Marine Seismic Surveys Research Articles

Can you hear the noise? Environmental licensing of seismic surveys in Brazil faces uncertain future after 18 years protecting biodiversity

Marine Seismic Surveys are an important source of concern for marine biodiversity conservation worldwide. In Brazil, Environmental Federal Agency IBAMA has developed a considerably advanced mitigation/monitoring requirements package in 18 years of environmental licensing practice, with standardized guidelines since 2005. Adding to global efforts aiming at filling knowledge gaps over the impacts on biodiversity, IBAMA has been able to foster important marine research through environmental licensing requirements. Better communication of research findings to the international scientific community remains a challenge to be addressed. Nevertheless, current institutional and legal reforming initiatives jeopardize the evolution of environmental control of Marine Seismic Surveys in Brazil.

Multiple field-based methods to assess the potential impacts of seismic surveys on scallops

Marine seismic surveys are an important tool to map geology beneath the seafloor and manage petroleum resources, but they are also a source of underwater noise pollution. A mass mortality of scallops in the Bass Strait, Australia occurred a few months after a marine seismic survey in 2010, and fishing groups were concerned about the potential relationship between the two events. The current study used three field-based methods to investigate the potential impact of marine seismic surveys on scallops in the region: 1) dredging and 2) deployment of Autonomous Underwater Vehicles (AUVs) were undertaken to examine the potential response of two species of scallops (Pecten fumatus, Mimachlamys asperrima) before, two months after, and ten months after a 2015 marine seismic survey; and 3) MODIS satellite data revealed patterns of sea surface temperatures from 2006–2016. Results from the dredging and AUV components show no evidence of scallop mortality attributable to the seismic survey, although sub-lethal effects cannot be excluded. The remote sensing revealed a pronounced thermal spike in the eastern Bass Strait between February and May 2010, overlapping the scallop beds that suffered extensive mortality and coinciding almost exactly with dates of operation for the 2010 seismic survey. The acquisition of in situ data coupled with consideration of commercial seismic arrays meant that results were ecologically realistic, while the paired field-based components (dredging, AUV imagery) provided a failsafe against challenges associated with working wholly in the field. This study expands our knowledge of the potential environmental impacts of marine seismic survey and will inform future applications for marine seismic surveys, as well as the assessment of such applications by regulatory authorities.

Application of FK Filtering for Coherent Noise Removal in High Frequency Shallow Marine Data

The use of seismic reflection to study earth subsurface is commonly troubled by the presence of the coherent noise in the seismic data. This unwanted signal were generated by various sources and therefore had their own noise characteristic. The acquisition of the shallow marine seismic survey were done using a sparker source to obtain a high frequency seismic data. This however results in a stronger noise level in the recorded data. The application of FK filter were used to eliminate this unwanted noise using the data obtained from the Malacca Strait and Terengganu Bay. The result obtained prove to be successful in reducing the coherent noise while preserving the frequency content of the seismic data.

Utilizing the R/V Marcus G. Langseth's streamer to measure the acoustic radiation of its seismic source in the shallow waters of New Jersey's continental shelf.

Shallow water marine seismic surveys are necessary to understand a range of Earth processes in coastal environments, including those that represent major hazards to society such as earthquakes, tsunamis, and sea-level rise. Predicting the acoustic radiation of seismic sources in shallow water, which is required for compliance with regulations designed to limit impacts on protected marine species, is a significant challenge in this environment because of variable reflectivity due to local geology, and the susceptibility of relatively small bathymetric features to focus or shadow acoustic energy. We use data from the R/V Marcus G. Langseth’s towed hydrophone streamer to estimate the acoustic radiation of the ship’s seismic source during a large survey of the shallow shelf off the coast of New Jersey. We use the results to estimate the distances from the source to acoustic levels of regulatory significance, and use bathymetric data from the ship’s multibeam system to explore the relationships between seafloor depth and slope and the measured acoustic radiation patterns. We demonstrate that existing models significantly overestimate mitigation radii, but that the variability of received levels in shallow water suggest that in situ real-time measurements would help improve these estimates, and that post-cruise revisions of received levels are valuable in accurately determining the potential acoustic impact of a seismic survey.

Shooting over the seismic spread

Conventional marine seismic surveys typically mobilize a single vessel towing two airgun source arrays in front of a spread of ten or more streamers. The data acquired in this way are narrow-azimuth and lack near offsets owing to the distance between the sources and the streamers which can be in the range of 100 to 200 m for the inner cables and up to 500 m for the outer cables. Several solutions, such as coil shooting (French, 1984; Ross, 2008) or advanced multi-vessel operations (Mandroux et al., 2013), have been proposed and deployed to improve azimuth coverage and fold. Although these are excellent solutions for achieving wide-azimuth data, they are generally expensive and/ or time-consuming, and none of them record zero-offset data. Near- and zero-offset data are, however, especially critical for imaging shallow geological targets and of great benefit for multiple attenuation. In this paper, we present a tailored solution to this challenge that allows the recording of both zero-offset data and dual azimuths in an effective and safe way. We call this acquisition solution TopSeis. This solution was created in close co-operation between Lundin Norway and CGG and is designed to deliver excellent broadband (2.5-200 Hz) imaging of shallow to intermediate targets at depths of up to 3000 m or more.

Advanced processing for UHR3D shallow marine seismic surveys

ABSTRACTTraditionally, commercial ultra‐high‐resolution 3D multichannel seismic surveys have used sources with relatively low‐frequency content (up to 600 Hz) with corresponding relatively low resolution. The new generation of multi‐tip sparkers, however, utilising negative discharge technology, is able to produce repeatable high‐frequency signals (above 2.5 kHz) with the potential to image hundreds of metres of the shallow stratigraphy at < 50 cm vertical resolution. These sources would be especially suited to ultra‐high‐resolution 3D studies such as the foundations of wind turbines installations for offshore wind farms.However, simple downsizing of conventional processing sequences, designed for sources of significantly lower frequency content, is not enough. To preserve the signal properties of the sparker source and produce accurately positioned seismic images, new strategies are needed. The implementation of processing solutions that accurately determine source and receiver depths for all shots is also paramount to correctly reduce the data to the desired vertical datum.A basic methodology for deriving geometry and shot corrections for wave motion and tides from the 3D data themselves has already been demonstrated. With Differential Global Positioning System (DGPS) navigation available on both source and streamers and the correct integration of the seismic recording and positioning systems, this concept has been further developed and refined to be more precise. Being able to derive components of the streamer position and depth is of great importance for effective broadband processing from the slanting streamers that are deployed in acquisition. New procedures and processing techniques have been developed, and their efficacy is demonstrated on case studies of recently acquired 2D and 3D multichannel data.

Marine seismic acquisition: efficiency and environment, new technologies applied in Australia

The design of successful marine seismic surveys is driven by many factors, two prime issues being efficiency and environmental impact. Efficiency is primarily driven by reduction of non-productive time and creating the largest sub-surface illumination area possible in the shortest time. In addition, public opinion and governmental regulations are requiring the industry to minimise their environmental impact. One aspect is reducing the overall sound exposure level (SEL) of the source into the marine environment. Using recent Australian examples, we will discuss and demonstrate the use of two new technology groups that address these concerns. The first is the use of a new type of seismic air-gun with optimal output over the range of frequencies commonly used in seismic exploration, while limiting potential environmental effects from unnecessary high-frequency emissions. The second is continuous data acquisition along the entire boat traverse, including the turns, thereby reducing non-productive vessel time. Both are described with examples from a recent survey acquired offshore north-west Australia.

First results of inaugural deployments of the Australian National Ocean Bottom Seismograph Fleet

The Australian National Ocean Bottom Seismograph Fleet is part of AuScope’s Australian Geophysical Observing System (AGOS) — an initiative of the Australian Government funded through the Education Investment Fund. These instruments will greatly contribute to the understanding of the crust beneath oceanic basins surrounding Australia. In 2014–15 the Australian National OBS Fleet was utilised by the petroleum industry on a number of seismic surveys. High-quality data were recorded at all OBS deployment sites, often to offsets sufficiently large to detect Pn phases — refractions from the upper mantle. Analysis of earthquake data recorded during marine seismic surveys suggests strong interaction between anthropogenic signals (airgun source, vessel noise) and the natural environment, and allows arguing that in some instances earthquake energy contaminates marine reflection data in the frequency pass-band needed for petroleum exploration. Recording earthquake and airgun signals at fixed locations opens up a completely new possibility for calibration and comparison of those signal strengths and spectral compositions.

Kraken 3D - acquisition to interpretation on the edge of the Browse

The broadband Kraken 3D Marine Seismic Survey was acquired during 2013 in the outer Browse Basin exploration permit WA-314-P with the specific goal of assessing risk and volumes at the Elvie prospect. The survey was acquired over a highly rugose sea floor, comprised of deep slump canyons that overlie a steeply prograding Miocene carbonate sequence. Multiple attempts at processing the seismic data have already been made; including a post-stack time migration (a fast-track volume), pre-stack time and pre-stack depth migration. Conventional processing and pre-stack depth migration approaches were unable to fully resolve short-wavelength velocity anomalies below the sea floor that cause obvious residual imaging problems and impact upon depth conversion and seismic amplitude interpretation. A geomechanical pre-stack depth migration now underway to hopefully address the remaining imaging concerns. Overall, the Kraken 3D is considered to be a significant improvement over the pre-existing 2D seismic. Interpretation was performed largely in the depth domain, although ties to nearby wells were made in the time domain using legacy 2D and 3D seismic. Mapping has further matured the Elvie prospect, which is a robust 4-way dip closure located on the divide between the Caswell and Seringapatam Sub-basins. The survey provides strong evidence for a thick top seal in the form of deep-marine muds of Miocene age, although there is evidence of minor seepage through a thin flank of the sealing unit. These shallow amplitude indicators, nearby surface seeps and pockmarks near the sea floor provide additional support for a working petroleum system. The Elvie structure appears to be draped by potentially high quality turbidite reservoirs of most-likely Paleocene age.

Use of distributed fibre-optic sensing for marine seismic measurements

Heterodyne distributed vibration sensing (hDVS) and, more generically, distributed acoustic sensing (DAS) are technologies that rely on traditional fibre-optic cables to provide inline strain measurement. In recent years, hDVS has found application in seismic monitoring; in particular vertical seismic profiling (VSP) greatly benefits from the advantage of using permanent fibres cemented behind casing (Mateeva et al., 2013). VSP measurements were also demonstrated using fibre installed on production tubing (Barberan et al., 2012) and deployed within a wireline cable (Hartog et al., 2014). Another potential application for hDVS is flow monitoring, where, owing to the fibre’s large spatial coverage, it is possible to monitor vibrations and waves along the length of the well. Baihly et al. (2015) offered an insight on this use of hDVS. Furthermore, Molteni et al. (2016) evaluated microseismic monitoring as another compelling hDVS application. This paper evaluates the viability of using hDVS for distributed dynamic pressure sensing in marine seismic acquisition while reviewing the readiness of hDVS technology for this application. An initial concept of including hDVS in a streamer was disclosed by Kragh et al. (2012). Here, a possible design of a towed streamer using hDVS was discussed along with experimental results showing the potential of the technology to acquire pressure measurements in marine seismic surveys. We show the results of using hDVS to measure the pressure wavefield in two different experiments, and the results are compared with reference data acquired with traditional hydrophones. The use of a distributed measurement in towed streamers would provide a new type of measurement. The characterization of the pressure wavefield would be continuous along the cable with a denser spatial sampling. Similarly, straight fibres deployed along the streamer could characterize the tugging noise with increased spatial resolution. Furthermore, the use of cost-effective and lightweight fibres would greatly reduce the electronics embedded in the streamer and the total weight of the cable. The use of hDVS would also have new challenges – for example in how to segment the streamer into shorter sections without the fibre-optic connectors compromising the quality of the backscattered hDVS signal.

The suppression of coherent noise from another airgun source in marine multi-channel seismic data

During seismic investigations, multiple and unexpected sources may cause serious interference on seismic records, and coherent noise generated by another unwanted active source could result in extremely poor data quality. Because airgun arrays have been widely used as the sound source in marine seismic surveys, the noise generated by another airgun array usually has similar characteristics to the primary signals in both frequency bands and wave forms, so the suppression of this type of coherent noise is very difficult. In practice, seismic crews try to avoid conducting multiple surveys simultaneously in a same area, so the source interference problem normally does not occur, and suppression of coherent noise from another active source has rarely been discussed and proposed before. This paper presents a dataset in which part of the records are contaminated by shot noise from another seismic vessel, and proposes a hybrid approach to suppress the coherent noise from that unwanted seismic source. Noise subtraction and primary signal preservation within different data properties are considered to begin the noise suppression. Based on different noise characteristics from various source directions and wave propagation paths, coherence noise can be separated from primary signals in frequency–wave number (F–K), frequency–time (F–T) and intercept time–slowness (tau–p) domains, respectively. This hybrid coherent noise suppression approach involves applying three different filters, F–K, F–T and tau–p, to the contaminated dataset. Our results show that most of the coherent noise generated by another seismic source could be suppressed, and seismic images could be substantially improved.

Inter-pulse sound field from a marine seismic survey in the Arctic and its potential effects on marine mammal acoustic masking

It has been documented that reverberant fields from marine seismic surveys elevate sound levels, and thus has the potential to mask acoustic signals that are important to marine mammals. In this study, we investigated one-third-octave (OTO) bands of airgun inter-pulse sound fields from a shallow water (<15 m) seismic survey in the Beaufort Sea. The results show that at close distances less than 3 km, peak energy of the OTO band levels were concentrated around 80-110 Hz and the median sound levels were 40-50 dB above ambient. The higher frequency components (>3 kHz) of inter-pulse sound field at these close distances ranged 5-20 dB above ambient. At distances beyond 3 km, the received acoustic energy becomes more uniform up to 1000 Hz. The median OTO band levels at these distances were approximately 10-20 dB above ambient. For higher frequencies, sound levels at these distances were approximately the same as ambient. These results imply that low-frequency bowhead whales could be affected by acoustic masking from seismic airgun inter-pulse sound fields, especially at relatively close distances. However, potential acoustic masking effects to mid-frequency beluga whale echolocation signals are expected to be less profound.

Airborne gravity gradiometer surveying of petroleum systems under Lake Tanganyika, Tanzania

The Lake Tanganyika South petroleum exploration block covers the southern portion of the Tanzanian side of Lake Tanganyika and is located within the East African Rift System. The rifting process has formed rotated fault blocks which provide numerous play types in the resulting basins. Interpretation of 2D seismic data from 1984 indicated that sufficient sediment thickness is present for hydrocarbon generation. The prospectivity of the lake sediment sequence is enhanced by large oil discoveries further north along the rift system at Lake Albert in Uganda. Airborne gravity gradiometry (AGG) has been used in the Lake Albert region to delineate the structural framework of sedimentary basins.Based on this analogy, in 2010 Beach Energy commissioned CGG to fly a FALCON AGG and high-resolution airborne magnetic survey over the Lake Tanganyika South block to provide data for mapping the basin architecture and estimating the depth to magnetic basement. A total of nearly 28000 line kilometres of data were acquired.The subsequent interpretation incorporated the AGG and magnetic data with available 2D seismic data, elevation model data, bathymetry, Landsat and regional geology information.The integrated data interpretation revealed that the Lake Tanganyika rifting structures occur as half-grabens that were formed through reactivation of Precambrian fault structures. Two major depocentres were identified in the magnetic depth-to-basement map in the north and in the west-central part of the survey area with sediment thicknesses in excess of 4 km and 3 km, respectively. Smaller, shallower depocentres (with less than 3 km of sediment) occur in the south-western region. This information was used to plan a 2100 km 2D marine seismic survey that was recorded in 2012. An interpretation of the results from the seismic survey confirmed a rifting structure similar to that encountered further north at Lake Albert in Uganda. Several targets were identified from the seismic sections for follow-up.

Marine soundscape during a shallow-water seismic survey in the Arctic Ocean

For noise generating activity that lasts for an extended period of time, an overall increase in noise levels and change of soundscape within a larger area (over tens of km2) can be expected. This study analyzed the sound field characteristics during a shallow-water marine seismic survey in the Beaufort Sea of the Arctic Ocean. Three bottom mounted acoustic sensors were deployed in the survey area: two outside the barrier islands in water depths about 12 m, and one inside the barrier islands in water depth of 2.8 m. Averaged 1 min sound pressure levels (SPLs) in broadband, 100–500 Hz, 1–5 kHz, and above 10 kHz bands were computed for periods when airguns were active and inactive. The results showed an 8-dB increase during the period when airguns were active in the 100–500 Hz band for two locations outside the barrier islands. However, there was no noticeable difference in SPLs during periods airguns were active and inactive inside the barrier islands. This is probably due to higher natural ambient noise and low-frequency cut-off of airgun pulses in this extreme shallow location.

A new dawn for Australian ocean-bottom seismography

The Australian government recently acquired 20 state-of-the-art broadband ocean-bottom seismograph (OBS) units. These instruments were designed and built by Güralp Systems Ltd. (U. K.) with the ability to withstand extreme ocean depths to 6000 m and to remotely record data for as long as 12 months. The Australian national OBS fleet is part of AuScope's Australian Geophysical Observing System (AGOS) — an initiative of the Australian government funded through the Education Investment Fund (EIF). These instruments will contribute greatly to understanding of the crust beneath oceanic basins surrounding Australia. In 2014–2015, the Australian national OBS fleet was used by the petroleum industry on several seismic surveys. High-quality data were recorded at all OBS deployment sites, often to offsets sufficiently large enough to detect possible Pn phases — refractions from the upper mantle. Such information means that crustal thickness as well as the seismic-velocity distribution to the Moho can be determined. As a result, these data can provide important constraint for subsidence and hydrocarbon-maturation modeling. Analysis of earthquake data recorded during marine seismic surveys is ongoing and should provide new information on the interaction between anthropogenic signals (air-gun sources, vessel noise) and the natural environment. Recording earthquake and air-gun signals at fixed locations opens a completely new possibility for calibration and comparison of those signal strengths and spectral compositions. Access to the OBS fleet is open to industry, academia, and government organizations. Application to use Australia's OBS fleet is via a simple process through ANSIR ( www.ansir.org.au ).

Wave-equation multiple imaging (WeMi) — Swimming between the flags

Deghosting dual-sensor streamer seismic data requires separation of the total recorded wavefield into upgoing and downgoing components during seismic preprocessing. Imaging of multiples exploits the wavefield separation process by using the downgoing wavefield at each receiver location as a virtual source. Multiple imaging has been shown to effectively remove shallow-acquisition-related artifacts commonly seen on wide-tow marine seismic surveys and improve imaging quality of the shallow overburden. Acquisition design factors, imaging condition, and cross-talk noise are the main factors that determine the effectiveness of separated wavefield imaging (SWIM) of dual-sensor streamer data. (SWIM is a common nontrademarked acronym used in the literature to describe multiple imaging of separated wavefields sampled from dual-sensor streamer data.) In a field trial, acquisition design did not allow for proper sampling of the total wavefield, with the result that multiple imaging did not prove to be as robust a technique as examples in the literature might imply. Understanding the effects of acquisition design parameters on image quality is essential to more successfully apply multiple-imaging techniques with greater relevance on the North West Shelf of Australia.

Effects of seismic surveys on New Zealand fur seals during daylight hours: Do fur seals respond to obstacles rather than airgun noise?

The responses of New Zealand fur seals (Arctocephalus forsteri) during a three-dimensional marine seismic survey were recorded in the first quantified investigation of the effects of seismic exploration operations on otariid seals. The survey was over the continental slope off southern New Zealand, where fur seals dive to forage at night and rest on the surface during daylight hours. Data were restricted to daylight sightings from the source vessel while towed seismic gear was fully deployed, with comparisons made between fur seal responses when airguns were off and when airguns were operating at full power. Results were inconclusive. Comparisons were confounded because both sighting rate and distance first seen decreased with deteriorating sea state. The key finding of this study was that the source vessel and towed gear created physical obstacles that often generated responses from fur seals. A more thorough investigation is recommended in order to differentiate between responses to airgun noise and responses to physical obstacles, and improvements are suggested to rectify shortfalls in data collection that should generate conclusive outcomes.

Sound source localization technique using a seismic streamer and its extension for whale localization during seismic surveys.

Marine seismic surveys are under increasing scrutiny because of concern that they may disturb or otherwise harm marine mammals and impede their communications. Most of the energy from seismic surveys is low frequency, so concerns are particularly focused on baleen whales. Extensive mitigation efforts accompany seismic surveys, including visual and acoustic monitoring, but the possibility remains that not all animals in an area can be observed and located. One potential way to improve mitigation efforts is to utilize the seismic hydrophone streamer to detect and locate calling baleen whales. This study describes a method to localize low frequency sound sources with data recoded by a streamer. Beamforming is used to estimate the angle of arriving energy relative to sub-arrays of the streamer which constrains the horizontal propagation velocity to each sub-array for a given trial location. A grid search method is then used to minimize the time residual for relative arrival times along the streamer estimated by cross correlation. Results from both simulation and experiment are shown and data from the marine mammal observers and the passive acoustic monitoring conducted simultaneously with the seismic survey are used to verify the analysis.

Airgun inter-pulse noise field during a seismic survey in an Arctic ultra shallow marine environment.

Offshore oil and gas exploration using seismic airguns generates intense underwater pulses that could cause marine mammal hearing impairment and/or behavioral disturbances. However, few studies have investigated the resulting multipath propagation and reverberation from airgun pulses. This research uses continuous acoustic recordings collected in the Arctic during a low-level open-water shallow marine seismic survey, to measure noise levels between airgun pulses. Two methods were used to quantify noise levels during these inter-pulse intervals. The first, based on calculating the root-mean-square sound pressure level in various sub-intervals, is referred to as the increment computation method, and the second, which employs the Hilbert transform to calculate instantaneous acoustic amplitudes, is referred to as the Hilbert transform method. Analyses using both methods yield similar results, showing that the inter-pulse sound field exceeds ambient noise levels by as much as 9 dB during relatively quiet conditions. Inter-pulse noise levels are also related to the source distance, probably due to the higher reverberant conditions of the very shallow water environment. These methods can be used to quantify acoustic environment impacts from anthropogenic transient noises (e.g., seismic pulses, impact pile driving, and sonar pings) and to address potential acoustic masking affecting marine mammals.

Mineralogy and Geotechnical Properties of Ultrasoft Soil from a Nearshore Mine Tailings Sedimentation Pond

An engineering geological study was undertaken to determine the engineering properties, and mineralogy of ultrasoft soils (USS) obtained from a nearshore mine tailings sedimentation pond. The USS is a high plasticity clay of high water content and low shear strength. Marine bathymetric and seismic reflection surveys were undertaken in the sedimentation pond located in the foreshore of the Eastern part of the Republic of Singapore. Specimens collected from the bore holes were tested to determine the engineering and mineralogy properties of the USS. Field vane shear tests were undertaken just adjacent to the sampling bore holes to determine the shear strength properties of the USS. The mineralogical properties of the USS were determined using X-ray diffraction and scanning electron microscope techniques. The USS is under consolidated soil where higher density and lower water content were found at deeper depth. The USS had three different compression indices under three log cycles of effective stress between 1–10, 10–100, and 100–1,000 kPa. This is the main characteristic of USS, which diverts from reconstituted soil. The outcome of this research is fundamental for understanding the compression behavior and subsequently the development of a constitutive model for USS, typical found in sedimentary pond.