Andrew Greenwood

Ore-body delineation using borehole seismic techniques for hard rock exploration

Over recent years, seismic methods have emerged as a potential imaging technique for delineation of ore-bodies and for mine planning. The application of surface seismic methods in hard rock environments is however challenging due to various effects such as energy attenuation and scattering. Borehole seismic methods can be used to reduce these effects. The methods offer higher resolution at target depths, thus allowing better delineation and understanding of reflections from ore deposits. We present a synthetic study to understand the ability of the cross-hole seismic method to delineate ore bodies. Three variations of a simple scenario typical of nickel deposits found in the Yilgarn Craton were considered. Of the three models, two consist of volcanics overlying a granite body and a thin sulphide mineralized zone along the contact but at different locations relative to the source and receiver boreholes. The third consists of only the rock units with no sulphide mineralized zone along the contact. Synthetic shot records were produced and wavefield separated. Up-going wavefields were then used to create depth migrated images. The resulting images correlate well with the volcanic-granite contact and massive sulphide lens, showing the potential of using the cross-hole seismic method to delineate ore bodies.

Comparative study of crosshole seismic reflection and VSP imaging

The application of conventional surface seismic methods for mineral exploration is often challenged by environmental factors such as rough terrain and thick vegetation cover. To image deep-seated vertical structures by conventional surface and VSP methods requires considerable horizontal offsets and surface sources are often degraded by highly attenuating regolith. The Crosshole Seismic Reflection (CSR) method in conjunction with Kirchhoff migration is an alternative higher resolution seismic imaging method in such challenging situations. Using a simplified model of magmatic nickel sulphide mineralization in the Kambalda region of Western Australia, the CSR imaging method has been evaluated by comparing migration images with those of multiple-offset VSP (MVSP). Kirchhoff VSP migration was adapted to create Pre-stack Depth Migrated images outside of the inter-borehole space. High resolution images of the ultramafic-basalt contact and a steeply dipping fault were obtained in both methods. Because of the large surface source aperture, the MVSP geometry provided greater coverage of the ultramafic-basalt. However, the CSR geometry provides similar coverage with a higher fidelity image and greater depth imaging. Our case study show that the CSR method serves as a suitable alternative to conventional surface seismic and VSP where there are surface mobility restrictions.

Cross-hole reflection seismic to delineate a relatively thin volcanogenic massive sulphide deposit in shale hosted environment

The seismic reflection method is a high resolution technique that can be used in many exploration environments including mineral exploration. However, mountainous terrain, depth of burial and the steepness of ore bearing structures pose a challenge to the application of surface seismic in mineral exploration. The cross-hole seismic method may present an alternative approach under such conditions. Presented here is a synthetic study examining the capability of the cross-hole seismic method to delineate a volcanogenic massive sulphide ore body in a shale hosted environment. A simple model typical for volcanogenic massive deposits in Tasmania has been considered. There, an elongated steeply dipping volcanogenic massive sulphide deposit with an average thickness of 10 m is seated within a shale rock. The primary aim of the modelling is to test the capability of the technique to delineate relatively medium sized, steeply dipping volcanogenic massive sulphide lens in shale hosted environment. A second objective is to use the technique to prospect for extensions to mineralization along steeply dipping reflectors. Synthetic cross-hole seismic records were generated using a 120 Hz energy source. Kirchhoff VSP migration was applied to wavefield separated shot records and Pre-stacked Depth Migrated images created. The resulting migrated images correlate well with the position and dip of the ore body demonstrating the potential of the cross-hole reflection technique to delineate steeply dipping ore structures in challenging environments.

Cutting the line in wireline with an Autonomous Sonde

Rock core has long been one of the pillars of mineral exploration strategy. This strategy, however, is becoming less viable as the depth of exploration targets continue to increase. Exploration strategies based on physical and chemical attributes of the rock-mass measured in-situ have the best chances to deliver efficient exploration programs by providing new data channels that can be used to improve the models of the deposits. Unfortunately, the logistic costs of acquiring these data using conventional wire-line methods have precluded their widespread use in the mineral industry. The autonomous sonde concept presented in this work drastically reduces the logistics costs of acquiring in-situ measurements. The autonomous sonde has been developed to integrate fully with the normal operations of current drill rigs. As such, it requires no specialised operator or equipment and no rig modifications. In this work, we present the results of field trials of the autonomous sondes at two Australian field sites. In the first experiment, we show that a pressure transducer can be used to evaluate the position of the sonde and to depth register the natural gamma data. In the second experiment, we show data acquired when the autonomous sonde protrudes through the bottom of the drill string and is brought back to surface by pulling up the rods. The results show a good repeatability between logging runs and data quality compares favourably to traditional wireline data.

Logging during diamond drilling ? Autonomous logging integrated into the Bottom Hole Assembly

Logging total count gamma data while diamond drilling an HQ borehole has been achieved using an autonomous shuttle. The shuttle is integrated into the Bottom Hole Assembly (BHA) prior to drilling. Logging is initiated at the beginning of each core run and the shuttle unit continuously logs at 1 second intervals. Continuous logging combined with the relatively slow rate of penetration of diamond drilling results in high fidelity logs at 1-5 cm intervals. The data is collected by the drilling crew, who download and email the data at the end of each core run for near real time analysis. Little to no interruption to the normal drilling process is experienced once the Shuttle has been integrated into the BHA. Autonomous logging while diamond drilling enables the collection of in-situ rock property measurements, without the risks and costs associated with later wireline logging. This value is added to the drilling process at little expense.

Logging While Drilling Via Autonomous Sonde

We present a method of geophysical logging while tripping drill rods to produce logs similar to wireline logs. The process is autinmous in that it does not require any significant changes by the driller and does not require any modifications to the drill rig. Using starting depth, rod length, and sensors on the sonde a geophysical log can be created when the drill rods are retreived upon hole completion. Thus, this methodology is very suitable for slimhole and diamond drilling methods that do not case the completed hole. Such holes often collapse and are too expensive to log via wireline logging.

The Application of Borehole Hydrophone Arrays in Hardrock Environments

The geometry of a VSP survey allows us to understand the characteristics of both the transmitted and reflected wavefields. As such, VSP is an “in-field seismic laboratory”, necessary for understanding the origin of seismic events. VSP enables calibration of surface reflection images and the survey can be designed to produce an image around the borehole at a much higher resolution than the surface reflection method. The main drawback of the method with respect to the mining community is the high logistic cost. Hence the main objective of the research presented here is to look into alternative ways of implementing VSP surveys that are cost effective, readily implementable in slim holes and pose lower risk to equipment in unstable uncased mineral exploration boreholes. As shown in this work, these objectives have been met using a borehole hydrophone array. Presented are two field trials in the Agnew-Wiluna and Kambalda regions of Western Australia. The results of these field experiments demonstrate that a borehole hydrophone array is capable of imaging structure in a complex geologic environment. These results, however, are not easily achieved because of the high sensitivity of hydrophones to acoustic modes in the borehole and the passive coupling to the formation.

Estimation of seismic attenuation from zero-offset VSP acquired in hard rock environments

Understanding of seismic attenuation plays an important role in successful application of seismic imaging and subsurface characterisation techniques based on amplitude analysis. Zero-offset vertical seismic profiling (VSP) is one of the principal tools which can be used to study seismic attenuation. Apparent attenuation estimated from seismic data analyses comprises of scattering and intrinsic components. Scattering mechanism can play significant role in hard rock environments in areas associated with fracture zones or other complex structures. As such seismic attenuation can be an important seismic exploration attribute. Meanwhile attenuation analyses from VSP data are almost routinely done in oil and gas industry they are still uncommon in mineral exploration. In this study we analyse zero-offset VSP data acquired in Western Australia on one of DET CRC test sites using both hydrophones and 3C geophones as receivers. We compare several methods for apparent attenuation estimation and evaluate their applicability to VSP data acquired in crystalline rocks. Extensive wire line log coverage for the well allows us to investigate relative contribution of different attenuation mechanisms.

Direct Targeting of VMS through Amplitude Consistent Seismic Imaging

Based on physical property measurements of core samples and the often observed difference in elastic properties from these there should be a significant difference in seismic amplitude between mineralisation and the surrounding host rocks. These results indicate that relative amplitude preservation processing may be of importance in the use of seismic data for the targeting of mineral resources, particularly in the case of massive ores. Such ‘true relative amplitude’ processing is not easy to achieve due to intrinsically low signal to noise ratio in hard rock environments, complex 3D geology, steeply dipping structures, high seismic velocities and often patchy and poor reflectivity. To help reduce the ambiguity in targeting and increase the likelihood of success we have worked on careful re-processing of 3D seismic data with the application of true amplitude preservation. We compare the anomalous amplitude zones that are related to massive sulphide bodies using a true amplitude seismic cube and a conventionally processed cube with the application of AGC (automatic gain control). A higher level comparison is conducted after seismic calibration with boreholes. The zonation and precision of targeting is discussed in this paper.

Diffraction Imaging in Hard-rock Environments

Herein, we expand and use the method used by Landa and Keydar (1998) to detect local heterogeneities. The method relies on computing local semblance along the diffraction offset-traveltime curves. We expand the method by taking into account the phase change occurring for edge diffractions. We demonstrate the usefulness of the method on hard-rock seismic data.