Estimating SHmax azimuth with P sources and vertical geophones: Use P-P reflection amplitudes or use SV-P reflection times?

We compared two methods for extracting the azimuth of maximum horizontal stress (SHmax) from 3D land-based seismic data generated by a P source and recorded with vertical geophones. In the first method, we used the direct-SV mode that is produced by all land-based P sources. P sources generate SV illumination that radiates in all azimuth directions from a source station and creates SV-P reflections that are recorded by vertical geophones. Unless stratigraphy has steep dip, SV-P raypaths recorded by vertical geophones are the reverse of P-SV raypaths recorded by horizontal geophones. Thus, SV-P data provide the same S-wave sensitivity to stress fields as popular P-SV data do. In the second method, we retrieved P-P reflections and then performed an amplitude-versus-incident-angle (AVA) analysis of the amplitude-gradient behavior of P-P reflection wavelets. We did this analysis in narrow azimuth corridors to determine the gradient of reflection-wavelet amplitudes as a function of azimuth. This P-P AVA amplitude-gradient method has been of great interest in the reflection seismology community since it was introduced in the late 1990s. Each of these methods, AVA analysis of the gradient of P-P reflection amplitudes and azimuth-dependent arrival times of SV-P reflections can be used to determine the azimuth of SHmax stress. We compare the results of the two methods with ground truth measurements of SHmax azimuth at a CO2 sequestration site in the Michigan Basin. SHmax azimuths were determined from P-P and SV-P data at three major boundaries at depths of approximately 3500 ft (1067 m), 5500 ft (1676 m), and 7500 ft (2286 m). Two estimates of SHmax azimuth (one using SV-P data and one using P-P data) were made at each stacking bin inside a 24 mi2 (62 km2) image space. The result was approximately 98,000 estimates of SHmax azimuth across each of these three boundaries for each of these two prediction strategies. Histogram displays of PP AVA gradient estimates had peaks at correct azimuths of SHmax at all three depths, but the spread of the distributions widened with depth and split into two peaks at the deepest boundary. In contrast, each histogram of SHmax azimuth predicted by azimuth-dependent SV-P traveltimes had a single, definitive peak that was positioned at the correct SHmax azimuth at all three boundary depths.

Integration of Geophysical Technologies in the Petroleum Industry

The most utilized technique for exploring the Earth's subsurface for petroleum is reflection seismology. However, a sole focus on reflection seismology often misses opportunities to integrate other geophysical techniques such as gravity, magnetic, resistivity, and other seismicity techniques, which have tended to be used in isolation and by specialist teams. There is now growing appreciation that these technologies used in combination with reflection seismology can produce more accurate images of the subsurface. This book describes how these different field techniques can be used individually and in combination with each other and with seismic reflection data. World leading experts present chapters covering different techniques and describe when, where, and how to apply them to improve petroleum exploration and production. It also explores the use of such techniques in monitoring CO2 storage reservoirs. Including case studies throughout, it will be an invaluable resource for petroleum industry professionals, advanced students, and researchers.

Submarine geomorphology

Submarine geomorphology underwent significant development in the second half of the 20th century, largely thanks to advances in technology by the military, navigation and hydrocarbon industry, which were later transferred to the academic and commercial sectors. In this chapter we summarise the development of the key methods used in submarine geomorphology between 1950 and 2000, which include sidescan sonar, multibeam echosounder, reflection seismology, seafloor sampling and marine robotic systems. We then highlight the progress in our understanding of seafloor processes and landforms made using these methods, focusing on continental shelf landforms, slope instability, submarine canyons, submarine fans and channels, and current-controlled landforms.

Detection of S-wave reflectors beneath aftershock area of the 2016 Kaikoura earthquake

<p>S wave reflectors in the crust may be caused by strong heterogeneous structures such as ones containing  fluid. Especially, fluid around an earthquake fault could play an important role for initiation of the earthquake rupture as a mechanism for reducing fault strength. The location and geometry of the reflector can be determined from the travel time of the reflected phases. For detecting the reflections, we need to observe at stations located close to the hypocentre because of sufficient phase separation of the small lapse time of the reflected phases due to a reflector in the crust from direct S wave. In this study, we attempted to detect reflected waves in observed seismograms at the seismic stations in and around the 2016 Kaikoura earthquake (Mw7.6). Seismic records were obtained from the permanent GeoNet stations as well as from seismic stations deployed before the Kaikoura earthquake in the northern South Island. We applied reflection seismology techniques to the data obtained by the network. We used seismograms with smaller epicentral distance than 30 km and obtained dip move-out sections for each station. We detected several reflectors in the mid and lower crust from the sections. Strong reflected phases were observed at the southern edge of the focal area (from a reflector with depth about 20 km). Weak reflectors were detected in/beneath the aftershock area (in the mid- to lower-crust). In addition, the subducting slab might be imaged with dipping angle 20 degree.  Reflectors parallel to the slab were also found below the interface.</p>

Evaluation of the 2D reflection seismic method toward the exploration of thrust-controlled mineral deposits in southwestern Fujian Province, China

The southwestern region of the Fujian Province is one of the major ore districts in China. The current model states that mineral deposition is highly controlled by thrust structure, which means that there may be concealed deposits located deep within overlapping thrust areas. Reflection seismology, which has great depth penetration and higher resolution than other geophysical methods, has great potential to delineate complex structures and give some clues to mineralization. In 2015, an experimental 2D reflection seismic survey called “Fujian 2D” was conducted in this region. Data were acquired along a 13.8 km length, with a source interval of 60 m, and 691 identical receivers with an equal spacing of 20 m were used to record data for each source. Due to topographical restrictions caused by the source environment, the mass or position of some shots was changed. Despite the restrictions, the average fold number reached 64 for a 10 km distance along the middle of the survey line. During the data processing procedure, conventional technologies involving static correction, noise elimination, deconvolution, and iterative velocity analyses were applied. After the prestack time migration failed to obtain a high-quality imaging result, rugged prestack depth migration (PSDM) was introduced that resulted in a better quality image of the subsurface structure and which included near-surface parts of the thrusts. In addition, P- and S-wave velocities and density data were determined from two borehole cores. Forward modeling and imaging found that the Permian marble hosting the mineral deposits has lower velocity than the surrounding rocks, where contacts give rise to strong reflections. The final rugged PSDM also clearly delineated the thrust bodies and magma intrusion zones. Combining this forward modeling with the known geology of the investigated site, the Fujian 2D reflection seismic experiment demonstrates great potential for unveiling the main elements controlling mineral deposition, such as tectonic structure, stratigraphic contacts, and lithology. Our experimental results demonstrate that reflection seismology has a wide range of applications for future mineral exploration at greater depths.