Estimation of the Nuclear Magnetic Gyroscope Sensitivity to Inertial Influences

The factors influencing the stability of the zero offset of a nuclear magnetic gyroscope (NMG) are considered. As a result, an equation for the connection of these parameters with changes arising in the course of inertial influences on the NMG body has been created. In addition, the displacements caused by vibration effects on the NMG under operating conditions are analyzed. Based on these data, the changes in NMG accuracy are calculated and conclusions are drawn about the effectiveness of the NMG design.

First Look-Ahead VSP Guided Salt Dome Island Exploration Well Drilling in the UAE

An exploration well offshore UAE, which was the first of it's kind, was planned to be drilled from an island and within salt dome. Well planning was based on a structural model that was estimated using coarse 2D surface seismic (with no line crossing planned well location) and gravity measurements. This model, therefore, had a large uncertainty as to the salt location and geometry. Concerns of potential drilling hazards associated with salt required utilizing the ability of borehole seismic to look-ahead of bit to image salt and direct the well such that it would be sufficiently far away from salt face. Pre-job survey planning was first made assuming salt face to the northwest (based on gravity data) of wellhead and that the well would remain outside the salt. To ensure the well remains close, but not too close, Vertical Seismic Profile (VSP) was planned to include Salt Proximity Survey. Just prior to spudding, a surface core indicated salt was, in fact, southeast of wellhead, thus changing the objectives of VSP from locating how far away the well was from salt, to how soon will it exit salt. After survey modeling for four possible scenarios, Look-ahead Zero-Offset and Offset VSPs were acquired using vibroseis at the island, at each of four casing points and rapidly processed to guide drilling next sections. In the 26" section, the well started drilling in salt and there was concern that there would be problems with casing design if the well did not exit salt before 4000 ft. A Zero-Offset and Offset VSP were shot for reflection imaging off the salt face. The survey indicated the salt face was approaching the well but at low rate (due to dip) to ensure an exit before 4000 ft. The well was deviated southeast and it exited the salt at 3620 ft. In the 17.5" section, a second run of Zero-Offset and Offset VSP were acquired indicating the salt face was still moving away from the well toward the northwest. In the 12.25" section, a third set of Zero-Offset and Offset VSP was shot. This survey confirmed the salt face was moving continually northwest and it was suggested the well deviate northwest to remain closer to salt. A large reverse fault was also clearly imaged and confirmed by drilling. In the 8.5" section, the well was drilled northwest at high angle as could be tolerated until it was TDed below target formation "A". The final set of Zero-Offset and Offset VSP results showed the salt was, at the level of formation "A", farther northwest than could be imaged by these VSP. There has been little to no experience of drilling salt dome islands in Abu Dhabi. This paper demonstrated how look-ahead VSP guided exploration well drilling in the salt dome island. Out-of-the-box survey design and rapid turnaround processing successfully aided in imaging location of the salt face and allowed casing points to be made without having to plug back and sidetrack. Once out of the salt, VSP allowed the well to be drilled closer to salt without re-entering it.

Using Zero-Offset VSP to Identify Interbed Multiples Generators, A Case Study from South Oman

The presence of interbed multiples is a serious concern in surface seismic processing and interpretation. Its impact is huge especially if they are masking the desirable primary reflections such as the targeted reservoirs area. The conventional demultiple methodologies such as stacking, and deconvolution often fail to suppress all the interbed multiples. Therefore, a need for other measurement is crucial to eliminate the remaining ones (Burton and Lines, 1997). There are several approaches, data-driven or model-driven, currently available to predict the interbed multiples. However, they require an accurate identification of the multiple generators (Lesnikov and Owus, 2011). The identification of the origin of these multiples seems to be the most effective solutions to remove them, however it is not an easy task. The allure of Zero Offset Vertical Seismic Profiles (ZOVSPs) in having the receivers placed close to the subsurface horizons, allow both upgoing and downgoing wavefields to be recordable and separable. It's the combination of short window and long window deconvolution operators which are derived based on our knowledge of downgoing wavefield which help us to determine the multiples generators at their exact depths in the subsurface. This paper demonstrates how Zero offset VSP successfully helped to identify the major multiples generators in one of the exploratory fields in south Oman. These generators then used as an input to demultiple technique named as Extended Interbed Multiple Prediction (XIMP) that eliminates the multiples within surface seismic. As the result of the multiple elimination, the seismic to well tie tremendously improved and the reliability of the overall horizon interpretation is enhanced.

Ultra-low frequency AVA inversion of plane-wave primary reflection seismograms

An amplitude versus angle (AVA) inversion method is presented for estimating density and velocities of a stratified elastic medium from reflection seismograms in the intercept time-horizontal slowness domain. The elastic medium parameters are assumed to vary continuously with depth. The seismograms are Green’s function pre-critical incidence primary P-wave reflections of time length T assumed to obey differential equations of a model for elastic primary P-wave back-scattering, similar to seismograms representing the first term in the well-known Bremmer series/WKBJ iterative solution model. A relation is found between plane-wave Green’s function seismograms at each horizontal slowness and the medium properties in time. The Green’s function seismograms after NMO-correction are directly inverted for the medium parameters as function of zero-offset traveltime. It is documented theoretically and verified numerically that the signal at the fundamental frequency f=1/ T must be present in the seismograms for the AVA method to provide the parameter trends of the elastic medium, implying that ultra-low frequencies <1 Hz for T >1 s must be generated and recorded. Noise in the seismograms at ultra-low frequencies is not considered since the theoretical AVA model does not handle microseisms that would be measured in real data. The main mathematical findings are illustrated by using simple model seismograms.

Compensation Gyrocompass Based on MEMS

The study of an astatic compensating gyrocompass, built on the basis of a modulation micromechanical gyroscope (MMG) of a hybrid type, has been carried out. A kinematic diagram is given and the principle of operation of the device has describing. The device uses the modulation principle based on obtaining information about the angular motion of the rotor and creating control torques in a rotating coordinate system, which makes it possible to exclude such a significant disadvantage of MMG as "zero offset". A feature of the gyrocompass under consideration is the use of two channels for controlling the rotor of the MMG, namely: a channel for the formation of a guiding moment, striving to combine its main axis with the direction of the true meridian and a channel for compensating this guiding moment. A linearized mathematical model has building, on the base of which an effective algorithm for the operation of a compensatory astatic gyrocompass is proposed. The device under consideration can be used to determine the true azimuth of the longitudinal axis of a mobile ground object, it has a higher measurement speed compared to devices built on three-degree "heavy" gyroscopes, and has good resistance to external influences (vibrations, shocks, etc.).

Assessment of Skin Deep Layer Biochemical Profile Using Spatially Offset Raman Spectroscopy

Skin cancer is currently the most common type of cancer with millions of cases diagnosed worldwide yearly. The current gold standard for clinical diagnosis of skin cancer is an invasive and relatively time-consuming procedure, consisting of visual examination followed by biopsy collection and histopathological analysis. Raman spectroscopy has been shown to efficiently aid the non-invasive diagnosis of skin cancer when probing the surface of the skin. In this study, we employ a recent development of Raman spectroscopy (Spatially Offset Raman Spectroscopy, SORS) which is able to look deeper in tissue and create a deep layer biochemical profile of the skin in areas where cancer lesions subtly evolve. After optimizing the measurement parameters on skin tissue phantoms, we then adopted SORS on human skin tissue from different anatomical areas to investigate the contribution of the different skin layers to the recorded Raman signal. Our results show that using a diffuse beam with zero offset to probe a sampling volume where the lesion is typically included (surface to epidermis-dermis junction), provides the optimum signal-to-noise ratio (SNR) and may be employed in future skin cancer screening applications.

Monitoring Reservoirs from Subsea Wells: Ultra-Long Step out VSP Acquisition Using Enhanced Back-Scattering Fiber

While DAS VSP has become relatively standard in dry-tree applications, acquiring data in subsea wells has remained a technical challenge as umbilical can be tens of kilometers long, thereby reducing the overall quantity of backscattered light to the topside interrogator. This adds to the attenuation due to connectors at the wellhead and along the optical path. Yet, the need for subsea DAS interrogation is high, particularly with the onset of complex, deep-water projects that will require on-demand monitoring capabilities. In this article, we report on the successful acquisition and subsequent processing of a zero-offset VSP in an ultra-long step-out context. We simulated a subsea well with 69km worth of lead-in fiber to the wellhead, including attenuation at the wellhead mimicking the connectors. The attenuation was tackled by using an active, subsea amplifier (that would normally sit at the wellhead), and an in-house developed engineered fiber that provides a significant uplift in backscattered energy. We acquired this ZVSP both on fiber and with a standard wireline tool string for comparison. The approach presented here combines hardware and processing strategies to tackle the long step-out challenge. We demonstrate the ability to record seismic data even at very large step-out, a requirement for subsea well monitoring.