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

Gravity gradiometry offers multiple single components and possible combinations of components to be used in interpretation. Knowledge of the information content of components and their combinations is therefore crucial to their effectiveness and so a quantitative rating of information level is needed to guide the choice. To this end we use linear inverse theory to examine the relationship between the different tensor components and combinations thereof and the model parameters to be determined. The model used is a simple prism, characterized by seven parameters: the prism location, xc, yc, its width w and breadth b, the density ρ, the depth to top z, and thickness t. Varying these values allows a wide variety of body shapes, e.g. blocks, plates, dykes, rods, to be considered. The Jacobian matrix, which relates parameters and their associated gravity response, clarifies the importance and stability of model parameters in the presence of data errors. In general, for single tensor components and combinations, the progression from well- to poorly-determined parameters follows the trend of ρ, xc, yc, w, b, z to t. Ranking the estimated model errors from a range of models shows that data sets consisting of concatenated components produce the smallest parameter errors. For data sets comprising combined tensor components, the invariants I1 and I2 produce the smallest model errors. Of the single tensor components, Tzz gives the best performance overall, but those single components with strong directional sensitivity can produce some individual parameters with smaller estimated errors (e.g., w and xc estimated from Txz).

A special challenge of hard rock exploration is to identify targets of interest within complex geological settings. Interpretation of the geology can be made from direct geological observations and knowledge of the area, and from 2D or 3D seismic surveys. These interpretations can be developed into 3D geological models that provide the basis for predictions as to likely targets for drilling and/or mining. To verify these predictions we need to simulate 3D seismic wave propagation in the proposed geological models and compare the simulation results to seismic survey data. To achieve this we convert geological surfaces created in an interpretation software package into discretised block models representing the different lithostratigraphic units, and segment these into discrete volumes to which appropriate density and seismic velocity values are assigned. This approach allows us to scale models appropriately for desired wave propagation parameters and to go from local to global geological models and vice versa. Then we use these digital models with forward modelling codes to undertake numerous 3D acoustic wave simulations. Simulations are performed with single shot and with exploding reflector (located on extracted geological surface) configurations.

As the search for mineral deposits moves to greater depths, seismic methods, with its penetration ability and unmatched resolution power, are becoming more important tool for exploration of mineral resources underneath the deep cover. However the performance of seismic appears to be still inconsistent which prevents it from becoming the primary exploration method in the mineral sector, similar to its role in oil exploration. The performance of seismic methods is affected by complex geology, excessive ambient noise, access restriction, weak reflectivity and/or low signal to noise ratio, limited acquisition program due to cost restriction, etc. Two other factors are emerging recently as important: a) lack of correlation of seismic images and b) miss-match between survey design and target characteristics. The first one is the greatest threat to the affirmation of seismic in the mineral sector as it prevents seismic images to be utilised in any constructive way. The second one translates to the use of simplified 2D geometries to delineate complex 3D structures which may cause seismic to underperform. On the positive side there are clearly favourable cases for the application of reflection seismic. Those can be primarily related to the massive, concentrated mineralisation such are massive sulphides.

The less favourable geological settings require much more elaborate analysis to allow seismic method to perform. It appears that the lack of understanding of the complexity and the variability of seismic responses in different geological settings is what still prevents the widespread use of this method for mineral exploration. In general seismic could be useful in different ways, from regional to deposit scale and from exploration to production stage.

Magnetotelluric data was collected across the Habanero geothermal site in the Cooper Basin, South Australia, in view of delineating the crustal structure underneath the geothermal area and to monitor fluid injection of the Habanero 3 borehole. Two surveys were carried out. Initially, two perpendicular profiles have been stablished, each about 20 km long, to obtain 2D profiles across the Habanero site. The aim is to estimate the sediment thickness to allow for constrained 3D forward modelling for fluid injection scenarios. Furthermore, the broadband data with periods up to 1000s allows a view into the crust. Geochemical data from the Mound Springs shows that the seeping water contains a minor component of mantle CO2. MT can be used to image the fluid pathways if such a connection exists. The results will be compared to recent findings from fluid pathways in the Lake From embayment a few hundred km south of the Cooper Basin. Additionally, initial results from the fluid injection monitoring experiment of the Habanero 3 borehole will be shown. In November 2012, fluids were pumped at depth of around 4 km for a period of two weeks. The fluids exceed the amount introduced into the Paralana EGS in July 2011, however a pre-existing fluid reservoir from a prior fluid injection exists. We report on surface MT response changes due to the fluid reservoir at depth.

Tendaho is one of the high temperature geothermal areas in Afar depression in north east Ethiopia. A total of 129 MT sites were acquired from Tendaho high temperature field. The 2D inversion of MT data from Tendaho high temperature field revealed three main resistivity structures down to a depth of 10 km: low resistivity surface layer underlain by a resistive layer followed by good conducting structure. The low resistivity surface layer show areas with either sediments, lateral flow of geothermal fluids or zeolite-clay alteration zone. Below the conductive layer, is a high resistivity zone that can be correlated to Afar stratoid basalts or epidote alteration zone .The high resistivity structure has been associated with the deep reservoir of the geothermal system. The deep good conductive body is probable heat source of the geothermal system.

The application of equipotential surface curvatures and Poisson’s relation to airborne gravity gradient data is presented. The mean and differential curvature of the equipotential surface, the curvature of the gravity field line, the AGG Geometry Map, the zero contour of the Gaussian curvature of the equipotential surface and the zero contour of the determinant of the gravity gradient tensor should improve the understanding and geological interpretation of gravity gradient data.

Seismic vibrators are the preferred sources for land vertical seismic profile (VSP) surveys as they are relatively repeatable, controllable, have high energy, have a low environmental impact, and are cost-effective. Unfortunately, due to the mechanical and hydraulic constraints of the vehicles, the typical swept bandwidth has been quite limited, typically of the order of 3 octaves. In this paper we show that dramatic improvements in seismic data quality can be achieved by extending the bandwidth used for Vibroseis VSPs. These increases are dependent, however, on the use of modern vibrators, specialised low-frequency enhancing sweeps, and downhole sensors with an extended frequency response range.

Seafloor canyons and complex seafloor topography pose significant challenges when analysing seismic data from the North West shelf off the Western Australian coast. Several prolific gas fields in this area lie beneath the continental shelf break, which contains large canyons that cause significant seismic amplitude distortion and complex wavefield behaviour (e.g. scattering and wavefield multi-pathing), and lead to irregular and poor illumination, unreliable AVO analysis, and difficulties in velocity model building. To illustrate these issues we present 3D elastic finite-difference simulation results from a model of a region of the North West Shelf in Western Australia. Using a high-performance computing cluster we model elastic wavefields through complex seabottom topography. We simulate plane-wave propagation through a bathymetric model to generate a pseudo stack. We observe many expected complex effects including wavefield (de)focusing, diffractions and triplications. We also measure wavefield amplitude variations of a factor of four over scale lengths of a few hundred metres; this is sufficient to cause significant imaging issues. By accurately modelling full 3D wavefield effects we can now generate data to benchmark existing algorithms and develop new techniques/algorithms for handling complex bathymetry.

Consistent and successful mineral exploration requires substantial experience and a broad knowledge base that traditionally takes years of resources to develop through hands-on experience in multiple and varied exploration projects. This creates a critical industry issue that implies many exploration decisions are sub-optimal prior to attaining an appropriate level of experience.

To address this issue, we present the unique and novel e- learning environment, exSim, which simulates exploration scenarios where users can develop and test their strategies and assess the consequences of their choices. This simulator provides an engaging platform for self-learning and experimentation in exploration strategies, providing a means to build experience more effectively.

We present an initial prototype of exSim, displaying features to facilitate decisions in ground selection, geophysical surveys, drill testing, and interpretation.

With increasing demands on groundwater resources, concerns about the impacts of groundwater abstraction on groundwater dependent ecosystems and the new paradigm of large scale managed aquifer recharge, there comes a requirement for more robust definition for water resources. Seismic reflection surveying may offer this higher level of definition. Resolution at depth and the ability to map detailed structures cannot be matched with any other method. Seismic reflection is able to recover information that may contribute to revealing aquifer geometry and system fluxes. It also increases the value of existing borehole information. Take up of seismic reflection by the groundwater industry has been slow but is certainly gathering momentum with several high resolution basin scale surveys now providing clear examples of the value of seismic reflection. Seismic reflection surveying is expensive, so the key to success is strategic location of lines and selection of suitable acquisition parameters with sufficient resolution to answer key questions regarding the targeted hydrogeological system. We present several examples from the West Australian aquifer systems, compare the acquisition parameters selected and then clearly identify hydrogeological value of the outcome for each setting.

In October 2012 a series of known oil & gas fields in the North Sea were surveyed with the newly developed towed streamer EM system. This is the first commercial Controlled Source EM (CSEM) system where both source and receivers are towed in a similar fashion as 2D seismic, and the technology is also combinable with 2D seismic facilitating simultaneous acquisition from one vessel. One of the selected targets was an average size oil & gas field located 2,100 m below mudline, where approximately half of the recoverable oil has been produced, but with the gas cap still intact. The source was towed at 10 m and the 8,700 m long EM streamer was towed at a depth of 50 m. The resulting electric field was measured at 23 offsets ranging from 0 to 7,700 m. The towing speed was 4 kn, and the water depth was 110 - 125 m in the survey area. The rich sampling makes the towed streamer EM acquisition technology ideal for the recently introduced synthetic aperture processing. The accuracy in the frequency responses enabled the detection of the depocenter of the reservoir with a signal strength only 7-8% above background with an uncertainty of 5% in a non-optimized synthetic aperture processing. The improved sensitivity provided by optimized synthetic aperture processing increased the signal above background to 200%. Further, by focusing the energy on the target it will also de-emphasize the strength of the anomalies of no interest located in proximity to the reservoir under evaluation.

Simultaneous AVO inversion of seismic data is an integral part of oil and gas exploration. Traditionally this technique is applied to surface seismic data, relying on availability of various angle stacks as well as on presence of offset wells for low frequency models. This paper investigates how a Walkaway Vertical Seismic Profile (WVSP) can be utilized for localized AVO inversion. Traditionally WVSP are designed for one of the following three purposes: imaging, anisotropy and AVO/AVA analysis. This paper shows a set of conditions that allow for WVSP data to be inverted following a commonly utilized simultaneous seismic inversion technique. The advantage of performing a localized WVSP AVO inversion is that low frequency models are measured during WVSP acquisition. This significantly reduces the ambiguity of the background models for inversion. Another advantage is that the inversion input derived during WVSP migration comes from measured vertical velocities, thus allowing more accurate angle stacks, thus improving the quality of the inversion. Lastly, WVSP images are generally of higher frequency content, thus producing high frequency density, Acoustic Impedance and VpVs images in the vicinity of the wellbore. The results can assist in calibrating the surface seismic inversion once more well data are available as well as making quick drilling decisions for deepening or side tracking wells.

Subsurface resistivity is a key component of many mineralization models including unconformity-related uranium, palaeochannel hosted uranium and nickel sulphides. Other key applications of subsurface resistivity involve environmental aspects of the subsurface such as groundwater detection, and civil engineering applications including detection of buried pipes and cables. Measuring subsurface resistivity is the aim of electromagnetic (EM) geophysical techniques. It involves transmitting an electromagnetic field into the Earth, and then recording this field - and the Earth response - on a receiver. The transmitted field signal can be removed from the received field to determine the Earth response.

Airborne EM (AEM) is a geophysical technique that allows this process to be undertaken from an airborne (aeroplane or helicopter) platform. AEM exploration first commenced in South Australia with AFMAG (Audio- Frequency Magnetic technique) surveys in the 1960s, and VLF (Very Low Frequency) surveys from 1971. Numerous platforms including RepTEM, GeoTEM, Input, QUESTEM, HoistEM, TEMPEST and VTEM are now routinely used within South Australia. Each technique provides a different view of the subsurface dependant on the system parameters and the processing undertaken on the data.

Given the increase in AEM surveying within South Australia and the wide applications available for this data a review of this technique within the State has been undertaken. This poster presents a summary of AEM within South Australia, focussing on a number of significant surveys and their outcomes. Surveys in the Cariewerloo Basin and Fowler Domain in particular have been used to model uranium prospectivity and help define nickel deposits. All data reviewed is now downloadable online via SARIG.

Current models for regional mineral exploration targeting for nickel and gold mineralisation emphasize the significance of deep penetrating geological structures and the margins of cratonic blocks as areas of greatest prospectivity. As part of a study on regional prospectivity, magnetotelluric (MT) surveys have been completed in several prospective Proterozoic and Archean terrains in Western Australia. These data, which have been interpreted in associated with potential field, seismic, geological and geochemical data, demonstrate that MT surveys can be used to identify such prospective features based on variations in the electrical conductivity of the crust and upper mantle. For example, a survey in the southern Yilgarn Craton has identified lateral changes in deep crust and upper mantle conductivity structure consistent with palaeo-cratonic boundaries inferred from studies using isotope geochemistry. The MT data allow the boundaries to be accurately located; the isotopic results being limited by the spatial distribution of outcrops of suitable lithotypes.

The areas of interest in Western Australia are geographically remote and often environmentally and culturally significant. MT surveys represent a comparatively cheap means of evaluating regional prospectivity, whilst causing minimal cultural and environmental disturbance.

A modified one-way equation pre-stack depth migration of up-going and down-going pressure wavefields was applied to two datasets derived from 3D towed dual- sensor streamer data in offshore Australia and Malaysia. The primary objective was to mitigate the well-known cross-line acquisition footprint effects upon shallow data quality and interpretability.

The new methodology introduced here exploits the illumination corresponding to surface multiple energy and thus exploits what has historically been treated by the seismic industry as unwanted noise. Whereas a strong cross-line acquisition footprint affected the very shallow 3D data using conventional processing and imaging, the new results yield spectacular continuous high resolution seismic images, even up to, and including the water bottom. One implication of these results is that very wide-tow survey efficiency can be achieved without compromising shallow data quality if dual-sensor streamer acquisition and processing is used, even in very shallow water areas such as that discussed here. The imaging methodology can account for all degrees of lateral variability in the velocity model, full anisotropy, and angle gathers can be created to assist with velocity model building.

An innovative 3D towed streamer project in offshore Gabon used a dual-vessel continuous long offset streamer configuration to acquire 0-12 km offsets with ten dual- sensor streamers. Streamer control for the 6 km streamers was robust and avoided operational complications or logistical penalties. Simultaneous shooting maximized inline shot density for long record lengths, thus capturing unaliased deep target reflections from rugose base-salt and sub-salt horizons. Survey design benefited from prior 2D survey experiences with a variety of broadband source and streamer technologies, and the use of 2D streamers as long as 12 km. 3D illumination modelling further suggested that offsets as long as 16 km could be expected to yield useful base-salt and sub-salt reflections.

Wavefield separation processing yielded full receiver- side deghosting onboard, followed by an inversion-based separation of simultaneous shots onshore. The ultra-long 12 km offsets combined with strong amplitudes of deghosted low frequencies have yielded encouraging sub- salt and pre-salt imaging.

To interpret magnetic responses of different mineral settings in Papua New Guinea, field campaigns were conducted by the Geological Survey Division in two mineral fields in the Rigo District, Central Province.

In December 2011, geological mapping accompanied by a ground magnetic survey was conducted over iron-rich gossan within a volcanic sequence at Kore. In March 2012, an analogous program was undertaken in a sedimentary-hosted manganese field near Kemaea village.

Ground magnetic surveys in both areas comprised a series of north-south magnetic profiles, up to 1 km in length separated by 100 m.

The results of the magnetic survey show significantly different responses that are apparently related to the style of mineralisation.

The total magnetic response of a gossan within a volcanic rock unit is characterised by high frequency signals that require extensive filtering to outline the trend of distinct magnetic source. In contrast, the limestone-hosted strata- bound manganese deposit has a well-defined north- westerly structural trend that is easily distinguished by the response in the total magnetic field.

Applying a reduced to magnetic equator algorithm and generating an upward continuation of the magnetic field enhances these structural trends.

The results of the survey demonstrate that background knowledge of mineral systems, including major mineral composition and style of deposit is essential to interpret imagery from ground magnetic surveys of mineral deposits and that different styles of mineralisation generate unique magnetic responses.

Enhancement of potential field datasets using operators based on one or more of the spatial derivatives is common practice. The performance of these methods in the presence of noise is poorly understood; other than a general acceptance that they can be significantly affected, especially when higher order derivatives are used. Most published descriptions which involve noise tests use random noise and a dense and uniform sampling of the test region. More realistic tests of the effects of noise should account for the incomplete and anisotropic sampling within most datasets and also correlated noise such as due to incorrect levelling. An understanding of the effects of noise on the different methods of enhancement is particularly important when working with lower quality (older) and lower resolution datasets.

Interpretation of geophysical data from West Africa, as part of a major project on the prospectivity of the region, is being undertaken. Much of the data available is of relatively low quality and resolution. An important component of the work will involve determining how best to enhance the gravity and magnetic datasets. Initial results working on gridded data show that the “generalized derivative operator” is the most robust derivative based enhanced product for low resolution data.

Beam Migration has specific advantages, in its speed and high signal-to-noise levels, which make it suitable for both depth velocity modelling and final imaging. One property of Beam Migration is that wavelets can, based upon a combination of criteria, be weighted down or excluded from reconstruction. It is possible to reject wavelets that match a multi-dimensional multiple model. This model is based upon a combination of interpretation, normal moveout and spatial & temporal location.

Data examples from the North West Shelf, Australia, illustrate the flexibility of this approach and demonstrate its effectiveness in the complex inter-bed multiple area of Browse Basin.