Exploration Geophysics - Volume 19, Issue 4, 1988

The Dundee Ignimbrite has a distinct magnetic fabric, defined by susceptibility anisotropy. Magnetic foliation data for the ignimbrite mass at Dundee define a basinal structure. The magnetic lineation data which are argued to be flow lineations generally have a NNE-SSW orientation. This suggests that the source vent(s) for the ignimbrite is external to the mass.

When analysing magnetic data collected on a sloping survey plane, in areas with steep topography or in inclined boreholes, attention must be paid to the magnetic inclination and declination. An analysis of these quantities forms the basis of a new method of analysing magnetic data: the slope correction method. Two new quantities are used to describe the magnetic field. They are the apparent inclination and the apparent declination, both of which depend upon the magnetic field orientation, and the slope and the azimuth of the survey plane. They are readily computed for other orientations of the survey plane since they only involve the inclination and the declination of the induced and remanent magnetisations. A series of graphs showing how these elements vary with orientation of the survey plane, for the complete range of magnetic latitude, provide insight into the effect of the attitude of the survey plane on the measured field. The slope correction method has a wide range of applications which include analysis of magnetic survey data from boreholes and mine shafts, and ground and airborne surveys in rugged terrains. For a particular case of a ground magnetic survey over a steep mountain in Papua New Guinea, an application of the slope correction method revealed that the anomalous fields observed on two sides of the mountain could be produced by a single structure, and not two distinct structures as might be first thought from an inspection of the data. The method highlights a possible source of error in applying the reduction-to-the-pole filter to magnetic data from an undulating survey surface.

A two-dimensional, physical seismic model system has been developed at Flinders University for laboratory studies into elastic wave propagation through a variety of simulated geological structures. The scale model facility produces useful (and inexpensive) data for training purposes as well as for testing of new signal processing and imaging algorithms.

The digital recording equipment is under computer control which enables trace gathers to be acquired, and subsequently processed, quickly and accurately. The piezoelectric transducers have been designed to provide good resolution (100 kHz bandwidth) and stability, and facilitate the recording of both P and S waves. The entire system has been carefully calibrated to enable deconvolution of the seismograms for the effects of the various instrument components as well as the nature of the input source waveform. The simple reflection experiments which are reported here demonstrate the high performance of the ultrasonic facility in imaging a small cavity, a horizontal linear target of finite extent, a fault, and a basement mound. The resulting seismic sections clearly show the prominence of diffractions, and the importance of S waves (mode conversions) in structural interpretation.

The acoustic properties of natural unconsolidated marine sediments are essentially those of a suspension of solid particles in a fluid, with usually a small rigidity or shear modulus evident. Because of this rigidity the system may be modelled as a fluid saturated porous elastic solid. The theory of acoustic propagation in such a medium was first presented in the early work of M.A. Biot, which maintains a fundamental approach aimed at including all relevant physical mechanisms in a quantitative manner. Application of Biot theory to saturated porous solids yields 3 wave solutions — 2 dilatational waves and a shear wave — but in the case of unconsolidated marine sediments only one dilatational wave is observable. The properties of these wave solutions — their velocities, attenuations and characteristic impedances — are related to the many physical properties of the sediments invoked by Biot theory. The main problems of applying Biot theory lie in the large range of input parameters required to be specified. Some of these parameters are well defined but others are difficult if not impractical to specify. These limitations and the range of applicability of the theory to the study of marine sediments are further discussed. Each sediment can be shown to be characterised by two frequency regimes separated by a transition zone, the frequency of which depends primarily on the permeability and pore size of the sediment. In the “low” frequency regime there is complete coupling between the motion of the fluid and solid particles. The attenuation of a sound wave is then solely due to the visco elasticity of the sediment frame structure and the sound speed approaches that of a non interacting mixture of fluid and solid particles, the modulus of which is increased slightly by some structural stiffness. At “high” frequencies there is a partial decoupling of the motions of fluid and solid resulting in an increase in total wave attenuation due to viscous damping. There is also an increase in sound speed. Other properties of interest to underwater propagation studies, such as the reflectivities of interfaces, have been examined in terms of Biot theory. These analyses show some significant differences from the results of treating the sediment as a lossy fluid and have importance in bottom loss studies.