Exploration Geophysics - Volume 37, Issue 4, 2006

A completely new electro-mechanical marine Vibroseis concept is introduced that uses a flextensional shell with a uniquely developed form. Two controllable output resonances achieve a very high efficiency and repeatability, with a typical frequency bandwidth of 6-100 Hz. The marine vibrator can be used as a standard towed acoustic source in shallow water, or as a stationary source in transition zone environments. It is particularly significant that the vibrator requires only an electrical power supply, cannot leak hydraulic fluids, is easily transported, and is suitable for applications requiring very low environmental impact. A marine vibrator will provide several environmental advantages. Vibrator technology spreads the net source energy over a long period, reducing the acoustic power in comparison to impulsive sources. This is attractive for applications where high peak power may be problematic. This fact was established by the original hydraulic vibrators developed previously. An electrical marine vibrator offers even more advantages than hydraulic vibrators. There is no need for heavy equipment and hydraulic systems that can cause hydraulic oil spills. As the electrical vibrator requires only an electrical power supply it can be easily transported to different vessels and locations without any costly installations and potential environmental hazards. The marine vibrators discussed here have been tested as a standard towed acoustic source in shallow water, and as a stationary source in transition zone environments (4-6 ft of water). These applications are particularly well suited for this type of source.

Surface wave data acquisition by landstreamer with flat baseplates produces nearly identical dispersion curves to those from planted geophones. Dependence on geophone frequency and good coupling is negligible and repeatability is lair. The primary application of landstreamers is to produce 2D shear wave velocity profiles through ID inversion of a number of dispersion curves at high shot density, and numerical modelling shows that this workflow provides accurate subsurface images. However, in the vicinity of a shallow, soft-layer termination, its lateral extent can be overestimated by about 10% of the recording spread length. Shorter spreads show less wavefield scattering hut more dependence on source-receiver geometry, and the response is dominated by the material under the nearer-offset channels.

Full SH-wavefield modelling by the reflectivity method is developed for Love wave inversion. The modelling simulates field tests, first by generating time-distance shot gathers, followed by standard plane-wave transform to extract Love wave phase-velocity dispersion. All SH surface wave modes, plus body waves and wavefront spreading are accurately simulated.

When a thin low-velocity layer (LVL) is underlain by a much stiffer layer, Love wave modes are well separated and fundamental-mode identification is more straightforward than the equivalent Rayleigh waves. However, when the LVL is thicker and bounded by equally stiff layers, multiple dominant higher modes propagate, and their correct identification and inversion is essential to resolve the soft layer.

A first field test shows how multimode Love wave inversion can resolve a 2 m thick clay layer at 1 m depth in the lithological log when only both the fundamental and first higher modes are incorporated. A second field test, where a much thicker LVL, interpreted as a mud plume, is present, shows how several higher modes are required to resolve the soft layer LVL, whereas use of the fundamental or effective mode only fails.

We demonstrate the potential of a recently developed grid-based eikonal solver for tracking phases comprising reflection branches, transmission branches, or a combination of these, in 3D heterogeneous layered media. The scheme is based on a multi-stage fast marching approach that reinitialises the wavefront from each interface it encounters as either a reflection or transmission. The use of spherical coordinates allows wavefronts and traveltimes to be computed at local, regional, and semi-global scales. Traveltime datasets for a large variety of seismic experiments can be predicted, including reflection, wide-angle reflection and refraction, local earthquake, and teleseismic.

A series of examples are presented to demonstrate potential applications of the method. These include: (1) tracking active and passive source wavefronts in the presence of a complex subduction zone; (2) earthquake hypocentre relocation in a laterally heterogeneous 3D medium; (3) joint inversion of wide-angle and teleseismic datasets for P-wave velocity structure in the crust and upper mantle. Results from these numerical experiments show that the new scheme is highly flexible, robust and efficient, a combination seldom found in either grid- or ray-based traveltime solvers. The ability to track arrivals for multiple data classes such as wide-angle and teleseismic is of particular importance, given the recent momentum in the seismic imaging community towards combining active and passive source datasets in a single tomographic inversion.

Most body wave seismic imaging schemes only exploit information contained in the first arrival of a seismic record. Later arrivals in the wavetrain, however, contain additional structural information, as the corresponding rays tend to sample slower regions of a medium that are often avoided by first-arrival raypaths. Here we investigate a Lagrangian (ray-based) and an Eulerian (grid-based) approach for the calculation of later arrivals. The Eulerian approach is based on the level set method, which implicitly evolves a wavefront by solving a pair of PDEs over a gridded velocity field in phase space. Our Lagrangian solver also uses phase space and represents the wavefront by a set of points, which are progressively moved through the velocity field using local ray tracing, with linear interpolation used to maintain a constant density of points. We compare the two methods using a velocity model of the subduction zone in the Tonga region. In theory both approaches can provide traveltimes for later arrivals. Our results clearly show that the Lagrangian approach is currently superior to the Eulerian scheme for the prediction of multi-arrival traveltimes when computation speed, ease of implementation, and accuracy are considered. In our experiments the Lagrangian solver is up to 6000 times faster, and successfully predicts later arrivals for our source receiver configuration in the subduction zone example. We then demonstrate the robustness and efficiency of the Lagrangian solver by tracking later arrivals in a smoothed version of the Marmousi model. By placing source points in certain parts of this model, we are able to find more than 60 secondary arrivals at surface receivers.

A number of shallow refraction interpretation methods are compared in variable regolith conditions using synthetic and published field data. The synthetic model contained a low velocity zone in a depression at the base of the regolith. Independent interpretation with the Reciprocal Method was in reasonable with this model. The Generalised Reciprocal Method performed poorly on this model, both smoothing and considerably underestimating the depth to the simulated regolith base and greatly narrowing the low velocity zone. However, neither the Reciprocal nor Generalised Reciprocal Methods produced valid velocity analyses over the low velocity zone, as diffracted and non-critically refracted wave arrivals are used. Wavefront Eikonal Traveltime Tomography identified the rapid thickening of the regolith over the depression, but introduced an artefact near the low velocity zone, and the regolith base was not easily located. Low wave path densities over the depression identified by this method also indicated that the interpretation should be treated with caution.

The field example was over a variable regolith with a faulted contact between rocks of differing weathering characteristics. Visual Interactive Ray Tracing and Wavefront Eikonal Traveltime Tomographic interpretations were in good general agreement for this example. These interpretations differed considerably from the original Generalised Reciprocal Method interpretation that contained a wide low velocity zone at the contact. This is likely to be an artefact of the Generalised Reciprocal Method interpretation process.

While our comparisons are not definitive and all the interpretation methods that were compared have deficiencies and limitations they do offer some guidance to improving shallow refraction interpretation for regolith mapping. This is achieved by combining some of the methods and involves the Reciprocal Method, Wavepath Eikonal Traveltime Tomography, and Visual Interactive Ray Tracing. Interesting subsurface features and limitations are highlighted by joint use of ray path displays and wave path density diagrams together with various statistical goodness-of-fit measures to the field data. While this interpretation approach should be more robust, it does not eliminate personal bias nor overcome inherent limitations in the shallow seismic refraction method for regolith mapping, such as the delineation of laterally hidden low velocity zones.

We derive an unambiguous apparent conductivity from total magnetic field TEM data for fixed-loop geometry. For single-component fixed-loop TEM measurements, apparent conductivity is either dual-valued or undefined. The ambiguity or non-existence is particularly evident for readings taken outside the transmitter loop, both for step and impulse response data. Therefore, computing apparent conductivity from single-component fixed-loop TEM data can be problematic, especially at intermediate delay times. However, if multi-component fixed-loop magnetic field data is available, an unambiguous apparent conductivity can be derived from |B (t )| at all times, except in the inductive limit. Impulse response measurements can be time-weighted and summed to yield “quasi-|B |” data. Apparent conductivity derived from quasi-|B | amplitudes is dual-valued, but usually only one of the alternatives is geologically plausible. Computing apparent conductivity from |B | or quasi-|B | amplitudes expedites generation of conductivity-depth sections from fixed-loop TEM. A field data example with multi-component SQUID data shows significant improvement in the conductivity-depth section when |B (t )| is transformed rather than the vertical component, Bz, alone; the lateral extent of a conductive target is under-estimated in the Bz CDI

Helicopter electromagnetic systems typically have a transmitter-receiver coil pair in a towed bird that, in flight, behaves as a complex compound pendulum. Pendulum motions of the towed bird create not only a geometric and inductive effect in the measured signal, but they may also contribute to altitude error. The oscillation of the towed-bird pendulum can be observed from both video recordings and GPS positions and is most usefully broken down into two modes: one in the direction of travel (in-line) and one perpendicular to the direction of travel (cross-line). We analyse the electromagnetic and altitude data for a RESOLVE survey on the Chowilla flood plain, near Renmark, South Australia. The in-line and cross-line motions of the towed bird are shown to have different frequencies of oscillation. In-line motions, which cause pitching of the towed bird, and cross-line motions, which generate roll, create a systematic error in data easily evident in the dimensionless in-phase divided by inductive limit (R/G) domain. We show that the bird swing due to both modes of pendulum motion is responsible for altitude error, and that a filter designed to use the observed period of oscillation can largely correct the R/G domain data. The filtering process does not require any more measurements than are usually provided from a typical electromagnetic survey.

2.5D electromagnetic (EM) modelling computes the response of a 3D source from an arbitrary 2D geoelectrical model. As such, it is practical for airborne EM (AEM) data to be inverted using 2.5D modelling provided that the geoelectrical cross-section is relatively constant along a strike length that exceeds the AEM system footprint. The program ArjunAir is introduced for modelling and inversion based on a 2D finite-element method that enables the accurate simulation of 3D source excitation for full domain models inclusive of topography, non-conforming boundaries, and very high resistivity contrasts. Inversion is based on an iterative Gauss-Newton method that is solved using the damped eigenparameter algorithm. Examples are presented for synthetic and practical frequency and time-domain AEM surveys for which inversion run-times are on the order of hours.

Normal faults within sedimentary basins are commonly associated with subtle linear features in high-resolution, total magnetic intensity (TMI) data. Many of these anomalies arise from the tectonic juxtaposition of sedimentary units of differing magnetic properties. In detail, the anomalies can be quite variable in character, even along the strike of individual faults. To understand this variability, we examine the well-exposed San Ysidro Fault in the central Rio Grande Rift, USA, using detailed magnetic-property measurements, geophysical models based on geology, and Euler analysis. We find that along-strike anomaly variability arises mainly from (1) multi-levelled magnetic contrasts at the fault that are variably sampled by uneven levels of erosion, and to a lesser extent from (2) magnetic susceptibilities that vary along strike within individual units, and (3) variable throw and dip of the fault that produces differences in the extents to which contrasting units are in contact. The multi-levelled magnetic contrasts arise from the juxtaposition of different strata across the fault at discrete depths. Locations of magnetic sources along the fault estimated from Euler analysis of the TMI data reflect the variations in depths to the shallowest sources along strike. Variations in clustering of the Euler solutions suggest that the sources have variable geometry (structural index). The results at the San Ysidro Fault demonstrate the important and complex role of multi-level led magnetic sources in understanding anomalies associated with faulted geologic layers in general. The potential for multiple sources suggests that the use of simple model geometries to represent faults may not always be appropriate.

In 1995, the SALTMAP airborne electromagnetic (AEM) system was used to survey an area of around 50 000 hectares west of Broomehill in the southwest of Australia. It was the first practical application of a new AEM system developed specifically for mapping salinity. The project was a cooperative study between farmers in the area, World Geoscience Corporation, Western Australian Department of Agriculture, Water and Rivers Corporation, and the Cooperative Research Centre for Australian Mineral Exploration Technology (CRC-AMET). Previous AEM surveys in the southwest were carried out using INPUT and QUESTEM, but the results had not been incorporated into land management decisions. The aim of the Broomehill study was to develop the information products to enable farm plans to be created using information from the SALTMAP data. Inherent in the approach was the collection of a large range of complementary geospatial data using geophysical surveys, remote sensing, and other techniques, and incorporation of the data obtained into a geographic database. Various techniques were explored to ensure that as much information as possible from the geophysical surveys flowed through to the eventual land management plans. These techniques included manual interpretation of magnetic and radiometric data and the development of new smart data interpretation in GIS. Intermediate information sets such as ‘Salt Hazard’ and regolith maps guided the development of new farm plans to address land degradation in the area. Ten years later the Broomehill study remains as one of the few cases where farm plans were created based on a full range of geospatial data. This paper reports on the interpretation process which enabled an effective information transfer from geophysicist to farm planner and discusses how new interpretation techniques to define a new set of information could be used to guide land management decisions today.

Mineralogical and geological maps are now possible through a new generation of airborne and satellite systems including the 126-band airborne HyMap and 14-band satellite-borne ASTER imaging sensors. Data from both these imaging technologies were tested for their ability to map geological host rock and mineralogies associated with the Woodie Woodie manganese (Mn) mineral deposits. A detailed study was conducted over the deposits using processed ASTER and HyMap data in conjunction with field and laboratory data. In addition, processed ASTER products were also generated for other prospective mineralised terrains over a larger area within the surrounding Bangemall Basin.

Seamless maps were generated in this study from ASTER and HyMap imagery to represent surface abundances of either mineral groups or specific mineral identities. The ASTER imagery, at 15 to 90 m resolution, enabled the generation of maps designed to represent the abundance of broad mineral groups, including MgOH/carbonate, quartz/silica, ferric, and ferrous iron (within silicate or carbonate). However, the accuracy of these products was observed to be limited, mainly by the broad spectral resolution of the ASTER bands. Narrower spectral bands are generally required to discriminate and map specific minerals, and separate the effects of vegetation and the atmosphere. In contrast, 5 metre resolution HyMap data demonstrated accurate mapping of abundances of minerals such as ferric iron and dolomite/calcite, as well as dry and green vegetation. Image products representing ferrous-rich carbonate units and opaques (including manganese) were also generated.

The results of this study showed that the best products for mapping the Carawine Dolomite and Pinjian Chert Breccia units hosting the Mn mineralisation are obtained by using HyMap data and included: carbonate abundance; ferrous iron content within carbonate units; and ferric iron abundance. The ASTER quartz map product also proved useful to identify silicified dolomite. The geological interpretation of these mineral maps was aided by the comparison and integration with digital elevation model (DEM) and Thorium information from detailed airborne radiometric surveying.