Exploration Geophysics - Volume 23, Issue 1-2, 1992

Conductive regolith represents a geological-noise source which masks the transient EM response of conductive exploration targets in bedrock. Frequent effects are the loss of target response at early times, with a resultant loss of resolution in estimates of target size and depth, and presence of a subtle rather than distinct target response at late times.

Overburden masking can be effectively reduced by computing and stripping the theoretical response of an equivalent layered conductive overburden model from field data. This process enhances subtle signatures attributable to bedrock conductors and improves target resolution by extracting useable TEM signatures at earlier times than is possible with raw data.

Extraction of signatures attributable to deep bedrock targets requires temporal and spatial filtering of the data, followed by Hilbert transformation of vertical-component data to equivalent horizontal components. These processes facilitate identification of the subtle trends attributable to conductive targets below overburden when the processed data is presented either as stacked sections or in plan-view as images.

These enhancement procedures are demonstrated on data from the Eloise copper-gold deposit, near Cloncurry, NW Queensland, which lies below 70 m of highly conductive Mesozoic shales.

The Australian Mineral Industries Research Association (AMIRA) has sponsored Monash University to investigate the acquisition and signal processing of ground-penetrating radar (GPR) for shallow exploration and open-pit mining. This project had three objectives: (1) to acquire high-quality GPR data sets for shallow exploration and surface mining applications under a variety of geological conditions; (2) to develop signal-processing techniques to enhance radar profiles prior to detailed interpretation; (3) to assess the accuracy of GPR and associated processing techniques through comparisons with independent measurements.

Over one hundred kilometres of data were recorded at the various sponsors sites in a wide variety of geological conditions for different economic targets. Several examples are presented to demonstrate:

1) the hand-carried and vehicle-towed acquisition techniques,

2) target detection at depths down to 25 m,

3) the benefits of signal processing for good and poor quality data,

4) the use of dielectric measurements, petrophysical modelling and frequency-dependent synthetic radargrams,

5) the application of seismic interpretation concepts.

The project has demonstrated commercially acceptable rates of GPR data acquisition where the penetration is satisfactory. Some of the unfavourable Australian geological conditions for radar have also been identified. Although signal processing techniques have extended the applicable range of the radar technique, additional hardware enhancements have been identified to improve the commercial application of radar. The next stage is to assess the economic feasibility of radar by comparison with competing techniques such as drilling.

Exploration in the Timor Sea is hampered by the presence of many near-surface limestone reefs. Reef structures form complex velocity anomalies because of their internal velocity structure and because they produce a rapidly varying seafloor profile. Distortion of seismic data recorded over such reefs comprises event pull-up and poor imaging. Conventional common mid-point processing is not adequate to interpret prospective targets below the reefs.

Seismic distortion associated with reefs can be reduced using replacement techniques of varying complexity. Wave-equation datuming is an accurate method but computationally expensive, and replacement statics is an efficient method but only approximate. All replacement methods require an estimate of the velocity structure inside the reef itself, although this is not normally attempted in production processing.

The seismic image beneath surface reefs can be improved by integrating layer replacement with velocity analysis. The seismic data are first modified by replacing the water velocity with an average velocity of the reef. This involves two steps: downward continuing to the sea floor, and then upward continuing to the surface. A sufficiently accurate value for an average velocity can usually be obtained using conventional velocity analysis. This approach removes most of the velocity inhomogeneity associated with the water/reef interface and renders subsequent velocity analysis more accurate.

A model of a typical reef from Browse Basin, which includes strong lateral velocity inhomogeneity on the reef flanks, shows that velocity analysis post-replacement gives much more accurate RMS velocities inside and immediately below the reef. Events below the reef are imaged more faithfully. Conventional velocity analysis is not accurate enough to give interval velocities for the reef because the CMP method cannot image such strong lateral velocity changes.

The same model can be analyzed using reflection tomography based on the algebraic reconstruction method (ART). ART is sufficiently accurate to give useful estimates of the interval velocities within the model reef. Comparing results before and after replacement indicates that replacing the water layer prior to ray tracing gives closer estimates of the high-velocity reef flanks and reduces velocity smearing across the sea floor to water interface. Estimates of the interval velocities inside the reef are important for accurate imaging. These can be used to continue downward through the reef structure for a second pass of wave-equation replacement, or for applying a variety of pre-stack depth migration methods.

Inversion of surface seismic reflection travel times for both the velocity distribution above a reflecting horizon, and the reflector position, is known to suffer from problems. These include non-uniqueness, poor resolution and ambiguity between velocity and reflector position.

Often this inverse problem is treated by using a minimization technique, where the objective is to minimize the difference between modelled and observed travel times. Gradient-based methods such as Steepest Descent and Conjugate Gradients often are used to produce a model that best fits the observed data in some sense. These techniques use local gradient information of the objective function that measures the goodness-of-fit between observed and modelled travel times.

One way of avoiding problems associated with differences in physical dimensions in gradient-based optimization methods is to use a Petrov-Galerkin, or Subspace, method (Saad and Schultz, 1985; Kennett and Williamson, 1988), in which the gradient vector is split into two orthogonal component vectors, producing a two-dimensional subspace. The first component is the gradient vector associated with the slowness parameters, while the second is the gradient vector associated with the reflector position parameters. After making this decomposition, the minimum is sought in the subspace spanned by these two vectors rather than searching for the minimum along the combined gradient direction alone.

By producing contours of the objective function within the two-dimensional subspace described above, insight can be gained into some of the problems faced by travel-time inversion. The trade-off between velocity and reflector position is indicated by the trend of the contours. Non-unique solutions are indicated by the fact that a broad range of earth models can produce an equally good fit to the data. They also highlight the need for reasonable starting models, which often are not available in practice, as these significantly influence the final model.

In this paper, brief details of Petrov-Galerkin methods for optimization are given. In addition, objective-function contours, within the two-dimensional subspace described above, are produced for a synthetic travel-time inversion problem.

The Eloise Cu-Au deposit is located approximately 60 km south-east of Cloncurry in northwest Queensland. It is hosted within interpreted Early to Middle Proterozoic metasediments of the Soldiers Cap Group, and comprises a number of steeply plunging, structurally complex mineralised zones. The deposit is overlain by between 50 and 70 m of highly conductive, flat-lying Mesozoic age sediments.

Geophysical methods assumed leading roles in the discovery of the Eloise deposit. The systematic evaluation of linear aeromagnetic trends using moving-loop transient EM (TEM) profiling led to the initial recognition of the Eloise anomaly. Fixed-loop TEM surveying detailed a strong bedrock conductor broadly coincident with a moderately intense magnetic ridge. Drillhole END17, sited to test the conductive source, intersected 20 m of chalcopyrite-pyrrhotite mineralisation.

The geometry of the deposit limited the extent to which surface TEM methods could confidently define targets for subsequent drilling. As a consequence, downhole TEM was identified as an effective tool to guide exploration drilling. This technique proved extremely useful, both in detecting additional mineralised bodies and in helping to define the dip extent of the deposit.

Lines of gravity and dipole-dipole IP have also been completed over the deposit to further characterise its response. In both instances sizeable anomalies were recorded, attributable in part to a significant mafic alteration system associated with the mineralisation.

3-D pre-stack depth migration is a viable imaging process when implemented on parallel super-computers. Run times for complete output volumes that would have been years on conventional computers are now on the order of weeks. An iteration on the important velocity model, however, can be done with a small subset of the output volume and hence can be accomplished in a few days. Sub-salt imaging is a particularly difficult imaging problem that requires 3-D prestack depth migration. A 3-D sub-salt case history from the Gulf of Mexico illustrates the benefits of and effort behind 3-D pre-stack depth migration and migration-velocity analysis, and underscores the perils of 2-D pre-stack depth migration in imaging complex structure.

A potentially useful seismic technique for fault detection has been tested by finite-difference modelling of elastic wave propagation in heterogeneous media. It employs a surface-to-borehole vertical seismic profiling (VSP) geometry. Geophones are lowered into the borehole and seismic sources are fired at or near the ground surface. Small disruptions to a low-velocity zone (LVZ) bounded by higher velocity media can be detected by the technique with the analysis of seismic arrival patterns recorded at geophones in a nearby borehole. A disruption in the LVZ will act as a secondary source when an incident wave strikes it. Some of the energy radiated from the secondary source can be captured by the LVZ and will propagate as guided waves.

Both offset VSP and walkaway VSP recording geometries are considered for the numerical simulation. In the offset geometry, a spread of geophones is deployed in the borehole around the coal seam while a single source is fired. Both disrupted and continuous LVZ models have been considered for numerical simulations. Explosive sources (P-wave energy only) are used. Synthetic seismograms show that channel waves can be excited by the scattered energy at a LVZ disruption. The maximum amplitude of the channel waves is comparable to those of direct body waves, which clearly indicates that the disruption in the LVZ can act as an efficient source to generate guided waves.

In the walkaway VSP geometry, a single geophone is fixed in the LVZ and a spread of sources is fired on the surface away from the borehole. Synthetic seismograms demonstrate that the apex of the channel-wave arrivals pinpoints the location of the disruption of the LVZ for a layered model.

Although this technique has parallels with in-seam seismic coal exploration, it does not require that the source be placed within the low-velocity channel. This method is equally applicable to any exploration target which relates to a LVZ.

Random noise can cause problems with the interpretation of seismic sections and can degrade the performance of deconvolution, velocity analysis and migration. When the noise occupies the same frequency band as the signal the only way to attenuate the noise is to perform some ‘averaging’ process across adjacent traces. A very successful way of doing this on 2D data is with the method of FX prediction filtering.

This paper describes the advantages that are gained by extending FX spatial prediction filtering for random noise reduction to 3D. First of all we give an outline of spatial prediction filtering in both 2D and 3D, pointing out the detrimental effects of the process on weak events, curved events and faulted events. It is then shown how the true 3D version of the method (which we call FXY prediction) performs better than the 2D version with respect to the above problems, with examples given using synthetic data. We find that FX and FXY filtering are both capable of reducing random noise on 3D data to the same extent, but that FXY is preferable because it gives less distortion of the geology.

Most previous approaches to dimensionality-reduction of multi-channel seismic data assume a linear constraint surface, such as principal-component analysis. Such methods buy simplicity at a sacrifice of generality, which will not work with a non-linear constraint surface. Even with such a disadvantage, the traditional methods can, in the linear cases, eventually map the multi-dimensional data to an eigenspace of a lower dimension with no loss of useful information. One previous nonlinear dimensionality-reduction method based on neural networks can simply map the multi-dimensional data to a constraint surface whose proper dimension can be determined only by experiments when prior information about the constraint surface inherent in the multi-dimensional data is sparse. When the selected dimension is not proper, there will be much loss of useful information.

Based on the self-organizing properties of neural networks and the multi-layered perceptrons’ ability of internal representation of the input patterns in the hidden units, a more general method is proposed to achieve optimal dimensionality-reduction which can discover the optimal dimension and type of constraint inherent in the multidimensional data with the least loss of information, and map the data from a n-dimensional feature space onto a m-dimensional ‘generalized-eigenspace’ (m

This new dimensionality-reducer consists of two parallel neural nets which, respectively from opposite directions, search for the optimal dimension and type of the ‘generalized-eigenspace’. The improved error back-propagation algorithm is used to update weights between the units of the different layers. A modified simulated annealing procedure is used to change the sizes of the two middle hidden layers of the two parallel neural nets so as to achieve finally an optimal dimensionality-reduction.

Real multi-channel seismic data are used to test this new approach. The results demonstrate its usefulness and advantages over the previous neural dimensionality-reducer.

Since 1988, more than 800 magnetotelluric (MT) stations have been acquired in Papua New Guinea. Most of the stations have been acquired in the highlands where conventional exploration (seismic) is difficult owing to the outcropping limestone, intense karstification and severe topography. The seismic data that are acquired are expensive and, in many cases, poor to no-record.

In an effort to provide useful geophysical data in the exploration effort, several companies have acquired MT data. The survey size has ranged from nine to over one hundred MT stations. Most of the data have been acquired along profiles, with station spacing of between 200 m and 1500 m.

With the release of MT and drilling data by the PNG government, a comparison of MT data with well data can now be shown. MT data have now been acquired at more than 15 well locations, which were drilled either before or after the MT data surveys. These comparisons confirm the usefulness and application of the MT technique in tough exploration regions such as PNG.

Although MT does not have the accuracy of seismic, it can be very useful in predicting pre-drill depths and lithologies, at best to within 5% of depth. Perhaps MT is more useful in determining gross structure at depth, such as mapping repeats of thrusted section and structural style, which combined with geologic modelling can help to determine the best well location or, in some cases, lower the prospectivity of a prospect.

When recorded properly as closely-spaced stations along dip profiles, and interpreted using 2-D algorithms integrated with geology, MT can be very successful in predicting pre-drill depths and helping to determine well locations.

Interpretation of magnetic surveys in terms of geology is hampered by poor correspondence between broad lithological categories and magnetic properties, and by lack of knowledge of the geological factors that influence the magnetisation of rocks. Magnetic petrology is the integrated application of rock magnetic and conventional petrologic techniques to identify and characterise the magnetic minerals in rocks. This information elucidates the factors that produce, alter and destroy magnetic minerals and thereby influence the bulk magnetic properties of the rocks and their associated magnetic anomalies. Improved understanding of magnetic petrology is therefore essential for maximising the geological information that can be obtained from magnetic anomaly patterns.

The Bureau of Mineral Resources (BMR) undertook a regional marine seismic survey of the Gippsland Basin in 1988-89. During this survey, recording stations were deployed onshore by the BMR and the Department of Earth Sciences, Monash University, to record long-offset wide-angle reflection and refraction data from the standard marine seismic reflection air-gun shots. The purpose of recording this complementary data was to obtain deep velocities and regional structure, to map variations of crustal thickness, and to link the marine data with earlier onshore results. Arrivals from offsets up to 230 km were recorded, thus sampling the total crustal thickness. It was necessary to enhance the data at large offsets by filtering and stacking because of the relatively low power of the source and noisy conditions during the survey. The acquisition method is logistically simple, and a large amount of additional data can be gathered for negligible extra cost to a survey.

Velocities between 5.3 and 5.9 km/s were obtained for the basement and mid-crust. Lower crustal velocities were around 6.3 km/s, and upper mantle velocities were between 7.8 and 8.1 km/s. Sediment velocities between 1.8 and 4.5 km/s were obtained from sonobuoys. The velocity control constrains the identification of deep reflectors and allows accurate depth conversion of interpreted seismic reflection profiles. The deep basement geometry and crustal structure were interpreted along selected traverses from the long-offset data in conjunction with sonobuoy and reflection results. The sediments in the Central Deep reach a thickness of about 12 km. The total crustal thickness thins from about 36 km in Victoria, north of the basin, to about 23 km below the deepest sediments.

The boundary between the upper and lower crust, where the velocity increases from about 5.9 to 6.3 km/s, correlates to a reflector which has been previously interpreted as a major regional detachment surface associated with basin formation.

In exploration practice in the Western U.S., the two most commonly used IP/resistivity electrode configurations are the dipole-dipole and gradient arrays. Differences in the relative responses between the two electrode configurations have been noted and the relative merits debated, but published data are limited. A suite of dipole-dipole and gradient-array profiles have been computed for direct comparison of both conductive and resistive bodies. Profiles for thin vertical and thin horizontal tabular bodies (dykes and slabs respectively), are the primary basis for comparison. The anomalies at a typical resistivity and chargeability contrast are normalized by determining the ratio of amplitude to the background response. Plots of resistivity and chargeability illustrate the differences with depth for the two types of bodies.

For thin conductive dykes, dipole data give a strong characteristic response on both resistivity and chargeability parameters; gradient profiles, however, show a weak resistivity and nil chargeability response. Conductive slabs show excellent anomalous response for both arrays. Resistive dykes produce better relative resistivity and chargeability responses on the gradient array than the low amplitude, peculiar characteristics, responses of the dipole array. The resistive slab produces almost no chargeability response on the gradient array.

Overburden effects, multiple bodies, dip and several case histories are also examined to show the differences in interpretation potential.

An eventual recommendation for use of either array depends upon the interpretation required, geological assumptions of the target size, shape, and physical property contrast, and the cost differential of data acquisition.

Normally, special purpose drill holes are required to provide direct determinations of surface heat flow. These provide core samples for thermal conductivity measurements and allow access for measurements of a reliable geothermal gradient after long periods of equilibration. Different, indirect procedures are now required to provide estimates of heat flow for offshore basins.

Bottom hole temperatures can be used with suitable corrections to provide an indication of the regional geothermal gradient. However, the corresponding thermal conductivities are more difficult to determine. Geophysical logs can be used to detect the major lithologies but detailed statistical analysis is required for numerical estimates. Complex mixing models are now suggested for geothermal surveys linking physical properties to variations in the mineralogy of each well.

Coal is a low-cost commodity and coal miners profit margins are slim, such that mining companies must maintain tight cost control to achieve acceptable returns. The challenge to coal geophysics is to adapt to these economic constraints and provide cost-effective, reliable methods for exploration and mining requirements. The geophysical methods which have been used in the reconnaissance, pre-development, and operational stages of coal exploration and mining are reviewed in terms of their exploratory function and their typical frequency of application. Borehole logging is the one method which is highly utilised for both surface and underground coal mining exploration, although seismic methods (mainly high-resolution reflection, and in-seam), are also frequently applied in underground coal-mining investigations.

Because surface coal mining is constrained to shallow seams, most exploration is achieved by drilling methods, with geophysics playing a supporting but subordinate role. Also surface mining is less sensitive to seam disturbances such as faulting, rolls and intrusions, and can afford to set less strict requirements for prediction and resolution. The trend in underground mining is towards high-production longwall methods, which are much less tolerant of disturbance, and demand survey reliability and resolution standards which often stretch the limits of current exploration technology. A Workshop on Underground Coal Mining Exploration Techniques was conducted by the Australian Coal Association in 1991 to identify the strategies and priorities for future research and development. Most of the recommendations coming from the Workshop incorporated geophysical technologies, particularly the development of borehole logging methods which can be applied to in-seam drillholes. The recommendations also identified the need to selectively import and adapt analogous but more advanced technology from other areas, such as seismic reflection expertise from the petroleum industry, and to consolidate on the promising beginnings shown by techniques such as the radio-imaging method and in-seam seismic.

The future also holds opportunities for geophysics to assist in the development and application of innovative mining systems. High-wall mining, continuous surface miners, and the increasing trend towards high-production automated mining equipment, bring with them the need to provide geophysical monitoring sensors and guidance systems.

Seismic and magnetic data were collected along a 4-km traverse which crosses a contact between greenstone and granitoid rocks about 1 km north of the Nevoria Gold Mine in the Southern Cross Greenstone Belt, Western Australia. The locations of magnetic sources were determined from the magnetic data by Werner deconvolution. First-arrival traveltime data allow the recognition of two time-distance branches, indicative of two layers. The depth to the refractor (top of the second layer) was determined using the plus-minus method. In the greenstones, the refractor coincides with a zone of magnetic sources, about 30 m wide, which occurs at depths of between 5 and 50 m. In the granitoids, the refractor coincides with the base of a broad zone of magnetic sources that extends from the surface to depths of between 20 and 100 m. These results are provisionally interpreted in terms of contrasts in magnetic susceptibility and seismic velocity at the base of the regolith, suggesting that the protolith-regolith interface can be mapped using the magnetic and seismic refraction methods.

A section through the regolith and into the protolith is exposed in the Bottle Dump Pit of the Nevoria Gold Mine. Measurements of magnetic susceptibility and density have been made on samples of amphibolite and banded-iron formation from different depths in the pit. These data yield evidence for the postulated increase in magnetic susceptibility and density, and therefore probably seismic velocity, at the interface between the regolith and protolith.

However, the contrasts in physical properties are small because the regolith in the pit is unusually fresh for the area.

These data imply that magnetic profiling may provide a cheaper alternative to seismic refraction studies as a method to map the extent of sub-surface weathering.

The three-dimensional geometry of greenstone belts in Africa and Canada is reasonably well constrained by gravity, geoelectric and seismic reflection data. A comparison of greenstone belts throughout the world shows that their overall geometries are everywhere similar. They are typically about 5 km thick and have relatively flat bases. The magnitude and direction of the dip of their contacts with surrounding granitoid rocks is variable, probably owing to local differences in their tectonic histories. The greenstones are sometimes intruded by sheet-like granitoid bodies. Thicker granitoid intrusions which form basement ridges are also recognised.

Fewer geophysical studies are available from Western Australian greenstone belts. However, gravity data show that in some cases the granitoid-greenstone contact dips at a shallow angle away from the greenstone belt. There is also evidence that some of the granitoids are sheet-like. These observations carry the implication that mineralised greenstones may be present in the sub-surface below areas of granitoid outcrop.

Evidence from the Murchison Greenstone Belt in South Africa, and from the Eastern Goldfields, suggests that a spatial correlation exists between mineralisation and concealed granitoid bodies. As the latter granitoids may be revealed as gravity ‘lows’, regional gravity surveys may be a useful first-order exploration tool within greenstone belts.

In the Southern Cross Greenstone Belt, gold mineralisation is often associated with iron-rich sedimentary rocks. In the Burbidge area, a massive iron formation crops out on the eastern limb of the north-south trending Caudan Antiform. On the western limb, the iron formation is rarely exposed, although it has been proved at depth by drilling and has been mined in the Bronco South Pit. Geophysical surveys were undertaken to the south of Bronco South to locate the continuation of the iron formation and to optimise a drilling program. Results of these surveys are described herein.

Three phases of ground magnetic data were acquired. Despite high-amplitude noise originating within surface laterites, a magnetic anomaly of approximately 500 nT amplitude and 300 m wavelength was identified, coinciding with the postulated position of the iron formation. Following wave-number filtering to suppress the lateritic noise, the data from each line were modelled in terms of a two-dimensional, subvertical, sheet-like body at a depth of about 100 m. The iron formation in Bronco South is highly silicified and weathered, and has a very low magnetic susceptibility (< 0.00028 si units). as this is too low to cause the observed anomalies, a magnetic susceptibility of 0.12 si units was used, based on measurements of fresher iron formations exposed elsewhere in the area. we interpret the modelled body as representing unweathered iron formation below the weathering profile.

Based on the magnetic modelling, a series of structural features affecting the iron formation is recognised. Since gold mineralisation associated with the iron formation is known to be concentrated in such zones, these areas are considered to be the most prospective within the horizon. Subsequent drilling confirmed the inferred position of the iron formation and intersected gold mineralisation at moderate grades.

The mid-Proterozoic Mt Isa Inlier forms part of the Mt Isa Geophysical Domain, an 800 km long by 200 km wide entity defined by gravity and magnetic patterns. The exposed portion of the Inlier totals about one-third, or 50 000 km², of the Domain, and contains in known or previously mined deposits about 42 Mt of zinc, 25 Mt of Pb, 10 Mt of Cu, and 2.5 million ounces (77.8 t) of Au, in a variety of geological settings, and valued in 1992 dollars and metal prices at $A100 billion. Not surprisingly, this degree of metal endowment has stimulated geological and geophysical exploration of the 100 000 km² of the Inlier which is unexposed, and where several discoveries have already been made at depths of 30-50 m.

Three major tectonic units are recognised — a central basement block (Kalkadoon-Leichhardt block) flanked by the Western and Eastern Fold Belts. Basement granites and volcanics are dated at about 1860 Ma and older. The post-basement volcano-sedimentary successions, dated 1800 Ma to 1620 Ma, are related to periods of crustal extension and rifting, especially in the west — the Leichhardt River Fault Trough — and also in the Eastern Fold Belt, where the character of rift-related sequences may be masked by high-grade metamorphism.

Post-basement granites are dated at about 1740 Ma and 1600 to 1500 Ma. Major deformation episodes D1 to D3 and accompanying metamorphism occurred also in the period 1600-1500 Ma.

Filtered gravity data reveal a crustal structure of north-trending alternating belts of high and low density, representing belts of palaeorifting and high-density basic rocks juxtaposed with lower density sediments, felsic volcanics and granites. The filtered gravity linear features are offset by NW to WNW-trending cross-structures, which may be of broad exploration significance.

Regional aeromagnetic data are very valuable in definition of subsurface major fault structures, delineation of areas of basic volcanics in the Western Fold Belt as a guide to Isa-style Pb-Zn and Cu deposits, delineation of granite plutons with possible skarn envelopes and U-REE-Cu-Au potential, and identification of quartz-magnetite zones associated with firstly, the Cu-Au deposits of the Starra-type, and secondly with the chemogenic/exhalative sequences which characterise the Pegmont/Broken Hill style of mineralisation, very common in the Eastern Fold Belt. Some regional aeromagnetic patterns may be modified by alteration styles in basalt, by facies variation and changes in metamorphic grade, by zoning in granites, and by superposition of thrusted sequences.

Future exploration of prospective but concealed areas demands that geophysics and geology be used and integrated to the greatest possible extent — that for optimum success, geophysicists, geologists and their respective technologies, truly become brothers-in-arms.