SYMPOSIUM ABSTRACTS

Achievements and lessons from 3-D seismic surveys of subduction zones

Nathan Bangs, University of Texas Institute for Geophysics

Our understanding of subduction zones has advanced significantly with the application of 3-D seismic imaging to these complex deformational setting. Three-dimensional seismic images offer unprecedented views of complex accretionary wedge structure for interpreting deformational history.  However, accurate seismic reflection images also offer valuable information on rock physical properties from which we can infer hydrogeology, state-of-stress, and fault-rock interactions.  Particularly valuable has been the opportunity to map structures and rock properties in three dimensions to reveal variations in both strike and dip directions. 

Three 3-D seismic experiments illustrate achievements that would not have been possible with just 2-D imaging. The Barbados Ridge accretionary wedge was surveyed in 1992, using the R/V Ewing with a short (< 600 m) streamer.  These data map the reflection characteristics of a 5 x 25 km area of the plate-boundary thrust fault, the décollement, to reveal broad (~ 1 km wide) areas of a fluid-charged fault zone.  These data reveal both the role of high pressure fluids in maintaining an inherently weak fault, and the role of the fault as a conduit for deeply source fluids. 

The Nankai Trough 3-D survey imaged an 8 x 80 km area of the subduction zone thrust beneath the seaward most portion of the accretionary wedge.  We acquired these data in 1999 with the R/V Ewing using a long (6 km), 240 channel streamer and moderate seismic source for deep penetration.  These data reveal the down dip variation of seismic reflection amplitudes of the décollement from the trench across the aseismic-to-seismic transition to the updip edge of the seismogenic zone. Fluids within the décollement zone are evident at the trench and extend 30 km down dip into the subduction zone where fluid indicators become scarce and the décollement shows signs of increasing shear stress including seismogenesis. 

Hydrate Ridge, offshore Oregon, was surveyed in 2000 with a high-resolution seismic system using 2 GI-guns and a short offset (<600 m) streamer.  We imaged a 4 x 11 km area of the BSR and the deformed strata deposited on top of the accretionary wedge.  Stratigraphic horizons and faults comprise the plumbing system that feeds free gas from deep sources into the gas hydrate stability zone.  One particular horizon directly feeds free gas to the summit of Hydrate Ridge where the highest hydrate concentrations were found during Ocean Drilling Program Leg 204 drilling and free gas vents at the seafloor. 

Each of these surveys illustrates various challenges and offer lessons on ways to improve 3-D imaging for the future.  Most notably are: 1) the need for proper infill of the survey volume, 2) appropriate seismic aperture, and 3) sufficient sampling in the crossline direction to avoid spatial aliasing.  In the example of high-resolution 3-D imaging, statics from acquisition geometry and tides are potentially problematic.  This presentation will illustrate both the achievements of 3-D imaging in subduction zone, and present examples of challenges we have faced with lessons for future 3-D seismic surveying.

Joe Cartwright, Cardiff University

3D reflection seismic is one of the most exciting technology-driven developments in the Earth Sciences over the past century. 3D seismic data provide interpreters with the ability to map structures and stratigraphic features in three-dimensional detail to a resolution of a few metres over thousands of square kilometres. It is a geological ‘Hubble’, whose resolving power has already yielded some fascinating (and surprising) insights and will continue to provide a major stimulus for research into geological processes and products for many decades to come.  Academic and other research institutions have a major role to play in the use of this data by exploiting the enormous volume of geological information contained in 3D seismic surveys.

This presentation reviews some of the recent advances in basin analysis made using the medium of 3D seismic data, focusing on the fields of structural and sedimentary geology, fluid-rock interactions and igneous geology. It is noted that the increased resolution of the 3D seismic method provided the essential catalyst necessary to stimulate novel observations and discover new geological structures such as polygonal faults, giant pockmarks and sandstone intrusions, km-long gas blow-out pipes, and to capture the spatial variability of diagenetic fronts. The UK’s first impact crater was also discovered using 3D seismic data. The potential for future developments in this field of geophysical interpretation is considerable, and we anticipate that new discoveries will be made in many years to come.

IODP Site Survey requirements

Dale Sawyer, Rice University

The Site Survey Panel (SSP) of the Integrated Ocean Drilling Program (IODP) is charged to: 1) review the quality and adequacy of site survey data submitted in support of drilling proposals, 2) provide guidance to proponents regarding necessary site survey data, 3) assess, on the basis of the submitted data, whether the scientific objectives of each drill site can be achieved, 4) encourage the use of new site survey technologies, and 5) foster cooperation in the acquisition of site survey data. The SSP has developed a matrix to guide proponents in planning site survey activities. 3D seismic data acquisition is probably called for when proponents plan to drill holes of depth greater than 1000 m in sediment and in cases where riser technology is to be employed.

It is important to note that site survey data play many roles in the activities of the IODP. They are used by the proponents to plan the location of proposed drill sites and to defend their scientific merit. They are used by the Environmental Protection and Safety Panel (EPSP) to determine if the holes can be drilled safely and in an environmentally friendly manner. They are used by drilling engineers to plan the drilling of the holes. They are used by Co-Chief and other shipboard scientists to make decisions while at sea, Finally, and no less importantly, they are used after the drilling to help understand and extrapolate away from the hole the unexpected results often obtained when drilling.

Detecting stress field changes around compacting reservoirs using time-lapse seismic

Paul Hatchell, Shell

Production of oil and gas from buried reservoirs is often accompanied by a large reduction in the reservoir pore-fluid pressure (1000s of PSI). This drop in fluid pressure increases the effective stresses on the reservoir rocks resulting in reservoir compaction as large as several meters in highly compressible formations. The rocks adjacent to the reservoir deform to accommodate the compaction resulting in changes in stress and strain fields that extend a great distance away from the reservoir.

Time-lapse seismic monitoring of compacting reservoirs detects the changes that occur both inside and outside of the reservoir interval. These come in the form of changes in the seismic reflection amplitudes (the amplitude changes are usually confined to the reservoir interval) and time-lapse time-shifts that refer to changes in the two-way travel times.

The compaction-induced time shifts have opposite gradients on the inside and outside of the reservoir. Within the reservoir, the reduction in layer thickness and the expected increase in seismic velocity will reduce the seismic travel time across these layers. Outside the reservoir, the decrease in reservoir thickness is exactly balanced by surface subsidence and rock expansion. The expanding overburden produces increased layer thickness and slower seismic velocities that increase the seismic travel times.

Observations on real time-lapse seismic data over compacting reservoirs show that the positive time shifts that accrue in the overburden are larger than the negative time shifts that accrue inside the reservoir (the sign convention chosen is that positive time shifts result when the seismic travel time increases).  This indicates that the sensitivity of seismic waves is larger for rock expansion than for rock compaction.

Using geomechanical models and a simple relationship between seismic velocity and strain we can produce forward models of the seismic time shifts that accurately match our observations around the world. One result of this model is that time-lapse time shifts are proportional to the stretching of the overburden layers and that this is highly correlated with the reservoir compaction. Time-lapse time shifts are a good measurement of the reservoir compaction.

The time-lapse signals we observe over depleting reservoirs are small (the seismic time shifts are ~1 ms!). The success that Shell has had measuring these small changes is a direct result of methodically and deliberately repeating very carefully the original shot and receiver locations of the baseline seismic data. This is a struggle against the forces of nature (tides, winds, and currents) that construe to miss-position your streamer cables, combined with shallow earth layers waiting to distort your seismic wavefronts and magnify the positioning errors into noise that can be neither tolerated nor processed away.

Over/under acquisition and rough sea deghosting

Johan Robertsson, WesternGeco Oslo Technology Centre,
Ed Kragh, Schlumberger Cambridge Research, and
Robert Laws, Schlumberger Cambridge Research

Novel marine seismic acquisition technology allows for the decomposition of recorded data into up- and down-going waves.  This has several important applications including multiple suppression and reducing sea surface scattering noise.  I will be describing two acquisition methods and compare results from data acquired in a field test.  The first method is based on acquiring data using pairs of streamers in an over/under configuration enabling estimation of the vertical hydrophone pressure gradient.  Pressure and pressure gradient data are then combined to decompose the wavefield into its up- and down-going constituents.  The second method is based on data acquired in a more conventional single streamer configuration.  By recording very low frequencies of pressure, below 0.5 Hz, we are able to estimate the wave heights directly above each hydrophone in the seismic streamer, as a function of time. Because of the bandwidth separation, this can be done without affecting the simultaneously acquired seismic recordings. These wave-height estimates are used to derive deterministic deghosting operators that compensate for the rough-sea reflection response.

We apply the methods to a 2D line acquired in the North Sea using a 5m streamer tow depth. Initial stack results show a clear improvement in resolution after deghosting with improved reflector continuity, a more compressed wavelet, better low-frequency response, and improved signal-to-noise ratio.

Proposing and Completing NSF Active Source Marine Seismic Programs

C. Ruppel, NSF-OCE, Marine Geosciences Section 

In recent years, PIs, ship operators, and funding agencies have frequently faced permitting challenges when undertaking active source marine seismic programs, particularly in foreign waters.  This presentation will provide an overview of past and future issues facing seismic programs funded by MG&G, ODP, MARGINS, RIDGE, and Continental Dynamics and use an example of one recently completed project to describe the intricacies of the permitting and clearance processes from the perspectives of NSF, other federal agencies, and PIs.  We will also review key points of the guidelines that NSF-OCE released to potential seismic PIs in December 2004, provide updates about new federal-level activities on the seismic permitting front, and give advice about specific difficulties PIs have had or are likely to face with implementing their programs once funded.

Seismic oceanography 

Steve Holbrook, University of Wyoming

In this talk I will present a new application of marine reflection seismology --  the imaging of thermohaline fine structure in the ocean's interior (Holbrook et al., Science, 2003).  Surprisingly, the reflection method is ideally tuned to detect thermohaline fine structure in the ocean, with sensitivity down to temperature contrasts of ~0.05 degrees Celsius.  Spectacular images can result, showing the ocean in a way it's never been seen before. The images include many features relevant to mixing processes in the ocean, such as water-mass boundaries, intrusions, mesoscale eddies, internal waves, and turbulent boundary layers.  Seismic oceanography presents new opportunities for cross-disciplinary work between the MG&G and PO communities, which will include new types of 2D and 3D work on the R/V Langseth.  I will present an overview of results to date and a vision for future work on the Langseth.

Effects of airguns used for geophysical research on marine life

Peter Tyack, Woods Hole Oceanographic Institution

While airgun arrays are tuned to create specific signals on the downwards axis, the signals put into the water column and horizontal propagation are not well characterized. Airguns produce enough energy to be of concern both for injury, for avoidance, and for disruption of behavior. I will review data on responses of invertebrates, fish, and marine mammals to airguns, point out critical data gaps, and discuss the effectiveness of different methods to reduce adverse impacts.