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

Since the 1989 Newcastle ML5.5 earthquake, the city of Newcastle, Australia, has become a focus for earthquake risk assessment. Surficial geology in the area varies from deeper alluvial deposits near the Hunter River, to shallower soils overlying weathered rock on the valley margins. Ambient vibration techniques, based on the dispersion property of surface waves in layered media, are promising for assessing the subsurface geophysical structure, in particular the shear-wave velocity (Vs). Using one such technique, the Spatial Auto-Correlation (SPAC) method, we characterise soil deposits at 23 sites in and around the city of Newcastle. Results show that values for soil overlying bedrock ranges from 200 m/s to 1000 m/s, with the higher velocity values observed in shallow soils which are relatively consolidated and distal to the river. Bedrock depth varies from 6 to 56 m, but an accurate quantification is hampered by the low frequency picks (< 2 Hz) which are either unavailable or of dubious quality. Some Vs profiles show two abrupt changes, the first at ~ 4-15 m depth and the second ~19-56 m. Low Vs values are of particular interest as they may indicate areas of higher seismic hazard.

Uncertainty in a migration based approach to surface and near surface microseismic monitoring occurs in two ways: uncertainty in the validity of detected events and uncertainty in the estimated position of the event. Synthetic modelling and comparison to case studies show that sign-to-noise-ratio is a key indicator of both types of the uncertainties. In this paper we present an analysis of both types of uncertainty using synthetic modelling to illustrate the performance characteristics of the migration process in terms of signal detection and false-alarm rates, along with uncertainties in positional estimates.

Examples from two case studies will illustrate that this kind of performance is achievable in actual monitoring surveys. Signal-to-noise-ratio (SNR) is a key indicator of the uncertainty in migration based imaging of microseismic events. Reliability, in terms of the ability to detect the complete set of events is a nearly binary function of SNR. Events with SNR above a threshold of 2-3 are readily detected, while events with SNR below the threshold are missed. Positional uncertainties likewise are driven by SNR. While vertical uncertainty is more sensitive to noise, both horizontal and vertical uncertainties decrease rapidly with increasing SNR.

Microseismic monitoring is attracting great interest due to the application of passive seismic to shale play completion activities and the successful expansion of the method from downhole to surface and near-surface acquisition geometries. Fundamental to this application is the science behind the interpretations: The inherent capabilities and limitations of downhole, surface and near-surface recording systems and the processing and imaging applications enabled by these recordings must be appreciated to understand the results.

These considerations inform issues such as: microseismic event detectability and position uncertainty; the characterization of geological features; sensitivity to hydraulic fracturing methods; rock failure modes; and well to pad to field-wide implications of large scale ‘horizontally drill and hydraulically fracture’ development programs.

Ultimately interpretation workflows determine microseismic event pointsets, modelling of discrete fracture networks and calculation of stimulated rock volumes.

As microseismic monitoring matures understanding the relationship of recording geometry, imaging capability and interpretation workflows will fuel expanded utilization.

Here the basic issues surrounding passive seismic acquisition methods and microseismic interpretation will be reviewed and discussed.

Geophysical inversion employs numerical methods to minimise the misfit between three-dimensional petrophysical distributions and geophysical datasets. Inversion techniques rely on many subjective inputs to provide a solution to a non-unique problem, including use of an a priori input model or model elements (a contiguous volume of the same litho-stratigraphic package) and inversion constraints. Inversions may produce a result that perfectly matches the observed geophysical data but still misrepresents the geological system. A workflow is presented that offers multiple starting models to inversion: (1) simulations are performed to create a model suite containing a collection of geologically possible models; (2) uncertainty analysis is then performed using stratigraphic variability to identify low certainty model regions and elements; (3) ‘Geodiversity’ analysis is then conducted to determine the geometrical and geophysical extremes within the model space; (4) geodiversity metrics are then simultaneously analysed using principal component analysis to determine which models exhibit common or diverse geological and geophysical characteristics, enabling the selection of models that are subjected to geophysical inversion.

The Ashanti Greenstone Belt, southwestern Ghana in west Africa is used as a case study to test the value of this workflow. Analysis of inversion results are performed that finds a correlation between regions of geological uncertainty and geophysical misfit. This correlation strongly suggests that geological uncertainty can be used as a powerful geological constraint to optimise inversion processes and produce geologically reasonable models.

Erzincan (a small city in Eastern Turkey) is located in the conjunction of three active faults: North Anatolian, North East Anatolian and East Anatolian Fault Zones. Erzincan city centre is in a pull-apart basin underlain by soft sediments which significantly amplify the ground motions. Combination of the tectonic and geological settings of the region have led to destructive earthquakes such as the 27 December 1939 (Ms=8.0) and the 13 March 1992 (Mw=6.6) events resulting in extensive losses. In this study, in order to perform site characterization in the region, we employ the microtremor survey technique at different sites near the Erzincan city centre. The use of the microtremor method is a well-known passive seismic tool to estimate the properties of sedimentary overburden. A total of 9 sites were selected to be aligned in the north-south and east- west directions. At each site, we performed surface wave dispersion curve analysis using the Multi-Mode Spatial Autocorrelation (MMSPAC) technique on either single or nested arrays. The MMSPAC method uses primarily the fundamental modes for estimating S-wave velocity profile up to bedrock levels, and can also identify higher modes of the seismic energy if present. We present our results in the form of one dimensional velocity structure as well as fundamental frequencies obtained from the microtremor HVSR spectra computed at selected sites.

The most cost-effective way to acquire 3D seismic data offshore is by towed-streamer survey; the majority of these surveys are acquired using a vessel shooting a series of parallel straight lines to cover the survey area. When each line is completed, the vessel turns and starts the next line until the survey area is covered. Normally, data are not acquired during the turns due to difficulty in maintaining streamer lateral separation and the extra noise generated in the data. This means that, depending on the survey size and shape, there may be significant non-production time.

Advances in marine acquisition technology have made it possible to acquire usable data as the vessel turns, which increases productivity because there is little or no non- productive time associated with line changes. Streamer steering systems allow spread shape to be maintained during line changes, and new streamer designs and processing schemes mean that noise no longer presents a barrier to utilising these data.

We present some background on this technique and discuss a recent survey in New Zealand. In this survey, the straight line sections were shortened and turn data were incorporated into the survey. This enabled data to be acquired in shallow water areas. In addition, the data were acquired using a fan shooting technique to improve coverage and a variable depth cable with a multilevel source to produce ghost-free data, and therefore extend the data bandwidth. These three streamer configurations were combined to efficiently acquire broadband data in a challenging area.

The Idlewilde Intrusion (IWI) sits below ~150 m-thick sediment overburden and consists of a Palaeozoic basement of presumed Ordovician Macquarie Arc volcanics and the Nyngan Intrusive Complex. The geological setting is similar to the gold-rich Cadia and Northparkes porphyry-style deposits. Geophysical data include ground gravity, airborne magnetic and electromagnetic.

Gravity data define a negative ovoid with an elevated gravity inner zone. Magnetic data show a central high surrounded by a circular low zone as part of a broader regional low. Electromagnetic data outline: (a) ?150 m- thick dual conductor corresponding to the overburden; (b) a medium conductor associated with the volcanics; (c) a resistor linked to the intrusion and (d) a slightly conductive annulus around the resistor interpreted as alteration, but may instead reflect the lack of sensitivity below the conductive overburden. The central part of the intrusion associated to gravity and magnetic highs could conceivably correspond to: (1) a magnetite-altered potassic core characteristic of gold-rich porphyry, (2) eroded granite with volcanics close to the surface, (3) roof pendant volcanics above a barely-eroded granite, and (4) a late monzonite pulse.

From an exploration point of view, gravity data delineate the extent of the IWI. The regional low magnetisation suggests that the intrusion may be a later intrusive phase. Electromagnetic data identify the footprint of a granitic body which cannot be clearly associated with an alteration system because of conductive overburden. A mineralised system is yet to be identified, and one of the first company priorities is now to resolve the inner part of the IWI.

UAV aircraft differ significantly from manned aircraft. This paper investigates what the differences are and how that will affect decisions made by purchasers of airborne geophysical data.

Differences between manned and unmanned aircraft produce either positive or negative impact on the geophysical data due to a range of factors such as permanent, induced, eddy current, electromagnetic and microphonic noise. Flight characteristics, terrain to be surveyed, location of the survey, safety aspects and regulations are also a consideration.

Whether a purchaser of geophysical data chooses a UAV aircraft over a manned aircraft or a particular type of UAV, differs for every company. Every company will have different safety requirements, prospects, locations to survey and budget.

Over the last couple of decades, mineral geophysics has benefited from well documented advances in various technologies. Airborne gravity gradiometry, DGPS, GIS integration, radiometric noise reduction, 3D electrical / seismic acquisition, 3D inversions of potential field and electrical data are particular examples.

Less documented are the newer demands placed on mining company geophysicists. Examples include formulating safety management plans and auditing air safety compliance, monitoring field safety as much as technical details and complying with relevant legal restrictions in different jurisdictions. All the while the modern geophysicist must stay abreast of an increasing number of software applications, ensure quality communication with geologists on issues of target generation and method limitation, while geophysical staff numbers have typically decreased. The vast amount of data available in the 21st Century has added to the importance of “sorting the wheat from the chaff”.

Geological and geochemical ideas have also evolved over the last two decades, and geophysicists must understand terms such as IOCG, porphyry and low-sulphidation epithermal. Geophysical responses and tools need to be evaluated for each deposit style and an objective and critical consideration maintained to constantly review whether the exploration model is appropriate or too restrictive. Pushing the search to new frontiers by proposing new, deeper and more conceptual targets as a part of the exploration portfolio is important to help limit excessive revisitation of previous outcropping targets.

As for the next 20 years, some possibilities include more airborne methods, UAVs, and better models.

In recent years, a group of seismic interpretation techniques have emerged that aim to arrive at fully interpreted seismic volumes. Collectively, these techniques are known as: ‘global seismic interpretation techniques’. The classification ‘fully interpreted seismic volume’ is misleading as it suggests that no further interpretation is needed. In actual fact the fully interpreted volume marks the starting point for advanced interpretation workflows aimed at extracting more geologic information from seismic measurements. This paper gives an update on global seismic interpretation techniques and how they are used to add value to seismic data. Application domains include seismic sequence stratigraphy, model building & seismic inversion, geo- steering and geohazard interpretation.

The Common Reflection Surface (CRS) stacking has been established as an alternative to conventional data- driven imaging techniques. We have successfully applied the CRS stacking technique to real 2D and 3D seismic data. A case study performed in the St Ives mining camp, Kambalda, Western Australia (WA) demonstrates that CRS stacking significantly improves imaging results and increases the effectiveness of interpretation steps. The CRS approach is now routinely used in hard rock seismic processing.

The main objective of this study is to review the application of the CRS approach at the St Ives mining camp in Kambalda, WA.

Regional surveys using the TEMPEST208 airborne electromagnetic system were acquired for the Geological Survey of Namibia in 2011. The TEMPEST208is the lowest cost airborne electromagnetic system available currently worldwide and was selected to test a cost- effective means of covering the country with AEM and particularly to map thickness of the Kalahari Sequence to open new areas for mineral exploration.

This review of the data acquired showed that;

• The TEMPEST208 system did have some noise problems but significant improvements could be achieved with minor system modifications.
• TEMPEST208 can map areas of thin (0-60 m), to medium (~100 m) and thick cover (>150 m) Kalahari Sequence. In areas of thin cover, conductors can be detected in the underlying basement. In areas of medium cover an estimate of thickness of the Kalahari (with LEI) is probably possible but in areas greater than 150 m the system generally did not appear to detect the base of the Kalahari.
• Comparison with detailed surveys done with standard TEMPEST and VTEM shows that TEMPEST208 detected most of the features seen in surveys by the more sophisticated system.
• The data will be useful for explorers selecting best areas for exploration particularly where the Kalahari Sequence is less than 60 m.

One of the most hotly debated topics in Australian geology pertains to the tectonic relationship between Tasmania and mainland Australia. The focus of this study is ambient seismic noise data from 24 broadband stations, which span northern Tasmania, several islands in Bass Strait (King Island, Deal Island and Flinders Island) and southern Victoria, thus providing a dense coverage of surface wave paths that can be exploited to image the 3-D structure of the crust joining Tasmania and Victoria in high detail. The new results of this study will address fundamental questions regarding Tasmania’s tectonic provenance and its enigmatic relationship with the mainland. Furthermore, they will impose important constraints on the broad scale geology of a highly prospective region that hosts significant hydrocarbon deposits. To produce the highest quality Green’s functions, careful processing of the data has been performed, after which group and phase velocity dispersion measurements have been carried out using a frequency-time analysis method on the symmetric component (average of the casual and acasual signal) of the empirical Green’s functions (EGFs). The location of the experiment is such that the cross-correlations produce strong signal down to 1 s period, thanks to the proximity of microseisms in Bass Strait and the Southern Ocean. Group and phase dispersion measurements from the EGFs have been inverted to obtain Rayleigh-wave group and phase velocity maps at different periods, which are expected to shed new light on the structure beneath Bass Strait.

Fully volumetric interpretation needs to encompass 3D delineation of geological features beyond the extraction of top and base horizons. To address this issue 3D geobody delineation techniques based on thresholding and voxel connectivity have been developed. Such techniques have limited applicability as there is often insufficient information to enable the discrimination of the constituent components of a geological system based on the seismic data alone.

Understanding how we perceive objects in images is central to the development of better interpretation tools. What we perceive in data is strongly influenced by geological knowledge, previous experience and analogues. These are subjective factors but to produce geologically realistic results we need to find a way of incorporating them within 3D geobody interpretation. A large step in this direction has been taken with a technology known as “adaptive geobody delineation”. The adaptive geobodies technique combines an adaptive, classification based region growing method, with interactive 3D surface manipulation techniques. This enables delineation of 3D geobodies that are a best fit to the data whilst matching the interpreter’s view of what is geologically realistic.

We present the potential utility of these techniques applied to the delineation of a variety of geological elements from seismic data acquired from the North Carnarvon Basin, North-Western Australia.

Seismic reflection survey is the main technique for exploring the underground. While the position is not expected to change in the near future, the data quality in resolution and S/N ratio is always subject of improvement. The authors have developed the high resolution MEMS accelerometer for oil and natural gas exploration. In this research, the form of a MEMS element, size and electronic circuit were modified to enhance sensitivity of a prototype of MEMS to use in a MEMS accelerometer. The MEMS sensor made as an experiment by this research surpassed -130 dB/?Hz which is the world highest ever by the performance evaluation test carried out. We are striving towards disseminating the developed MEMS sensors.

A new 4D-repeatability indicator is proposed to appraise the quality of positioning during time-lapse marine surveys.

Repeatability of illumination between base and monitor surveys is assessed on selected reservoir horizons at fine discrimination scales, ranging from lines down to individual shots. Similarity between corresponding illumination imprints is evaluated from an adapted Partitioned Intensity Uniformity metric.

Such repeatability indicator can be used to assess jointly source and receivers positioning during 4D towed streamers surveys and under-shoots, or only source positioning over nodes surveys.

It provides a user-friendly tool to qualify the acquisition, or identify and rank preliminary re-shoots needs.

The anticipated almost doubling of world mine Cu production by 2030 will require a substantially increased output from existing porphyry Cu mines, along with production from as yet undeveloped mines and the discovery of new porphyry ore bodies. Assisting the potential for increased output from existing and new mines, and partly in response to declining ore grade, mass (large-scale) mining of porphyry Cu ore bodies is undergoing a major transformation, foreshadowing a significant increase in the size of some existing and future mining operations.

The discovery histories of two of the four Cadia, porphyry Au-Cu ore bodies in New South Wales, Australia offer insights into discovering deeper porphyry ore bodies. Induced polarisation geophysics (IP) contributed importantly to one of these discoveries (Ridgeway) by identifying the overlying ‘sulphur’ halo to the ore body.

It is proposed that IP, and possibly other geophysical methods, can play a greater role in discovering deeply- located porphyry ore bodies, when used as part of an ‘ore-system’ approach to discovery; particularly if the methods can be modified so as to ‘see’ much deeper than at present and used to identify a porphyry ‘sulphur’ halo, starting at a depth below surface of up to 1,000 m.

In the last decade the seismic industry has seen ever increasing application of wide, multi-azimuth, and rich azimuth surveys. Additionally, in last half decade the industry has seen a much greater emphasis on broadband acquisition and processing methods with the goal of achieving a seismic spectrum that spans both low and high frequencies. But are these technologies used only in specialized cases or are they the new normals in acquisition and processing?

There are significant costs associated with these acquisitions styles. So, value must be achieved by the use of technologies that provide improved subsurface images. The paper discusses some of the issues and highlights key technologies. This includes: imaging and velocity inversion in complex regimes, preservation of bandwidth for reservoir characterization, and estimation of the anisotropic and azimuthal properties of the subsurface.

Unconventional resource plays are currently driven by advances in drilling and completion technology, rather than in exploration technology. In order to increase the efficiency of operations in these fields, it is necessary to actually explore for the best places, the “sweet spots”, to drill and frac. This will require unconventional geophysics, to match these unconventional rock formations. In particular, the exploration techniques of acquisition, processing, and interpretation will recognize the fact that these formations are anisotropic, both seismically and electromagnetically.