional spin-down. Spatial and temporal distributions of mixing rates (i.e. diapycnal diffusivities) are estimated by spectrally analysing vertical displacements of automatically tracked reflections. Both internal wave and turbulent regimes are identifiable. Recovered diapycnal diffusivities are of \textit{O}($10^{-6}$--$10^{-2.2}$)~m$^{2}$~s$^{-1}$, consistent with hydrographically determined estimates. Mixing is suppressed and enhanced on the warm and cold sides of the front, respectively. Seismic Oceanography has considerable potential to quantify aspects of thermohaline circulation on multiple scales.

No description available.

The refraction convolution section (RCS) is a simple and efficient method for full trace processing of shallow seismic refraction data. It facilitates improved interpretation of shallow seismic refraction data through the convenient use of amplitudes as well as traveltimes. The RCS is generated by the convolution of forward and reverse shot records. The convolution operation effectively adds the first arrival traveltimes of each pair of forward and reverse traces and produces a measure of the depth to the refracting interface in units of time which is equivalent to the time-depth function of the generalized reciprocal method (GRM). The convolution operation also multiplies the amplitudes of first arrival signals. This operation compensates for the large effects of geometric spreading to a very good first approximation, with the result that the convolved amplitude is essentially proportional to the square of the head coefficient. The head coefficient is approximately proportional to the ratio of the specific acoustic impedances in the upper layer and in the refractor, where there is a reasonable contrast between the specific acoustic impedances in the layers. The RCS can also include a separation between each pair of forward and reverse traces in order to accommodate the offset distance in a manner similar to the XY spacing of the GRM. Lateral variations in the near-surface soil layers can effect amplitudes thereby causing 'amplitude statics'. Increases in the thickness of the surface soil layer correlate with increases in refraction amplitudes. These increases are adequately described and corrected with the transmission coefficients of the Zoeppritz equations. The minimum amplitudes, rather than an average, should be used where it is not possible to map the near surface layers. The use of amplitudes with 3D data effectively improves the spatial resolution by almost an order of magnitude. Amplitudes provide a measure of refractor wavespeeds at each detector, whereas the analysis of traveltimes provides a measure over several detectors, commonly a minimum of six. The ratio of amplitudes obtained with different shot azimuths provides a detailed qualitative measure of azimuthal anisotropy. Dip filtering of the RCS removes 'cross-convolution' artifacts and provides a convenient approach to the study of later events. The RCS facilitates the stacking of refraction data in a manner similar to the CMP methods of reflection seismology. It can improve signal-to-noise ratios.

Deep seismic exploration in the Gippsland Basin is hindered by strong, unidentified noise below the Latrobe Group coal sequence. Low velocity events that appeared in field data semblance analyses suggested that an elastic wave modelling study was required to understand the subcoal reflection response. The modified reflectivity method (Kennet, 1980) provided a means for constructing detailed and accurate synthetic seismograms from realistic depth models, under the assumption of an isotropic, plane-layered earth. A study into the effect of each part of an elastic depth model (upon an elastic depth model (upon an elastic synthetic seismogram) resulted in a set of guidelines for obtaining a field data comparison. Excellent ties were obtained at several wells, often using little more than a partial sonic log. The noise contributions to the synthesis were interpreted using additional synthetics computed from variations upon the depth model and by exercising control over the wave types modelled. Subsequent processing of the synthetics revealed three types of persistent noise in progressively deeper parts of the subcoal image: 1) mode converted interbed multiples (generated within the coal sequence); 2) S-wave reflections and long period multiples (generated between the coal sequence and the Miocene carbonates and 3) surface related multiples.

Deep seismic exploration in the Gippsland Basin is hindered by strong, unidentified noise below the Latrobe Group coal sequence. Low velocity events that appeared in field data semblance analyses suggested that an elastic wave modelling study was required to understand the subcoal reflection response. The modified reflectivity method (Kennet, 1980) provided a means for constructing detailed and accurate synthetic seismograms from realistic depth models, under the assumption of an isotropic, plane-layered earth. A study into the effect of each part of an elastic depth model (upon an elastic depth model (upon an elastic synthetic seismogram) resulted in a set of guidelines for obtaining a field data comparison. Excellent ties were obtained at several wells, often using little more than a partial sonic log. The noise contributions to the synthesis were interpreted using additional synthetics computed from variations upon the depth model and by exercising control over the wave types modelled. Subsequent processing of the synthetics revealed three types of persistent noise in progressively deeper parts of the subcoal image: 1) mode converted interbed multiples (generated within the coal sequence); 2) S-wave reflections and long period multiples (generated between the coal sequence and the Miocene carbonates and 3) surface related multiples.

Graduation date: 1994

Graduation date: 1997

Graduation date: 1987 / Best scan available for figures.