Exploration Geophysics - Volume 41, Issue 2, 2010

Rock physics models play a crucial role in seismic reservoir characterisation studies. The optimal rock physics model for a sandstone reservoir might be significantly different from that of a carbonate reservoir. There are several theories that compare the elastic properties of dry and saturated rocks. These models have mainly been explained by poroelastic theories or effective medium theories. The Gassmann’s model which is commonly used in petroleum rock physics is suitable for rocks with spherical and interconnected pores at low frequencies. These assumptions do not necessarily meet the conditions of carbonate rocks. In this work, two additional models, the differential effective medium (DEM) model and the self-consistent (SC) model have been examined for several carbonate samples. Ultrasonic 30 carbonate and 5 sandstone core samples from an oilfield in south-west Iran were measured in the laboratory. The results show that the DEM model gives the best compatibility with the dense and low porous carbonate samples. These results are confirmed by well log data from the same area.

Acquisition of teleseismic data in south-western New South Wales during 2007 formed the latest stage of a rolling deployment of seismometers over south-eastern Australia, and allowed a revised tomographic model to be constructed for the lithospheric mantle under Victoria and southern NSW. Our aim here is to link the observed distribution of upper-mantle P-wave velocity to the major geological features of the upper crust, which here comprise terranes of the Delamerian and Lachlan orogens. We have extended the definition of the boundaries of these terranes under cover by the use of the tilt-filter of total magnetic intensity, which provides an image with detailed resolution for sources at depths down to ~5 km. We proceed to infer the distribution of deeper sources in the middle and lower crust by two approaches to the use of potential-field images: we exploit the relationship between wavenumber and source depth, through the application of a 20-km low-pass filter to the total magnetic intensity grid; and we take advantage of the lower sensitivity of gravity anomalies to depth of source, compared to magnetic anomalies with dipolar sources, by defining broad features in the isostatic gravity grid. Our interpretation of the low-pass magnetic and isostatic gravity imagery confirms the relationship between high mantle velocity and the Proterozoic Delamerian Orogen, and indicates that a salient of high mantle velocity under the Palaeozoic Stawell Zone results from an underthrust wedge of Delamerian basement. High mantle velocity under the Palaeozoic Wagga-Omeo Zone may be a result of lithospheric thickening that is a corollary of mid- to lower crustal thrust faulting indicated by the potential field data. Low mantle velocity under part of the Melbourne Zone may result from thermochemical resetting of its Proterozoic microcontinental basement by the thermal event responsible for the extensive Cainozoic volcanism in western Victoria; low mantle velocity under the Hay-Booligal Zone, which also appears to be anomalous, may similarly be related to a heat pulse that engendered the swarm of diatremes that is distributed across the zone.

High-resolution aeromagnetic surveys are commonly used to locate subtle anomalies that are important in mineral and oil exploration. However, such anomalies, especially in highly populated areas, are often masked by undesirable magnetic signals from near surface man-made objects – known as ‘cultural noise’ – making post processing and interpretation of the aeromagnetic data difficult. Magnetic data need to be cleaned of this cultural noise before applying advanced processing and interpretation methods. Conventional algorithms for cultural noise removal tend to identify and remove noise signals, either manually or using non-linear filters. These methods are often combined with Fast Fourier Transform filters to smooth the result. These algorithms usually introduce artificial anomalies, have difficulty interpolating across edited sections and rarely yield clean data. For these reasons, we have developed a semi-automated two stage equivalent source approach to remove cultural noise and image subtle geological anomalies. A theoretical example that combines a magnetic anomaly due to a dyke with three cultural noise sources is used to test the effectiveness of the proposed method. Comparison of the equivalent source and conventional results shows that the equivalent source method more closely recovers the original magnetic data. We then demonstrate the practical utility of the two stage equivalent source approach using a high-resolution aeromagnetic dataset from Harberton Bridge, Ireland.

The identification of lineaments in potential field data is an important step in many interpretation projects. A method of extracting ridges and valleys based on maxima and minima of a balanced plan curvature dataset is suggested here. The method is compared with other ridge detection algorithms and is applied to aeromagnetic and gravity datasets from South Africa. Source code in Matlab format is available from the author on request.

Circular features in potential field or topographic data are of interest because they can be caused by impact structures or kimberlite pipes. A new method for enhancing circular anomalies has been developed, based on the generalised derivative filter. The method is compared to other existing techniques and is applied to gravity datasets from South Africa. The source code in Matlab format is available from the author on request.

The coastal area Marina di Capilungo located ~50 km south-west of Lecce (Italy) is one of the sites at greatest geological risk in the Salento peninsula. In the past few decades, Marina di Capilungo has been affected by a series of subsidence events, which have led in some cases to the partial collapse of buildings and road surfaces. These events had both social repercussions, causing alarm and emergency situations, and economic ones in terms of the funds for restoration.

With the aim of mapping the subsurface karstic features, and so to assess the dimensions of the phenomena in order to prevent and/or limit the ground subsidence events, integrated geophysical surveys were undertaken in an area of ~70 000 m² at Marina di Capilungo. Large volume voids such as karstic cavities are excellent targets for microgravity surveys. The absent mass of the void creates a quantifiable disturbance in the earth’s gravitational field, with the magnitude of the disturbance directly proportional to the volume of the void. Smaller shallow voids can be detected using ground-penetrating radar (GPR). Microgravimetric and GPR geophysical methods were therefore used. An accurate interpretation was obtained using small station spacing and accurate geophysical data processing. The interpretation was facilitated by combining the modelling of the data with the geological and topographic information for explored caves. The GPR method can complement the microgravimetric technique in determining cavity depths and in verifying the presence of off-line features and numerous areas of small cavities, which may be difficult to be resolved with only microgravimetric data. However, the microgravimetric can complement GPR in delineating with accuracy the shallow cavities in a wide area where GPR measurements are difficult. Furthermore, microgravity surveys in an urban environment require effective and accurate consideration of the effects given by infrastructures, such as buildings, as well as those given by topography, near a gravity station. The acquired negative anomaly in the residual Bouguer anomalies field suggested the presence of possible void features. GPR and modelling data were used to estimate the depth and shape of the anomalous source.