Near-Surface Geophysics Applied to Geologic Hazards I: Offshore Hazards
Presiding: M A Smith, U.S. Department of the Interior Minerals Management Service; N C Dutta, Schlumberger WesternGeco
NS13A-01 13:30h
3D AUV Microseismic Implementation for Deepwater Seabed Investigations
Autonomous Underwater Vehicle (AUV) technology, developed commercially over the past 5 years, allows for the geophysical investigation of the seabed on the deepwater continental slope at resolutions, data densities and timelines not previously attainable. High-resolution geophysical systems normally employed on deepwater survey AUVs consist of multibeam bathymetry, side scan sonar and subbottom profiler. Inertial navigation allows positioning accuracies on the order of plus or minus 3 meters in depths up to 2,000 meters. C & C Technologies, Inc. owns and operates the C-Surveyor I AUV, which has collected more than 40,000 km of geohazard survey data on the continental slopes of the Gulf of Mexico, Mediterranean Sea, Brazil and West Africa. The oil and gas industry routinely engineers deepwater platform-mooring systems and other bottom founded subsea systems for exploration and production developments. Resolute subbottom imaging of the foundation zone in order to identify the near-seafloor geologic conditions at these deepwater development sites is critical in order to maintain system integrity. The paper describes the methodology and post-processing techniques used to create a high-resolution (2-8 kHz) 3D seismic cube from subbottom profiler data collected from an AUV system. Data examples of the multibeam bathymetry, side scan sonar and 2D seismic profiles will be provided to complement the results of the 3D seismic cube processing. Examples of inlines, crosslines, arbitrary lines, seafloor amplitude extraction and time slices are presented for the 4-meter binned data set. Advantages, disadvantages and suggested improvements for the survey acquisition technique and post processing are discussed.
NS13A-02 13:45h
Shallow Water Flow in the Deepwater Gulf of Mexico: 20-Year Overview of a Near-Surface Geologic Hazard
Since 1984, shallow water flow occurrences have been reported in about 100 Gulf of Mexico lease blocks covering 80 oil and gas fields or lease blocks. These sands were deposited as continental slope/fan sequences during upper Pleistocene progradation. They include prodelta debris flows with a chaotic seismic character and, in some cases, rotated slump blocks. High sedimentation rates and an impermeable mud or clay seal from a condensed section are critical factors contributing to overpressures in shallow water flow sands. With a few exceptions, water flow incidents occur at water depths exceeding 1500 ft with a mean value at about 3800 ft of water. Water flow problem sands typically occur from 750 to 2000 ft below the seafloor. In the Mississippi Canyon and southern Viosca Knoll area, some of the shallower channel sands can be identified as part of a particular distributary system such as the old Timbalier channel, Southwest Pass canyon, or Einstein levee/channel system. Integration of high-resolution multichannel and reprocessed conventional 2D and 3D seismic data for the top-hole section is widely used to identify sand bodies with moderate or high shallow water flow potential. With the development of new geophysical technology and a better understanding of the controlling geological factors, instances of shallow water flow in recent years have been less severe and mitigating approaches to this geologic hazard have been successful in most cases.
NS13A-03 14:00h
Geophysical Evidence for Large Scale Fluid Vent Structure in the Abyssal Gulf of Mexico
This talk examines geophysical evidence for the appearance of a large-scale fissure or fluid vent structure in the abyssal Gulf of Mexico, located in Lund Area. We notice a near vertical fissure 20,000 ft tall and about 100km long, which directly underlies the thickest portion of the Mississippi River Fan. Disruption of the sedimentary layers below the fan resembles fluid vent structures normally seen as localized chimneys on seismic from deep water. In this case, the feature has developed into a long, linear structure, the upper portion of which appears to contain hydrates. We expect a relationship between the formation of the fissure and the rapid deposition of the Mississippi Fan during Pleistocene and Holocene glacial lowstands. The expanding fan would have led to uneven loading of the pre-existing, layered sediments of the abyssal plain. This may have caused a zone of low effective stress to develop at the leading edge of the levee, reaching deep into the subsurface due to the large spatial wavelength of the load, and bringing pore pressures close to the fracture gradient in much the same manner as envisioned by Dugan & Flemings, 2000 for formation of the box canyons off the New Jersey coast. Geophysically, we can document: 1) A long, linear time structure appearing at all levels underlying the Mississippi Fan and above the known source rock section (i.e., Top K through Pliocene). 2) Isopachs show no thinning implying a late, catastrophic event. 3) Banded, high amplitude reflections crowded into the top of the vent may be hydrates or alternating layers of hydrates and free gas trapped under the fan sediments. 4) Low impedance, low frequency, chaotic seismic "halo" 20-25miles wide surrounds the vent. 5) Near and far offset stacks of seismic data resemble one another enough to prove the feature is not just a shallow velocity anomaly. Far offset raypaths would undershoot a near surface feature causing deep reflectors from a far offset record to differ from a near offset record if it were simply a shallow velocity anomaly. 6) Congruence of the subsurface feature with the bathymetric maximum thickness of the Mississippi Fan combined with the apparent late-stage deformation suggests a causal relationship. Understanding the formation of this vent feature may provide insight into large-scale petroleum migration routes, the origins of overpressure in the shallow sedimentary section, effects on the distribution of hydrates, and possibly rates of petroleum sourcing.
NS13A-04 INVITED 14:15h
Assessing Deep Water Gas Hydrate Systems and Seafloor Stability
We demonstrate how four-component ocean-bottom-cable (4-C OBC) seismic data acquired in deep water can be used to study near-seafloor strata and the geologic characteristics of fluid and gas expulsion systems that extend to the seafloor and become thermogenic sources of gas hydrates. We document the importance of the converted-shear (P-SV) mode extracted from 4-C OBC data. We show that P-SV data provide a spatial resolution of deep-water, near-seafloor strata that is an order of magnitude better than the resolution of the compressional (P-P) mode. Shear wave velocities less than 100 m/s in unconsolidated near-seafloor sediments produce scattered SV wavelengths of meter scale even when long-range surface-based air guns illuminate the seafloor with frequencies that do not exceed 100 Hz. These short wavelengths allow the P-SV mode to define geologic detail that cannot be detected with P-P scattered data. The geomechanical properties of the seafloor strata are determined by transforming seismic measurements of compressional and shear velocities into estimates of compressional and shear moduli. Current 4-C OBC technology available from major seismic contractors allows deep-water gas hydrate systems and seafloor stability to now be studied over large areas of many hundreds of square kilometers.
NS13A-05 14:30h
Mudflows Triggered by Hurricane Ivan on Mississippi Delta
High resolution seismic surveys obtained before and after passage of Hurricane Ivan in September 2004 show pipeline displacement and downslope mud flow movements. Side scan sonar mosaic and multibeam bathymetry reveal quantitative details of slump morphology and pipeline displacement processes. Subbottom profiles and bathymetry reveal displacement and accumulation volumes. Triggering of mud slides on very low slopes by the passage of storm waves are confirmed and illustrated by remote sensing seismic tools.
NS13A-06 14:45h
Regional Trends in Undercompaction as a Predictor of Shallow Water Flow and Overpressure in the Deepwater Gulf of Mexico
Shallow water flow and overpressure, arising primarily from rapid sedimentation rates generated by the Mississippi River depocenter, represent major drilling hazards in the deep water Gulf of Mexico. Based on data from geotechnical wells, drilled in several different prospects in the central deepwater Gulf of Mexico, Ostermeier et al (2001) report that where significant overpressures are present, they begin at or close to the mudline, and that regional trends in overpressures correlate with the incidence and severity of shallow water flow. For shallow sediments in the deepwater Gulf of Mexico, compaction disequilibrium is the most important cause of overpressure. For sediment to compact, pore water must be expelled. If the sedimentation is too rapid compared to the time for fluid flow, the pore fluid will become overpressured and will support part of the overburden load. As a result, the porosity of the sediment will be higher and the velocity of elastic waves will be lower than in normally pressured sediments. Overpressures resulting from compaction disequilibrium may therefore be predicted, given sufficiently accurate seismic velocities (Sayers et al., 2002). In this paper, all available checkshots released by the MMS for the entire Gulf of Mexico are inverted for velocity versus depth below mudline, then kriged to populate a 3D Mechanical Earth Model with both velocity and expected uncertainty. The 3D velocity cube thus obtained is used to infer the regional variation in undercompaction of shallow sediments and overpressure. By applying a threshold to the predicted kriging error, a map of undercompaction and overpressure can be restricted to areas of greater reliability. The results of this analysis are expected to find wide application in the drilling of safe and economic deepwater wells. References: Ostermeier, R.M., Pelletier, J.H., Winkler, C.D. and Nicholson, J.W. (2002) Trends in shallow sediment pore pressures - Deepwater Gulf of Mexico, SPE/IADC 67772. Sayers, C.M., Johnson, G.M. and Denyer, G. (2002) Pre-drill Pore Pressure Prediction Using Seismic Data, Geophysics, 67, 1286-1292.