80th EAGE Conference & Exhibition 2018 Workshop Programme

Innovative seismic technologies enable us to achieve higher quality seismic images and reservoir models while reducing the cost of seismic acquisition and processing. A great example of these innovative methods is blended acquisition, in which multiple sources are firing within a shorter time window that result in significantly shorter acquisition time. The utilization of surface multiples, which can be considered as a natural form of blending is not yet very well established by the industry. However, recently much more attention has been drawn to the possibility of utilizing surface multiples in seismic imaging, rather than removing them. Surface multiples can be considered as secondary sources that illuminate the subsurface many times at different angles. Therefore, they have a significant potential to overcome the shortcomings of seismic acquisition by filling the illumination gaps resulting from imperfect acquisition, especially in sparse acquisition geometries. In this work we discuss the possibilities of improving the quality of seismic images in sparse node acquisition by utilizing surface multiples. We investigate two separate acquisition scenarios, namely, sparse node coverage and sparse node sampling. The impact of including surface multiples will be analyzed for these scenarios.

What is the business value that internal multiple technology can bring at the seismic quantitative interpretation stage? This is the question that we ought to start answering, as we move the internal multiple prediction and attenuation technology from a mostly academic pursuit to a standard part of seismic processing. For years, internal multiples have been neglected in seismic processing. This means that a large majority of the data used for interpretation and analysis are contaminated with these events. This contamination is not always easy to identify. For example, it can appear too weak compared to nearby primaries, interfere with them, or have enough strength and coherency –increasing the risk of interpreting them as real horizons. Furthermore, interference can create false dimming or brightening of events, obstructing quantitative analysis. We believe that there is untapped business value in utilizing and further developing internal multiple technology.

Long-period internal multiple suppression is difficult when severe short-period multiples are present in flat geologic environments. The propagating signal is bell-shaped followed by a higher-frequency noise coda. Standard deconvolution processes are suspect. Starting with these geologic conditions, an interpretation process is proposed that allows validation of long-period multiple suppression at well locations. Primary-only and multiple-only synthetics with time-varying propagating wavelets are created for validating the multiple technique. Since short-period multiples are not suppressed, well ties are facilitated by morphing multiple synthetics to primary-only synthetics. These principles are part of an interpretation workflow applied to various worldwide basins that contain both short- and long-period multiples.

Increased interest in seismic survey’s possible disturbance of marine life and masking of biologically-relevant sounds means that the accuracy of acoustic propagation models at ranges well beyond auditory injury distances is important. The models are used to predict the effects of noise before a survey occurs as well as to model the noise exposure of tagged animals from actual surveys. The definition of long range depends on the animals of concern – it could be 5 km for porpoise, or 100 km for baleen whales. This abstract summarizes two projects where precise measurements of seismic airgun sounds were made over a wide range of distances from the source. We present typical data collected over ranges of .1-150 km, the frequency content as a function of range, and how the modeled levels compared to the measurements. For data collected in Baffin Bay, the model accuracy is highly dependent on the accuracy of the bathymetric data. For measurements at the edge of the Gran Banks, high resolution date-specific sound speed profiles are required to accurately model received sound levels.

The interest in the environmental impact of marine seismic sources has steadily increased over recent years as the risk of potentially harming or disturbing marine life has been brought into stronger focus. Designing a seismic source setup that simultaneously fulfill geophysical requirements and minimize the environmental footprint is often required up front of a seismic survey, but is difficult to assess. While the geophysical requirements of a survey are depending on geology and survey objectives, the output of a given source configuration does not. We have assessed, through modelling of the seismic source and analytical homogeneous propagation modelling, the decrease in environmental impact of the most frequently considered mitigation measures as a non-survey dependent comparative study. We show that many of the most frequently used mitigation measures have limited effect while other measures, especially changing the source setup to consist of triggering individual source elements separated in time, will result in a relatively stronger decrease of the potential environmental impact. This quantitative assessment gives a platform for understanding and evaluating different mitigation measures to lower the environmental impact of the seismic source.

The early tens saw a growing interest in searching for hydrocarbon deposits in offshore Greenland Waters. Deeper Arctic waters have propagation conditions and a marine mammal fauna different from better-studied temperate or shallow-water regions, and there are therefore many unknowns when it comes to negative impacts on marine mammals when conducting seismic surveys in Greenland. The Greenland Environmental Agency for Mineral Resource Activities (EAMRA) therefore established a rather strict regulation of seismic surveys to protect marine mammals during seismic surveys. The regulation is built on four pillars; 1) an annual application cycle, 2) modeling of noise emissions, 3) time-closure areas for vulnerable species and 3) a requirement of noise monitoring during seismic surveys. In 2012 four seismic surveys were planned for Baffin Bay, Greenland. The surveys were accompanied by three studies to examine the effects on the soundscape and narwhals: 1) Propagation of airgun pulses, 2) aerial surveys to examine narwhal distribution, 3) an interview study with narwhal hunters to examine potential changes to the narwhal hunt. The overall results will be presented at the workshop.

A modelling approach is described in which time-domain underwater-acoustic source functions are generated using a model that predicts the motion of air-bubbles generated by mini-airguns operated at various pressures. These time-series are Fourier-transformed to produce frequency-domain descriptions of the source. These are combined with complex transfer functions describing the propagation of sound in shallow-water waveguide geometries so that source spectra can be converted to received spectra by a simple multiplication process for specified source and receiver positions. Received spectra can be inverse-Fourier transformed to produce time-series of received signals. Calculations are performed to yield three acoustic variables: one pressure and two particle velocities (in the radial and vertical directions). Time-series are shown for a test environment in which total acoustic fields are made up of contributions from waterborne and bottom-interacting paths.

Many believe that environmentally friendlier seismic acquisition means worse data quality and increased cost, but the opposite can be shown. Existing geophysical methods in combination with new and upcoming technologies makes it possible to save survey costs, have less environmental impact, and record better data. The main cost-driver is vessel time and reducing it using passive methods or other geophysical methods to reduce the target area is the highest priority. Then one can focus on structures with highest priority by applying active seismic with novel methods and new technologies. Reducing the Sound Pressure Level, avoiding unnecessary frequencies and reducing saillines with Marine Vibrators will be the standard acquisition requirements in the future.

Seismic surveys are now subject to an increasing number of precautionary environmental risk mitigation measures ranging from pre-survey risk modeling to visual and acoustic monitoring for marine life, and even exclusion from some areas, shutdowns, and more. The scientific support is often inferred indirectly from other sound sources due to a lack of data on seismic sound itself. Direct injury from sound was an early concern (Finneran, 2015; NOAA 2016). But as concern about injury diminishes, attention has shifted more toward effects on behavior and health. The biological consequence of such disruptions is usually expressed in population terms, including effects on commercially exploited species. The initial regulatory response was to place observers on seismic ships and to move or shut down the source when protected marine animals were detected nearby. The logistic and financial burdens of these added obligations have become part of an increased cost of exploration. More recent regulatory responses such as closing areas to seismic surveys or seeking alternative sound source technologies offer even greater technical and financial challenges to the industry.

Anthropogenic sound emitted into the sea has the potential to impact marine mammal by masking relevant sound, causing behavioural changes and, in extreme cases, induce auditory injury or death. It is often required to monitor an operational area of offshore projects for the presence of marine mammals in order to reduce the risk of potential impact. This kind of monitoring is traditionally conducted by trained visual observers, which scan the sea surface for marine mammals. This method, however, is restricted to daylight hours and fair weather conditions. The talk presents a review and evaluation of monitoring methods that could now, or in the near future, be used for marine mammal monitoring for mitigation purposes in periods of low visibility during which the effectiveness of visual observers is reduced.