Using Active and Passive Near-Field Hydrophones to Image the Near-Surface in Ultra-Shallow Waters Offshore Abu Dhabi

Seismic surveys are generally designed to image deep reservoirs, which leaves the near-surface woefully under-sampled. This is particularly a challenge offshore Abu Dhabi, where a complex near-surface – with karstic collapses and meandering channels – contaminates the seismic image with strong footprints. To mitigate these effects, we use near-field hydrophone data, primarily designed to QC the airgun source, for near-surface imaging. Near-field hydrophones (NFH) are positioned about a meter above each airgun and are designed to record the source near-field pressure. They immediately capture dysfunctional or out-of-spec guns, which alerts the recording crew. Yet, in a shallow water environment, they unintentionally record seismic reflections from the near-surface, which we will use for seismic imaging. Streamer vessels usually use two source arrays, 50 meters apart, which shoot in a flip-flop mode. The active NFH refer to the recordings directly above the shooting guns, while the passive NFH refer to the recordings from the array that is not shooting. Because the passive NFH are less contaminated by the source near-field, they are typically the preferred choice for near-surface imaging. Waters are too shallow in offshore Abu Dhabi to use streamer vessels. Instead, seismic surveys involve ocean-bottom cables (OBC) or nodes (OBN) and smaller airgun arrays. The shooting vessels can be single-source or dual-source. While a single source vessel has only active NFH, a dual source vessel has both active and passive NFH. However, even if a dual-source vessel is used, the 50 m distance between the shooting source array and the passive NFH is too large to capture the water-bottom reflection for water-depths shallower than 25 m. For these reasons, we propose to combine both measurements, using active NFH for the very shallow section and passive NFH for the deeper section. We have applied this technique to a recent node survey acquired offshore Abu Dhabi. By combining the active and passive NFH, a very high-resolution shallow image was obtained, which allows the interpretation of geological layers just below the water bottom. Comparisons with high resolution 2D site survey images show good agreement. Given the NFH do not require any additional acquisition and are delivered as a byproduct of standard seismic surveys, we have demonstrated that proper use of NFH can provide high quality images for pre-site survey interpretation, which reduces the need for additional – and expensive – geotechnical surveys. This is the first published use of combined active and passive NFH in Abu Dhabi shallow waters for the purpose of imaging. The resolution of the shallow formation images allows detailed interpretation not achievable using conventional seismic data. In the long term, this technique may reduce the need for additional site survey acquisitions.

Seasonal change in heat flux at the water-bottom sediment boundary in a small lake

Heat exchange with bottom sediments is the main component of the thermal regime of ice-covered shallow lakes of the temperate zone, which explains the importance of its study and parameterization for inclusion in numerical models. Circulations arising in ice-covered lakes due to heat exchange with bottom sediments, and existing for several months, can make a significant contribution to the transport of dissolved and suspended particles along the water column. The aim of this work was to study the seasonal variability of the heat flux at the waterbottom boundary in a shallow lake during the under-ice period, including the period of spring under-ice convection. Based on the analysis of data from high-frequency (minute) long-term measurements of water temperature in the bottom area of a small lake in the temperate zone, a wide range of variability of the heat flux across the water-bottom boundary during the winter from minute to daily fluctuations was established. The role of the spring under-ice heating in the change in the heat flow at the water-bottom boundary is shown. It is shown that shallow areas of the lake bottom, falling into the zone of influence of spring subglacial convection, can accumulate heat already at the end of the ice period. The comparison of temperature fluctuations in the deep-water part of the lake and tin he area with depths close to the average is carried out. It is shown that the spectrum of temperature fluctuations has similar periods, however, in time, sharp temperature jumps in different areas of the lake do not coincide.

Relationship Between Mercury Concentration in Water, Bottom Sediment And Two Mollusc Species (Crassostrea Gasar and Tympanotonus Fuscatus) From a Lagos Creek in Nigeria

Concentration of mercury was investigated in the flesh and shell of two species of benthic mollusc, Crassostrea gasar and Tympanotonus fuscatus and in water and sediment from Makoko Creek, adjacent to the Lagos Lagoon between January to September 2019. Values obtained for physicochemical parameters in Makoko Creek (water temperature- 28.92±0.1°C; pH- 7.73±0.02; salinity- 14.23±0.05 ppt; dissolved oxygen- 5.34±0.02 mg/l; biological oxygen demand- 7.780±0.1 mg/l and chemical oxygen demand- 12.34±0.02 mg/l) were within the acceptable levels for survival, metabolism and physiology of aquatic organism. The concentration of mercury followed decreasing order as sediment >water >flesh >shell across locations for both species. For all the tested samples of C. gasar and T. fuscatus, biowater accumulation factor in flesh and shell were higher than those of bio-sediment accumulation factor. The coefficients of variance (CV %) in shells were lower than those of the flesh for both investigated mollusc species. It was shown that mercury contents of flesh or shells of C. gasar and T. fuscatu are directly affected by those of water and bottom sediment. J. Bio-Sci. 29(1): 143-151, 2021 (June)

Potential of a Non-linear Full-Waveform Stacking Technique in Airborne LiDAR Bathymetry

Airborne LiDAR bathymetry is an efficient measurement method for area-wide acquisition of water bottom topography in shallow water areas. However, the method has a limited penetration depth into water bodies due to water turbidity. This affects the accuracy and reliability of the determination of water bottom points in waters with high turbidity or larger water depths. Furthermore, the coverage of the water bottom topography is also limited. In this contribution, advanced processing methods are presented with the goal of increasing the evaluable water depth, resulting in an improved coverage of the water bottom by measurement points. The methodology moves away from isolated evaluation of individual signals to a determination of water bottom echoes, taking into account information from closely adjacent measurements, assuming that these have similar or correlated characteristics. The basic idea of the new processing approach is the combination of closely adjacent full-waveform data using full-waveform stacking techniques. In contrast to established waveform stacking techniques, we do not apply averaging, which entails low-pass filtering effects, but a modified majority voting technique. This has the effect of amplification of repeating weak characteristics and an improvement of the signal-noise-ratio. As a consequence, it is possible to detect water bottom points that cannot be detected by standard methods. The results confirm an increased penetration water depth by about 27% with a high reliability of the additionally extracted water bottom points along with a larger coverage of the water bottom topography.

Refined Geometric Modeling of Laser Pulse Propagation in Airborne LiDAR Bathymetry

To achieve a geometrically accurate representation of the water bottom, airborne LiDAR bathymetry (ALB) requires the correction of the raw 3D point coordinates due to refraction at the air–water interface, different signal velocity in air and water, and further propagation induced effects. The processing of bathymetric LiDAR data is based on a geometric model of the laser bathymetry pulse propagation describing the complex interactions of laser radiation with the water medium and the water bottom. The model comprises the geometric description of laser ray, water surface, refraction, scattering in the water column, and diffuse bottom reflection. Conventional geometric modeling approaches introduce certain simplifications concerning the water surface, the laser ray, and the bottom reflection. Usually, the local curvature of the water surface and the beam divergence are neglected and the travel path of the outgoing and the returned pulse is assumed to be identical. The deviations between the applied geometric model and the actual laser beam path cause a coordinate offset at the water bottom, which affects the accuracy potential of the measuring method. The paper presents enhanced approaches to geometric modeling which are based on a more accurate representation of water surface geometry and laser ray geometry and take into account the diffuse reflection at the water bottom. The refined geometric modeling results in an improved coordinate accuracy at the water bottom. The impact of the geometric modeling methods on the accuracy of the water bottom points is analyzed in a controlled manner using a laser bathymetry simulator. The findings will contribute to increase the accuracy potential of modern ALB systems.

ATENUASI WATER BOTTOM MULTIPLE DENGAN KOMBINASI METODE SURFACE RELATED MULTIPLE ELIMINATION DAN TRANSFORMASI RADON DI LAUT SERAM, PAPUA BARAT

Metode seismik refleksi digunakan dalam eksplorasi minyak dan gas di laut dengan memanfaatkan gelombang suara yang menjalar ke dalam batuan dasar bumi. Penelitian ini menggunakan data seismik 2D laut berupa data lapangan hasil akuisisi di Perairan Seram, Papua Barat. Data lapangan hasil akuisisi masih bercampur dengan multiple yang disebabkan oleh perbedaan impedansi akustik dari lapisan-lapisan bawah permukaan bumi. Keberadaan multiple dapat menyebabkan kerumitan pada saat interpretasi karena menimbulkan efek reflektor semu. Oleh karena itu perlu diterapkan metode atenuasi multiple yang tepat, untuk mengurangi derau multiple. Penelitian ini menerapkan kombinasi antara Surface Related Multiple Elimination (SRME) dan Transformasi Radon untuk menghasilkan penampang seismik yang bebas dari multiple. Hasil dari pengolahan data menunjukkan bahwa kombinasi metode Surface Related Multiple Elimination dan Transformasi Radon efektif untuk menghilangkan multiple periode panjang pada zona near offset, middle offset dan far offset. Penerapan kombinasi metode ini juga menghapus beberapa bagian yang tidak signifikan pada reflektor utama oleh karena bercampurnya sinyal dan multiple dalam domain moveout yang sama.

Maximising the Value of Multi-Sensor Streamer Data via MAZ Processing

The North West Shelf of Australia contains a late Paleozoic to Cenozoic sedimentary succession, which attains a thickness of over 10 km and is dominated by Triassic to Lower Cretaceous sediments. The deeper plays exist at multiple stratigraphic levels including oil-prone Jurassic sediments and faulted gas-prone Triassic sediments. The area has been proven difficult as far as seismic imaging is concerned, particularly over the Madeline trend. The presence of a hard, rugose water bottom, strong reflectors beneath the water bottom, and shallow Tertiary carbonates make the Dampier Sub-basin vulnerable to multiple contamination, amplitude distortion, lower signal-to-noise ratio (S/N) and unreliable AVO response. Poor seismic quality in the data has been a significant barrier to reducing exploration risk. In the 1990s, East Dampier (1992, blue polygon in Figure 1) and Keast (1997, yellow polygon in Figure 1) seismic data were acquired in East-West and North-South directions respectively, in an effort to better understand the impact from the shallow complex overburden. To address these challenges, the Demeter survey was acquired in 2003 (black polygon in Figure 1) with a denser acquisition grid. The overall seismic quality was improved, but the results still contained a significant level of residual multiples. Later, the Fortuna survey, the most comprehensive multi-sensor seismic survey on the North West Shelf of Australia to date, was acquired in 2014 with the aim to provide better subsurface imaging (pink polygon in Figure 1) from different acquisition perspectives. The data was processed with advanced processing technology, including shallow water demultiple, deghosting and high definition tilted orthorhombic velocity model building (Birdus et al., 2017). However, the final results were still suffering from a number of challenges, specifically: 1) strong residual multiple in near offsets, 2) low S/N ratio, particularly at reservoir level, and 3) inconsistency from near to far stack resulting in unreliable AVO. In this paper, the Dixon area (green polygon), considered as the most challenging area in the Dampier Sub-basin, was chosen as the testing area for our work. By integrating high-end imaging technology, for example dual-sensor deghosting, multi-survey surface related multiple elimination (MAZ-SRME), and multi-azimuth processing (MAZ stack), we will illustrate how we have overcome many of these imaging challenges.