Ultrasonic and seismic constraints on crystallographic preferred orientations of the Priestley Glacier shear margin, Antarctica

Crystallographic preferred orientations (CPOs) are particularly important in controlling the mechanical properties of glacial shear margins. Logistical and safety considerations often make direct sampling of shear margins difficult and geophysical measurements are commonly used to constrain the CPOs. We present here the first direct comparison of seismic and ultrasonic data with measured CPOs in a polar shear margin. The measured CPO from ice samples from a 58 m deep borehole in the left lateral shear margin of the Priestley Glacier, Antarctica, is dominated by horizontal c-axes aligned sub-perpendicular to flow. A vertical seismic profile experiment with hammer shots up to 50 m away from the borehole, in four different azimuthal directions, shows velocity anisotropy of both P-waves and S-waves. Matching P-wave data to the anisotropy corresponding to CPO models defined by horizontally aligned c-axes gives two possible solutions for c-axis azimuth, one of which matches the c-axis measurements. If both P-wave and S-wave data are used, there is one best fit for azimuth and intensity of c-axis alignment that matches well the measurements. Azimuthal P-wave and S-wave ultrasonic data recorded in the laboratory on the ice core show clear anisotropy that matches that predicted from the CPO of the samples. With good quality data, azimuthal increments of 30° or less will constrain well the orientation and intensity of c-axis alignment. Our experiments provide a good framework for planning seismic surveys aimed at constraining the anisotropy of shear margins.

PROCESSING OF SEISMIC DIFFRACTIONS FROM CARBONATE NODULES IN CLAY FORMATIONS

The problem of localizing small (relative to wavelength) scatterers by diffractions to enhance their use in identifying small-scale details in a seismic image is extremely important in shallow exploration, to identify interesting features such as fractures, caves and faults. The conventional approach based on seismic reflection is limited in resolution by the Rayleigh criterion. In certain acquisition geometries, such as crosswell surveys aimed at obtaining high resolution signals, the availability of suitable datasets for effective migration depends on the spatial extent of the available source and receiver data intervals. With the aim of overcoming the resolution limits of seismic reflection, we studied the detectability, response, and location of meter- and possibly sub-meter-dimension carbonate concretions (septaria) in the Boom Clay Formation (potential host rocks for radioactive waste disposal) by diffraction analysis of high-frequency signals. We investigated diffraction wavefields by signal separation, focusing, and high-resolution coherency analysis using the MUltiple Signal Classification (MUSIC) method and semblance. The investigation was performed for two different surveys in Belgium, a shallow and high resolution Reverse Vertical Seismic Profile (RVSP) and a near-offset crosswell application at Kruibeke and ON-MOL-2 sites, respectively. The data analysis is supported by synthetic wavefield modeling. The multi-offset RVSP provides the appropriate geometry to observe and investigate the septaria diffractions both from depth and the surface. The crosswell approach, calibrated using synthetic data in the analysis of wavefield patterns in 2D, shows promising imaging results with field data of a selected diffraction zone in the interwell area.

Q estimation based on the logarithmic spectral area double difference

The Q factor is an essential parameter describing the characteristics of medium absorption within a material during wave propagation. When a seismic wave propagates within the attenuating media, its amplitude decreases and frequency band narrows, resulting in a variation in its logarithmic spectral area. Based on these effects, we calculate the logarithmic spectral area difference (LSAD) before and after attenuation and set a division point to divide the LSAD into two parts. We then compute the difference between the two LSADs to derive a new Q-estimation formula based on computation of the logarithmic spectral area double difference (LSADD). To improve the noise robustness of the Q estimation, we select multiple different division points to calculate the Q factors and consider their average value as our final estimate. We then compare and analyze the noise robustness and bandwidth sensitivity of our technique with other commonly used methods. These results demonstrate that our approach is the most accurate and robust, and least sensitive to the frequency band when processing noisy synthetic seismograms. Finally, we apply our methodology to field vertical seismic profile (VSP) and seismic reflection data, further illustrating the effectiveness of this method to estimate the Q factor.

Seismic-while-drilling by drill-bit source and large-aperture ocean-bottom array

Seismic while drilling (SWD) by drill-bit source has been successfully used in the past decades and is proven using variable configurations in onshore applications. The method creates a reverse vertical seismic profile (RVSP) dataset from surface sensors deployed as arrays in the proximity of the monitored wells. The typical application makes use of rig-pilot reference (pilot) sensors at the top of the drill-string and also downhole. This approach provides while-drilling checkshots as well as multioffset RVSP for 2-D and 3-D imaging around the well and prediction ahead of the bit. For logistical (sensor deployment) and cost (rig time related to technical installation) reasons the conventional drill-bit SWD application is typically much easier onshore than offshore. We present a novel approach that uses a network of passive-monitoring sea bottom nodes pre-deployed for microseismic monitoring to simultaneously and effectively record offshore SWD data. We study the results of a pilot test where we passively monitored the drilling of an appraisal well at the Wisting discovery in the Barents Sea with an ocean-bottom cable deployed temporarily around the drilling rig. The continuous passive recording of vibration signals emitted during the drilling of the well provides the SWD data set, which is treated as a reverse vertical seismic profile. The study is performed without rig-pilot signal. The results are compared with legacy data and demonstrate the effectiveness of the approach and point to future applications for real-time monitoring of the drilling progress, both in terms of geosteering the drill bit and predicting formation properties ahead of the bit by reflection imaging.

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.

Enhancing subsalt imaging through advanced identification and suppression of interbed multiples and mode-converted reflections — Gulf of Mexico and Brazil case studies

In our case studies, the success of subsalt exploration and development wells depended heavily on the characterization of highly heterogeneous lacustrine microbial carbonates. Acoustic and elastic inversions have proved to be a good proxy for identification of reservoir quality variation for exploration and development well placements. However, qualitative and quantitative usage of subsalt seismic amplitudes requires proper illumination and good signal-to-noise ratio. If properly imaged, mode-converted reflections and interbed multiples can be complementary to the P-wave image. But, in conventional P-wave-oriented imaging, both types of events cannot be imaged correctly. They appear as coherent noise and negatively impact the overall exploration and development project outcomes, especially in areas with poor illumination. This paper consists of two parts: first, we investigate the potential problems resulting from converted waves and interbed multiples in data from two different basins — the Gulf of Mexico and the Campos Basin in offshore Brazil — and show our approach to attenuate them to reveal the true structures. The second part focuses on advanced identification of interbed multiples in modeling and migration methods. To facilitate the various strategies to attenuate interbed multiples, “interpretation” of the various events plays a significant role. Vertical seismic profile (VSP) data are excellent for the purpose; however, these data are only available at well locations, if they are recorded. As a result of many years of technology advancement, pseudo VSP data can be constructed effectively from standard streamer survey data. Two methods are highlighted in this paper for building pseudo VSP data in a full two-way sense, based on a typical Brazil-type salt model: Marchenko-based processing and full-wavefield migration. Major subsalt plays in the Gulf of Mexico and emerging plays in Brazil should benefit significantly from elimination of these kinds of coherent noise.

Marine forearc structure of eastern Java and its role in the 1994 Java tsunami earthquake

We resolve a previously unrecognized shallow subducting seamount from a re-processed multichannel seismic profile crossing the 1994 Mw 7.8 Java tsunami earthquake rupture area. Seamount subduction occurs where the overriding plate experiences uplift by lateral shortening and vertical thickening. Pronounced back-thrusting at the landward slope of the forearc high and the formation of splay faults branching off the landward flank of the subducting seamount are observed. The location of the seamount in relation to the 1994 earthquake hypocentre and its co-seismic slip model suggests that the seamount acted as a seismic barrier to the up-dip co-seismic rupture propagation of this moderate-size earthquake.

Waterflood Analysis in Compartmentalised Reservoir Using Multiwell Retrospective Testing and Tracers

The study field consists of four oil pays and is currently going through a waterflood trial. Due to a presence of high amplitude faulting it becomes crucially important to understand the geology of the field and reservoir connectivity prior to progressing the waterflood project. The results of the cross-well tracers have indication (some strong and some vague) of communication between a trial water injector and all oil producers in the same and adjacent compartment. Since the wells were equipped with permanent downhole pressure gauges it was possible to decipher the cross-well communication using the Multiwell Retrospective Testing (MRT) technique based on multiwell deconvolution algorithm (MDCV). The results of MRT study were showing no traceable communication between trial water injector and offset wells in adjacent compartment except one producer which showed a strong response across the fault. By correlating the MRT results with seismic profile and well completion it became possible to establish how exactly the main pay is communicating between the compartments. It also carried few learning points on how to interpret results of cross-well tracers and MRT in terms of reservoir properties.

Multiwell 3D distributed acoustic sensing vertical seismic profile imaging with engineered fibers: CO2CRC Otway Project case study

The 4D surface seismic monitoring is a standard method for reservoir surveillance during the production of hydrocarbons or CO2 injection. However, land 4D seismic acquisition campaigns are often associated with high cost and disruptions to industrial operation or agricultural activities in the area of acquisition. An alternative technique for time-lapse monitoring of the subsurface is the 3D vertical seismic profiling (VSP), which becomes particularly attractive when used with distributed acoustic fiber-optic sensors (DAS) installed in wells. The advantages of 3D DAS VSP include its relatively low cost, minimal footprint on the local area during acquisition, and superior spatial resolution compared to the resolution of geophones. The potential of this technique is explored by processing and analyzing multiwell 3D DAS VSP data acquired at the CO2CRC Otway Project site in Victoria, Australia. The DAS data were recorded using an engineered fiber with enhanced backscattering cemented behind the casing of five wells. The data from each well are processed individually using the same processing flow and then migrated using a 3D migration code tailored to DAS data. Having DAS along the full extent of multiple wells ensures adequate seismic coverage of the area of CO2 injection. The migrated images provide detailed information about the subsurface up to 700 m away from a well and up to 2 km depth. The images are consistent with previously acquired geophone VSP and surface seismic data. The quality of the 3D DAS VSP imaging is comparable or superior to the quality of conventional imaging using geophone data. Therefore, 3D DAS VSP is a demonstrably optimal solution for reservoir monitoring.