ASEG Extended Abstracts - ASEG2013 - 23rd Geophysical Conference, 2013

We tested plausible initial 3D density models across the eastern Cook Inlet basin to constrain the geometry of the Border Ranges fault system (BRFS). The BRFS forms the structural boundary between the forearc-arc structures and accretionary terranes of the Aleutian subduction zone in southern Alaska. We used 2D density cross-sections as starting “a priori” models to constrain the 3D inversion model. We computed the theoretical gravity based on the inversion, compared it to the observed gravity, and determined the gravity misfit. We tested the 3D inversion solutions on the lowland regions where the densest spacing of gravity points is located using three surfaces defining the topographic ground level, the top and base of the sediment layers within Cook Inlet basin to build the preliminary 3D models. Preliminary results show the 3D inversion is able to constrain the simple shape of Cook Inlet basin with geologically reasonable densities. We will produce several alternative starting models for the entire Cook Inlet forearc basin and the BRFS. The most reasonable models will be used as starting models for the structure of the entire study area.

The Newer Volcanic Province (NVP), western Victoria, Australia, represents the most extensive and youngest volcanism of the entire intraplate volcanic field of eastern Australia. The nature of, and mechanism(s) for, melting of the source magma of the NVP is still unclear. Previous teleseismic studies associate the magma genesis for the NVP to conduits of a mantle plume. Here we present data from a long-period MT array conducted over the same grid as the teleseismic survey, across the southern end of the Lachlan and the Delamerian Orogenies, western Victoria in a rectangular grid with nominal 270 km x 150 km dimensions. Forward modelling of MT data suggests that the lithosphere beneath the Lachlan orogeny is more conductive than the Delamerian counterpart by several orders of magnitude, perhaps associated with thinning of the lithosphere beneath the Lachlan orogeny. The phase tensor analysis illustrates that there is an increasing conductivity trend beneath the Central Highlands, observed up to 500 s, that is perhaps associated with NVP magma source region. Furthermore, the geoelectric strike direction beneath the Central Highlands is aligned parallel to the NW-SE Mesozoic-Cenozoic fracture zones, which coincides with the highest density of eruptions of the volcano field.

The Center for Geological Survey (CGS) of Geological Agency, Indonesia, known as Pusat Survei Geologi, commissioned an airborne magnetic and radiometric survey covering the Indonesian Papuan Central Highlands Region and the southern side of the highlands during 2010 and 2011. The survey was funded by the State Revenue and Expenditure of the Ministry of Energy and Mineral Resources of The Republic of Indonesia and covered an area of 156,964 square kilometres. Geological mapping in this area is recognised as being difficult predominately due to the extreme terrain and weather. The objective of the survey was to map the surface geology as well as sub-surface geology, identify and map the structure of the region, to model new geophysical data along with previously collected ground gravity data, and integrate all the results with the previously known geology. The expected outcome would be a better understanding of the geology and structure in the region and an increase in mineral and oil and gas exploration for Indonesia. The survey consisted of a helicopter based magnetic and radiometric survey in the Central Highlands Region at 500 m spacing totalling 20,045 km and a fixed- wing aircraft based magnetic and radiometric survey over the south side of the highlands at 1,000 m spacing totalling 155,530 km. The results highlighted and extended the geological, structural, and tectonic evolution knowledge of the region and identified areas for further mineral and oil and gas exploration. The modelling of the magnetic and gravity data supported the interpretation and added further information to the depth analysis of the data.

One of the limitations in seismic waveform inversion is noise, which plays a role in impeding that the FWI converges on the global minimum solution. We first analyse noise behaviours in the inverse problem and limitations of the conventional FWI strategy for noisy data. For a more robust inversion algorithm, we propose applying a frequency-depth scaling strategy. The frequency-depth scaling strategy is designed using the denoise function for both the spectral scaling and the flexible damping factor of the Levenberg-Marquardt method for the depth scaling so that the influence of strategy that combines the spectral scaling strategy using the denoise function suggested by Oh and Min (2012) and the depth scaling strategy using the constraint of the Levenberg- Marquardt method (Lines and Treitel, 1984).

Geophysical methods including electrical resistivity and MASW are used for delineating seawater inflow through sea dike and detecting the abnormal compactness region that could affect the structure of dike. To identify the part of anomalous seawater inflow through the sea dike, we periodically carried out 3D resistivity survey along the dike including anomalous regions. 3D resistivity survey and monitoring for the dike can define effectively the low resistivity zones lower than 1 ohm-m, which may indicate seawater inflow through the dike. For detailed analysis of 3D resistivity monitoring data, time-lapse inversion method was adopted in this study. Time-lapse inversion is effective for identifying the subtle changes over time and for suppressing inversion artifact. The results of time-lapse inversion method show no significant changes in the sea dike with time. Shear wave velocity profiles obtained by MASW periodically indicate less compacted layer that could be originated by the loss of dredge sand or the bad compactness condition during the construction and would be predicted the possibilities of subsidence. From this study, 3D electrical resistivity surveys using time-lapse inversion approach and periodic MASW surveys are revealed to be effective for identifying seawater inflow pathway through the sea dike and investigating its safety, respectively.

We develop an inversion strategy for seismic full waveform inversion (FWI) for 2D vertical transversely isotropic media (VTI). In our strategy, we use Lamé constants (? and μ) instead of C33 (= ? + 2 μ) and C44 (= μ). For C11 and C13, we redefine them by Lamé constants and residuals between anisotropic parameters (C11 and C13) and Lamé constants. Accordingly, these residuals represent anisotropic characteristics. In FWI, we invert Lamé constants and residuals, and then we extract C11, C13, C33 and C44 from the inverted parameters. We validate our new strategy by comparing inversion results obtained by the new inversion strategy to those obtained by the isotropic inversion and the conventional VTI inversion for a modified version of the overthrust geological model. Results obtained by the isotropic inversion are reliable only for isotropic layers. On the other hand, anisotropic layers cannot be recovered well by only ? and μ. In case of the conventional VTI inversion, C33 and C44 are well inverted. These results agree well with the isotropic inversion results. The C11 and C13 models inverted by the conventional VTI inversion are reasonable, but their resolutions are poorer than those of the C33 and C44. On the other hand, the C11 and C13 models inverted by the new inversion strategy are much improved compared to those of the conventional VTI inversion.

From these results, we note that anisotropic properties in subsurface media cannot be recovered by the isotropic inversion, whereas the developed inversion strategy can provide reasonable anisotropic characteristics. In addi- tion, elastic parameters by the new inversion strategy are more precise and stable than those of the conventional isotropic and VTI FWIs.

An area exists on the South Sea of Korea where the magnetic compass is said to turn round and round not giving a fixed direction to the navigator.

We newly assembled a three axis magnetometer system composed of a three axis fluxgate magnetometer, a GNSS compass, a two axis clinometer, and a multi-channel data logger, to investigate if there really exists such an area.

We supposed that the horizontal component of the vectorially measured magnetic field on such an area will be too small to maintain the magnetic compass’s needle to one fixed direction. We processed the measured data mainly through two steps. Firstly we transformed the random coordinate system into a fixed coordinate system, i. e., into the geographical coordinate system. Secondly we performed an inversion to eliminate the effect of the ship itself from the measured data.

On the finally achieved anomaly map we could not find such an area where the horizontal component is so small that it could not maintain the magnetic compass’s needle to a fixed direction.

We conclude that such statement about the existence of the so-called magnetic north immeasurable area on the South Sea of Korea does not have a strict scientific base.

We report on our experience gained in applying the multiple-mode spatially-averaged coherency method (MMSPAC) at 25 sites in Newcastle (NSW) for the purpose of determining shear-wave velocity profiles as part of an earthquake hazard study.

The MMSPAC technique is logistically viable for use in urban and suburban areas, both on grass sports fields and parks, and on footpaths and roads. A set of seven recording systems designed for earthquake after-shock observations plus a team of three personnel is sufficient to survey three sites per day.

The uncertainties relating to local noise sources from adjacent road traffic or from service pipes contribute to loss of low-frequency SPAC data in a way which is difficult to predict in survey design. Coherencies between individual pairs of sensors should be studied as a quality-control measure with a view to excluding noise- affected sensors prior to interpretation; useful data can still be obtained at a sites one sensor is excluded.

The combined use of both SPAC data and horizontal: vertical spectral ratio (HVSR) data in inversion and interpretation is necessary in order to make effective use of low frequency data (typically 0.5 to 2 Hz at these sites) and thus resolve shear-wave velocities in bedrock below 20 to 50 m of unconsolidated sediments.

We have developed WiBand, a de-ghosting and broadband processing method that can be applied to conventional streamer data acquired with single component streamers. We review two case studies of WiBand processing. In the first case study, data with streamers towed at different depths is used. We apply WiBand to each track independently to remove the effects of the source and receiver ghosts. We compare the WiBand results from a deep tow track to conventionally processed result from a shallow tow track to validate the phase fidelity of the de-ghosting process. We conclude from the results that WiBand correctly removes the ghosts and recovers broadband data from deep tow streamer data. In the second case study, we compare inversion results from conventional processing and WiBand processing, and observe improved correlation with well logs.

The oil and natural gas energy supply situation changed dramatically in the United States and in North America between 2000 and 2013. A new ability to extract economically both natural gas and oil from shale resource rocks has recently made the United States the world’s largest producer of natural gas and reversed a long-term decline in oil production. The economic impact of these changes is not only being felt in North America, but throughout the world, as the United States supplies more of its energy needs from domestic sources and prepares to export liquefied natural gas (LNG) globally.

The technology and business models that created this revolution are now being tested to various degrees throughout the world. Even if the export of these models is only partially successful, it will have a profound impact on many of the current producers and consumers of fossil fuels, and on industries which support shale resource exploitation.

A major beneficiary of the shale resource explosion is microseismic monitoring technology. Monitoring fracture stimulations in shale is a large and growing service industry with rapidly evolving capabilities, and concerns about induced seismicity associated with the disposal of waste water from fracture stimulation operations are starting to create a similar demand for long-term microseismic monitoring services.

In January 2012, the storm clouds of Tropical Cyclone Iggy were gathering over the Timor Sea as acquisition of the 500 km2 Zeppelin 3D seismic survey began in the Vulcan sub-basin permits AC/P 50 and AC/P 51. Despite the high seas and the adoption of a conventional survey design, the Zeppelin 3D appears to have set a new benchmark for seismic data quality in the Vulcan sub- basin, which has traditionally been considered a ‘difficult data area’.

Reprocessing some of the overlapping legacy Onnia 3D seismic suggests that much of the imaging improvement stems from a multiple-suppression similar to that used recently in other ‘difficult’ Australian basins. The combination of shallow water demultiple, SRME and gap deconvolution followed by a stratigraphically-controlled velocity analysis appears to account for much of the improvement. Shooting in the (dominant) dip direction also appears to have helped although had 3D SRME been applied this might have been rendered less important.

Using the reprocessed Onnia 3D and the Zeppelin 3D, our seismic interpreters have been able to develop a detailed structural model which underpins the distribution of fault-bend fold related prospectivity. Enhanced resolution enables sequence stratigraphy to be delineated. A series of structures within 3 separate plays (Triassic, Jurassic and Cretaceous) define a significant increase in the assessed oil and gas volumetric potential thereby justifying the early investment in seismic.