Advances in Hydrographic Systems Aid Hydrocarbon Explorationand Geohazard EvaluationA paper presented to the HGS Northsiders Luncheon on April 19, 2005.ABSTRACTSeafloor mapping systems, originally developed for defense purposes, were applied to academic projects as early as the 1970’s. Such systems were first applied commercially to cable route surveys with the increase in global telecommunications traffic and explosion in fiber-optic cable laying in the 1980’s. In the 1990’s, two separate trends lead to an increase in hydrographic surveys for the oil industry. First, the resolution of mapping systems improved significantly, and second, the push to deeper water made such systems relevant to oil industry problems. Marine geologists and geophysicists are accustomed to the 100% seafloor image that results from a good water bottom pick from 3D seismic data. Hydrographic systems can provide a similar image. The water bottom obtained from seismic can proved bathymetry and amplitude (a function of impedance), whereas the water bottom obtained from a hydrographic system can provide bathymetry and backscatter (a function of impedance and roughness, as well as volumentric scatterring from below the mudline). 3D seismic can of course provide a volume of data, allowing sub-surface horizons and faults to be picked with confidence. Hydrographic surveys provide a comprehensive image of only the seafloor horizon. Hydrographic surveys, however, can be conducted rapidly, although the amount of area surveyed per day will decrease as a function of water depth (800 sq. km. per day in water deeper than 1000m; 400 sq. km. per day in water depths of ~500m; 100 sq. km. per day in water depths of ~100m). Data can be processed within 12 hours of acquisition, and cost approximately 100 times less than 3D seismic (per sq. km.). In this presentation, we will discuss the application of hydrographic techniques to both exploration and geohazards. ExplorationIn exploration, a hydrographic survey can be the foundation of a two-stage field program whose objective is to reduce exploration risk or rank prospect areas, typically in a block with little previous exploration and no known discoveries. We typically begin with an analysis of satellite derived synthetic aperture radar (SAR) images, which can image sea surface slicks. Such slicks may provide an indication of hydrocarbon seepage to the sea surface, and the location of the slicks can be tied to the possible sea-floor origination points of the seeps. Our approach for the first stage of the survey is to combine a hull-mounted survey system (mulutibeam) with other acquisition programs that are amenable to a ’mowing the lawn’ survey strategy; this can include the acquisition of gravity, magnetics, and sub-bottom profiler. The combination of these data sets can define the tectonic fabric within a block, indicate the presence of volcanics or the location of thick basin sequences, image shallow faults and indications of fluids at and immediately below the mud-line (Figure 1).Such ’mowing the lawn’ surveys can be conducted at 10 knots, with short (15 minute) line turns. We can also acquire 2D seismic data during these surveys, although this requires a slower survey speed (4.5 kt. max), and the 2D is typically a hybrid between the traditional long-cable 2D and the short-cable used for geohazard surveys. In previous programs, we have used a 4.5km streamer and 1700 cu. in. gun array to acquire 8 second data (25m bin) at a survey speed of 4.5 knots. Figure 1: Hydrographic survey (multibeam bathymetry) of a mud volcano province in an exploration region. Colors indicate depth (warmer colors = shallower), whereas brightness indicates slope (edge map). Note the broad seafloor dome, and the numerous circular highs in the field of view. These circular highs indicate the presence of “mud volcanoes” up to 1500m across, which may provide conduits for fluids from depth. Regions of rapidly changing slope suggest geologic “youthfulness”, and can be used to guide sampling strategies when combined with multibeam backscatter. Inset: photograph of a small positive relief ‘gryphon’ (side vent) on the Dashkili mud volcano, Azerbaijan, showing actively seeping (bubbling) hydrocarbon-rich fluids. Knife for scale. Note small mud flow in background.100% seafloor maps of an area can provide valuable information regarding the tectonic and sedimentary processes that are active today, or that have been recently active and impact the seafloor (e.g.: Figure 2). For example, the location of seafloor slumps may provide information on the distribution of shallow gas charging or hydrate, or the location and style of slope canyons can provide information on the style of sediment by-passing of the slope to basin floor fans. ”The present is the key to the past”, and we have found in previous projects that the high-resolution data we acquire at the mudline can be applied to exploration relevant questions. Figure 2: Multibeam data maps of an exploration block approximately 40 km x 80 km in the central Makassar Straits, Indonesia (after Decker et al., 2004). On the left is a high resolution bathymetric map (25m bin size; no interpolation or smoothing; water depths range from 2250 to 2450m) and on the right is a backscatter mosaic (5m pixel) from the same area (backscatter ranges from a high of -5 db (light) to a low of -60 db (dark)). The survey of this block documented an unexpected (based on bathymetry alone) and spectacular basin floor fan, which was subsequently sampled to evaluate the origin of the backscatter and the sediment distribution within the channel-fan system. The first stage data can be processed and interpreted to identify possible sites of seafloor fluid seepage. By applying a stacking approach similar to that used for seismic, on hull-mounted surveys acquired at up to 10 knots we have been able to achieve a bathymetric bin size of 25m, and a backscatter pixel size of 5m in water depths up to 2500m deep. This resolution is sufficient to identify relatively small sites of fluid seepage on the seafloor. The acquisition systems can be adjusted to acquire more square kilometers per day (wider swaths), athough this will result in lower resolution data.