ASEG Extended Abstracts - ASEG2010 - 21st Geophysical Conference, 2010

Spatial variations in the transmission properties of the overburden cause seismic amplitude attenuation, wavelet phase distortion and seismic resolution reduction on deeper horizons. This poses problems for the seismic interpretation, tying of migration images with well-log data and AVO analysis. We developed a prestack depth Q migration approach to compensate for the frequency dependent dissipation effects in the migration process. A 3D tomographic amplitude inversion approach may be used for the estimation of absorption model. Examples show that the method can mitigate these frequency dependent dissipation effects caused by transmission anomalies and should be considered as one of the processes for amplitude preserving processing that is important for AVO analysis when transmission anomalies are present.

Gas hydrate, a complex compound, formed in the special condition of low temperature and high pressure, has been considered as very high potential new energy resources. In the gas hydrate exploration, bottom simulating reflector (BSR), defined as the boundary between gashydrate and free gas zone, is considered as the most important indicator for the gas hydrate exploration by seismic reflectivity method. Additionally, the location of BSR can help to estimate the thermo-dynamic parameters for gas hydrate stability zone. Spectral decomposition methods such as STFT (Short Time Fourier Transform), CWT (Continuous Wavelet Transform) and MPD (Marching Pursuit Decomposition) for seismic data has been proposed and applied in the various rock reservoirs to characterize hydrocarbon indicators as positive anomalies in the spectrum. Because the BSR has several specific characteristics that create strong reflectivity pattern in the seismic section, the time-frequency decomposition could be used to distinguish this boundary in the gas hydrate seismic data by taking the high energy position at the frequency gather slice. The output of seismic data processing for gas hydrate exploration in the Ulleung basin, Korea, will be used for time-frequency analysis and integrated to the well logging data in order to locate the BSR in seismic data. These results showed the high energy position at the time of 0.25-0.30 second below the sea floor reflector at the frequency gather slice of 60 Hz that would indicate to the position of BSR.

Spatial aliasing happens in some seismic data acquisition and affects severely the performance of multichannel data processing and interpretation. Conventional methods (e.g. antialias filtering) remove high-frequency information from data in hand and distort the signals. The procedure we present here produces a unaliased estimation of all frequency components within the original seismic wavefield. The method is benefitted from the advantages of the Wavelet and Radon transforms, for instance the overlap of information between wavelet scales at the same frequency, in decomposing the seismic time series. We demonstrated the performance of the developed dealiasing algorithm in wavelet-Radon domain by applying on synthetic seismic data.

Currently geophysical electromagnetic exploration for metal ores is facing some problems such as shallow probing depths, low accuracy and resolution, and poor anti-interference ability. To solve these difficulties, it is a feasible approach to make use of the existing mine tunnels, the electromagnetic transmitter on the ground and the recorders at the underground (borehole) or ground (prefecture) to compose a quasi-three-dimensional arrangement of measurement technology. This combination method of ground exciting and underground receiving can carry out observations from different directions to obtain subsurface electrical conductivity of the large amounts of information, and conduct quasi-three-dimensional imaging, allowing us to increase detection depth and resolution, and to reduce non-uniqueness in data interpretation and provide a new method for metallic ore exploration. The high-power single-frequency transmitter developed by our work can provide a single or multi-frequency inverter square wave, launching the signal frequency from DC to 9600Hz; the maximum emission voltage of 700V, and power supply current of 60A. This apparatus uses the GPS clock and real-time clock (RTC) module to ensure a regular and on time launching at scheduled frequencies. It can also make use of wireless Bluetooth and serial communication interface to control transmitting operation and edit the control parameter file and acquire a variety of supplementary status information. The experiment proves that the equipment is stable and reliable and can meet the requirements of conventional electromagnetic exploration and "tapping the deep and blind exploration of successive resources" for the exhausted mines.

Magnetic surveys are routinely used for detection of discrete magnetic targets, such as unexploded ordnance (UXO). Gradient tensor measurements have a number of benefits over measuring total field intensity or its vector gradient: they provide detailed information about a target in a single pass, without necessarily passing directly over the target; they determine the location of the target using a direct method, rather than the indirect inversion method required with other measurements; and they determine directly the magnitude and orientation of the magnetic moment, as well as the location of the target, aiding discrimination between sources and characterization of the munitions.

The CSIRO is building a magnetic tensor gradiometer, designed for underwater deployment, based on high temperature superconducting quantum interference devices (SQUIDs). Technical challenges for the project include: designing a high temperature SQUID device that can achieve the required gradient sensitivity in motion; developing a gradiometer housing that can deal with the boil-off gas from the liquid nitrogen cryogen while underwater; identification and removal of wave-induced magnetic noise; compensation for magnetic noise caused by motion and the platform itself; and developing a dipole-tracking algorithm for gradient tensor measurements that is both robust and computationally undemanding. This paper presents the progress to date on this project including an evaluation of gradiometer performance in laboratory conditions, and describes some simple methods for localization and characterization of compact magnetic sources using gradient tensor measurements.

Identification and prediction of fracture and its developmental zone is meaningful to reservoir exploration and development. According to solid mechanics ,fracture strata is anisotropic media . Previous formulation for S-wave elastic impedance neglects seismic anisotropic. In order to incorporate anisotropic , we use an approximation of converted PS-wave reflection coefficients to deduce converted wave elastic impedance in weakly anisotropic media for the first time. Through numerical simulation, the effect of Thomsen parameter, incident angle and azimuthal angle on the formulation are described in detail. At the same time, we analyze the precision of new approximation formulation and compare the hydrocarbon detection effect of PP wave and converted wave elastic impedance. The future work is to select the newest nonlinear inversion algorithm and realize elastic impedance inversion in anisotropic media.

On the basis of sensitive fluid identification factor in isotropic media,we construct the new fluid identification factor in weakly anisotropic media, which is composed of PP wave and converted wave elastic impedance. The advantage of new fluid identification factor is to allow investigating AVO anomalies with the effect of anisotropic parameters .Then ,we can analyze the difference of fluid identification factor with azimuthal angle and anisotropic parameters in 3D space .Adding Thomsen parameter to the 25 formation models gived by Castagna and Smith, we analyze the identification effect. Numerical simulation results show that the new fluid factor can identify the gas sand and brine sand effectively.

Landsoner is the abbreviation of extremely elastic reflection wave recording continues profiling with extremely small offset and vary wide band frequency. It has following characteristics: (a) Small-offset single is used; (b) Under situation of using hammer source can be excited and be received reflection wave with frequency 10Hz-4000Hz of depth of 1 ~ 200m; (c) It is unnecessary that the geophones f ixed on the ground. On this characteristics the wave with different frequency band can be got that can greatly rise resolution, and can keep away from the noise of acoustic wave, direct wave, reflected wave, surface wave, specially can keep away from the noise of vibration of pedestrians, cars and the other machines on a city. When using landsoner method survey line with broken line can be used. There is no need to statics in mountainous region for this method. On the time-section of figure of Landsoner a karst cave can clearly be reflected. Landsoner method has been successfully used in shallow high resolution survey in mountainous region or in a busy city, in survey karst caves, in geological prediction forward from working face in a tunnel construction and in examining quality ofconcrete structures.

A key strategy to characterize subsurface materials is the integrative analysis of tomograms of several physical properties provided by different geophysical data. Among the recent strategies to produce these tomograms is the joint inversion. Unfortunately, most joint inversion techniques are based on hypothetical property relationships that hinder the site-specific property correlations that characterize individual materials. Differently, a recently developed cross-gradient joint inversion technique is underpinned by the hypothesis of structural resemblance and permits the coexistence of natural property correlations that provide the key to unravel the signatures of subsurface materials. In the present work, electromagnetic, seismic and potential field data are jointly inverted for several field sites with near surface targets. The results show not only a clearer disposition of the mapped units in structural terms, but also a sound evidence of their actual fluid-mineral characteristics. In general, the results demonstrate the power of analyzing multiple-property images when they are structurally consistent.

Prestack depth migration is used to image for complex geological structure. In this case, the surface reflection data are used as an input data in general. However, the surface reflection data have some problems in imaging the subsalt and the salt flank due to the complex wavefields and multiples which come from overburden. To overcome the defect of the surface reflection data, we used the virtual source in terms of seismic interferometry to image the subsurface. Inhomogeneous velocity models were developed and virtual source gathers were generated ocean bottom seismic numerical modelling. Cross-correlation gathers were generated by the crosscorrelation between the reference trace and others. The virtual source gathers were made by the integration at the stationary phase interval.

Numerical test showed that the virtual source gathers integrated at the stationary interval are superior to that of all sources. To verify the possibility of subsurface imaging, prestack depth migration was applied for the virtual source gathers. This prestack depth migrated section re-produced the velocity model below receivers.. Especially artificial interface by multiples were suppressed without any further data processing. The results of imaging obtained from inhomogeneous velocity model below receivers also showed that the artificial geological interfaces were significantly reduced comparing to the homogeneous velocity models case.

Geoscience Australia (GA) has recently completed two regional-scale Airborne Electromagnetic (AEM) surveys: one in the Paterson Region, WA; and the other in the Pine Creek region, NT. These surveys provide AEM data at line spacings of 200 m to 6 km covering an area greater than 110 000 km². The surveys were designed to promote more detailed investigations by the mineral exploration industry. An inherent risk in using AEM surveys is that the depth of penetration of the primary electromagnetic field is highly variable. Although forward modelling is undertaken before the AEM campaign, the depth to which we can reliably invert the AEM signal to generate conductivity models is not known until after the survey is flown. In order to estimate the penetration depth of the AEM surveys, we calculate the depth of investigation (DOI) based on the GA layered-earth inversion algorithm, which is influenced by both conductivity measurements and reference model assumptions. We define the DOI as the maximum depth at which the inversion is influenced more by the conductivity data than the reference model. We present the DOI as a 2D grid across both the Paterson and Pine Creek AEM surveys. Labelled the “AEM go-map”, the DOI grid helps to promote AEM exploration by decreasing risk when industry undertakes follow-up surveys within these regions.

Obtaining reliable predictions of the subsurface will provide a critical advantage for explorers seeking mineral deposits at depth and beneath cover. A common approach in achieving this goal is to use deterministic property-based inversion of potential field data to predict a 3D subsurface distribution of physical properties that explain measured gravity or magnetic data. The non-uniqueness of inversions of potential field data mandates careful and consistent parameterization of the problem to ensure realistic solutions. Including all prior geological knowledge as constraints on the inversion also helps ensure that the recovered predictions are consistent with both the geophysical data and the geological knowledge.

We review how potential field inversions are best applied for mineral exploration problems using the UBC-GIF inversion algorithms. We use examples to emphasise the importance of mesh design and applying appropriate data processing, and identify the approach for defining key parameters such as data uncertainty, potential field weighting functions, and numerical parameters that approximate prior geological knowledge of in situ trends, geometries and properties. Consistent application of these techniques will ensure the most reliable predictive physical property models for explorers.

We have developed a petrophysical joint inversion between magnetotellurics (MT) and gravity. It utilises Archie’s Law and the porosity-density relationship. Through these equations it can be shown that both conductivity and density are dependant on porosity. Porosity then forms the crucial link between the two techniques used by the joint inversion.

The approach was tested using synthetic models. The results demonstrate the joint inversion produces a better representation of the subsurface, as compared to individual MT or gravity inversions. It shows sharper boundaries and more accurate parameter values. Our joint inversion is viable in real Earth applications and this was demonstrated through a case study of the Renmark Trough, South Australia.

In early 2009 AWE Limited (AWE), as operator of the Tui field, conducted a review of original 3D seismic data processing done in 2003 and 2005 and identified a number of additional elements in the workflow that could, as a result of new processing technology, lead to improvement in the data quality of this 3D survey.

The additional elements included new processing technology that was not available in 2005 and also a redesign of the processing sequence to include or modify the steps used in the original processing project. These processes included surface multiple attenuation, tidal statics, bin centring and additional Radon demultiple.

The work started with test processing over a key 100 sq. km. area of the field. Good results from this test volume led to full reprocessing of the entire 3D data set.

This paper describes the redesign of the 3D processing sequence and shows examples of the improvements we obtained using a current state of the art workflow to reprocess the data set.

The propagation of seismic wave through viscoacoustic media is affected by the attenuation that is caused by the quality factor Q, resulting in significant loss of signal strength and bandwidth. Gas trapped in sediment is an example of such media. Seismic images of geological structures underneath shallow gas often suffer from resolution degradation and the effect of amplitude dimming, making the identification and interpretation of the structures difficult. This in turn affects the ability to accurately predict the reservoir properties. Thus, there is a need to compensate the attenuation anomalies due to Q.

In this paper, we present a workflow of Q estimation and compensation that is based on our previous work on amplitude tomography. Our new approach involves utilizing tomographic inversion for estimating Q from prestack depth migrated common image gathers that fully honours the wavepaths. By filtering the seismic data into different frequency bands and measuring the effect of attenuation on amplitudes in each band, the frequency dependent effect, which was ignored in our previous amplitude tomography work, of attenuation is fully taken into account, thereby allowing Q to be estimated from our tomographic method. By using the estimated Q volume in one of the migration methods that incorporates Q in the traveltime computation, we demonstrate, through realdata examples, that our workflow provides an optimal compensation solution that resolves amplitude, phase and bandwidth distortions due to seismic attenuation.

Multiples due to shallow water are observed in seismic data acquired in various places such as the Gippsland Basin of Australia. These short period multiple reflections often pose problems to the interpretation of geological structures. They are not easily handled by conventional surface-related multiple elimination (SRME) methods because the recorded primary waterbottom reflection, which is required by SRME, is often indistinct in shallow water situations due to the near offset gap. Hence, predictive deconvolution in the x-t or τ-p domain is frequently used for attenuating shallow water multiples. However, besides multiples, deconvolution also attenuates primary events that have a periodicity which is close to that of the water-layer.

In this paper, we present a workflow that involves first attenuating short-period water-layer related multiples (WLRMs) – a process that we term shallow water demultiple (SWD); and then suppressing other longperiod free surface multiples using conventional SRME. SWD is a wavefield-consistent method that first makes use of WLRMs in the data to reconstruct the missing water-bottom primary reflection and then uses the reflection for predicting shallow WLRMs. It is data driven and takes into account the spatial varying nature of subsurface structures. Since the WLRM model predicted by SWD has similar amplitude and phase as the input data, very short matching filters, which are not possible if deconvolution is used, can be utilised in the adaptive subtraction process.

We demonstrate, through real-data examples, that our workflow provides an optimal multiple attenuation solution in shallow water environment in comparison with conventional methods such as τ-p deconvolution or SRME alone.

Geoscience Australia has been acquiring deep crustal reflection seismic transects throughout Australia since the 1960s. The results of these surveys have motivated major interpretations of important geological regions, contributed to the development of continental-scale geodynamic models and improved understanding about large-scale controls on mineral systems.

Under the Onshore Energy Security Program, Geoscience Australia has acquired, processed and interpreted over 5000 km of new seismic reflection data [over what period?]. These transects are targeted over geological terrains in all mainland states which have potential for hydrocarbons, uranium and/ or geothermal energy systems, but also have implications for other mineral systems.

The first project was undertaken in the Mt Isa and Georgetown regions of North Queensland. Interpretations of these results have identified several features of interest to mineral and energy explorers: a previously unknown basin with possible hydrocarbon and geothermal potential; a favourable setting for iron oxide uraniumcopper-gold deposits; and a favourable structural setting for orogenic gold deposits under basin cover. Magnetic, gravity, and MT data were used to map some of these features under cover in 3D, areas away form the seismic line.

Seismic imaging of the full thickness of the crust provides fundamental data to economic geologists, to explain why major deposits occur where they do, and reduces risk for companies considering expensive exploration programs under cover.

A recent airborne electromagnetic (EM) survey in Papua New Guinea (PNG) has provided an opportunity to trial systematic measures of data quality in a similar manner to those routinely employed in airborne magnetic and radiometric surveys. These calibration checks are designed to demonstrate the integrity of the acquisition system from a client perspective.

AeroTEM IV data were acquired over two project areas in PNG during 2008/9, and a set of check flights were acquired over pre-determined test lines in each area. These were (a) pre-survey check flights and (b) repeat check flights. Pre-survey check flights consisted of a range of helicopter and bird manoeuvres carried out along line to assess system performance under varying survey conditions. These manoeuvres included: typical survey-type flight, bird swing, speed increase/decrease, ascent/descent, and flight at altitude with the transmitter on and off. Repeat check flights consisted of acquiring data along the same line at survey height and speed, at the commencement of each sortie.

The 45 check flights acquired during the survey are used to demonstrate the amplitude of noise due to aircraft manoeuvre, and the detection of system error using repeat data. Recommendations are made for the evaluation of data quality consisting of the pre-survey and repeat check flights listed above, plus a heading check. Spectral fast Fourier transform (FFT) plots can be usefully employed to identify unwanted noise sources. Inversion of one or more lines of data from each flight is recommended to check the geologic value of the data.

Measurements of achieved resolution on data recorded in 1941 show better resolution than typical data recorded in 2007, and the data in intervening years are generally consistent with the long-term trend, though there may be a slight increase in resolution from a low point in the 1970s. Possible explanations include the use of increasing reflection angles, increased use of surface sources, and the use of multiple-fold techniques.

Tarim basin is located in northwestern China and is the one of main basins with abundant oil and gas. Due to existence of thick low velocity-reducing layer in desert area of the basin, it is very difficult to gain good seismic datum. The main problem in collecting data include: 1) Bad shot and receiving situation cause very low signalto-noise ratio and low resolution; 2) Undulant hypsography make complicated static correction; 3) Surface deep sand stratum induce absorption and attenuation of seismic wave. To finish the geologic target, we introduce following methods: 1) Enhancing analysis and investigation of surface structure to perfect surface structure model which will serve for choosing better shot points; 2) Exploding under water table to guarantee enough shot energy, and embed the geophone to get the better receiving condition; 3) Selecting bigger diameter of dynamite to form better point explosive response; 4) Surveying the noise wave to design the best acquisition parameters; 5) Calculating better static correction by surface refraction tomography; 6) Applying the compensation of absorbed energy of seismic wave to gain better information. After these methods are introduced, the signal-to-noise ratio and resolution of main prospecting objective in collecting seismic datum are improved obviously and obtain correct geologic interpretation.