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Results
SUMMARY Marine controlled-source electromagnetic (CSEM) surveying using nodal ocean-bottom recorders has become a standard tool for offshore hydrocarbon exploration. A combination of low-noise receivers and transmitters emitting 1,000 amps or more allows data collection at sourceโreceiver offsets of up to 10 km, with depths of investigation reaching several kilometers. However, characterization of shallow geological structure (less than several hundred meters below mudline) is limited by the typical node spacing of 500 m or more. A 3-axis electric field receiver has been developed that is towed behind the EM transmitter in order to collect continuous constant-offset data, either as a stand-alone surveying technique or as a supplement to a node-based survey. Low frequency noise on the towed receiver is significantly higher than that for sea-floor nodes, but at 10โ100 Hz approaches that of sea-floor instruments when the shorter (1โ2 m) antenna length is considered. Early applications of this new technology were limited to sourceโreceiver offsets of a few hundred meters, for fear of the array fouling on the sea-floor. To address this we have developed a telemetry protocol that can be used on twisted-pair copper cables to distances of up to 4 km, allowing real-time monitoring of the array depth during towing. By careful trimming of the buoyancy we are able to "fly" an array of four receivers with offsets of up to 1,000 m at an altitude of 100 m above the seafloor, with only a few meters variation in depth across the array during level flight. Tests were carried out in the San Nicolas Basin, offshore southern California, over an area where a seismic bottom-simulating reflector (BSR) had been identified in heritage seismic data. Increases of 30% in amplitude and 20ยฐ in phase were observed when the array was over the BSR, suggesting minor amounts of hydrate above the BSR or free gas accumulation below the BSR.
- Geophysics > Seismic Surveying (1.00)
- Geophysics > Electromagnetic Surveying (1.00)
Summary Uncertainties in marine controlled source electromagnetic (CSEM) data consist of two independent parts: measurement noise and position uncertainties. Measurement noise can be readily determined using stacking statistics in the Fourier domain. The uncertainties due to errors in position can be estimated using perturbation analysis given estimates of the uncertainties in transmitter-receiver geometries. However, the various geometric parameters are not independent (e.g. change in antenna dip affects antenna altitude, etc.) so how uncertainties derived from perturbation analysis can be combined to derive error-bars on CSEM data is not obvious. In this study, we use data from the 2009 survey of the Scarborough gas field to demonstrate that (a) a repeat tow may be used to quantify uncertainties from geometry, (b) perturbation analysis also yields a good estimate of data uncertainties as a function of range and frequency so long as the components are added arithmetically rather than in quadrature, and (c) lack of a complex error structure in inversion yields model results which are unrealistic and leads to "over-selling" of the capabilities of CSEM at any particular prospect.
Marine CSEM of the Scarborough gas field, Part 1: Experimental design and data uncertainty
Myer, David (University of California at San Diego, BlueGreen Geophysics, LLC) | Constable, Steven (University of California at San Diego) | Key, Kerry (University of California at San Diego) | Glinsky, Michael E. (CSIRO Earth Science and Resource Engineering, University of Western Australia) | Liu, Guimin (BHP Billiton)
ABSTRACT We describe the planning, processing, and uncertainty analysis for a marine CSEM survey of the Scarborough gas field off the northwest coast of Australia, consisting of 20 transmitter tow lines and 144 deployments positioned along a dense 2D profile and a complex 3D grid. The purpose of this survey was to collect a high-quality data set over a known hydrocarbon prospect and use it to further the development of CSEM as a hydrocarbon mapping tool. Recent improvements in navigation and processing techniques yielded high-quality frequency domain data. Data pseudosections exhibit a significant anomaly that is laterally confined within the known reservoir location. Perturbation analysis of the uncertainties in the transmitter parameters yielded predicted uncertainties in amplitude and phase of just a few percent at close ranges. These uncertainties may, however, be underestimated. We introduce a method for more accurately deriving uncertainties using a line of receivers towed twice in opposite directions. Comparing the residuals for each line yields a Gaussian distribution directly related to the aggregate uncertainty of the transmitter parameters. Constraints on systematic error in the transmitter antenna dip and inline range can be calculated by perturbation analysis. Uncertainties are not equal in amplitude and phase, suggesting that inversion of these data would be better suited in these components rather than in real and imaginary components. One-dimensional inversion showed that the reservoir and a confounding resistive layer above it cannot be separately resolved even when the roughness constraint is modified to allow for jumps in resistivity and prejudices are provided, indicating that this level of detail is beyond the single-site CSEM data. Further, when range-dependent error bars are used, the resolution decreases at a shallower depth than when a fixed-error level is used.
- Overview (0.66)
- Research Report (0.40)
- Oceania > New Zealand > South Pacific Ocean > Lau Basin (0.99)
- Oceania > Fiji > South Pacific Ocean > Lau Basin (0.99)
- Oceania > Australia > Western Australia > North West Shelf > Carnarvon Basin > Exmouth Plateau > WA-1-R > Scarborough Field (0.99)
- (2 more...)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (1.00)
- Data Science & Engineering Analytics > Information Management and Systems (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Geologic modeling (0.93)