Exploration Geophysics - Volume 31, Issue 1-2, 2000

Airborne electromagnetic (AEM) surveys for mineral exploration generate vast quantities of data. Rigorous electromagnetic (EM) modelling of geologically realistic structures is very computationally expensive, and interpretation of local AEM anomalies is usually based on approximate or nomogram methods. These methods typically account only for vortex induction in a conductive target, and are only valid if the host medium containing the target is poorly conductive, or if measurements are made at long delay times.

When a target is either wholly or partly embedded in a conductive host, current channelling may enhance its response in comparison with that due to vortex induction, and the exciting and secondary fields are modified by ‘blanking’ effects as they pass through the host. The relative importance of current channelling and vortex induction depends strongly on the target and host parameters, the location of the AEM system relative to the target, and on the measurement delay time. In some instances, galvanic currents may dominate the entire target response, and models that account only for galvanic excitation can be applied. At the relatively early times employed in AEM prospecting, it is necessary to account for blanking of the current channelling response by the host medium, imposed as the secondary fields pass from the target to the receiver. Blanking effects can, however, be ignored if modelling is carried out at the resistive limit of the response.

Shallow regolith features, such as palaeochannels, may give rise to observable AEM current-channelling anomalies. A field example from the Lawlers area of Western Australia demonstrates that the AEM anomaly associated with a known palaeochannel can be largely explained by the galvanic response of horizontal thinsheet targets embedded in the overburden of a two-layered Earth. The host and palaeochannel parameters required to reproduce the observed anomaly are consistent with those derived from drilling, surface EM and DC resistivity surveys, and petrophysical measurements.

Electromagnetic modelling of 3D conductive targets using staggered grids results in matrix systems that are severely ill conditioned and unsuitable for many iterative solvers. Either a minimal residual solver, MINRES, or a restarted biconjugate gradient stabilised solver, BiCGSTAB(l), can be used to bring the solution to convergence. The BiCGSTAB(l), method, combining the bi-conjugate gradient method with a restarted minimal residual correction, is very efficient in solving the matrix system arising in our applications. To speed up the solver further we employ various preconditioning techniques. The use of a symmetric Jacobian preconditioner greatly improves the convergence of the MINRES method. In homogeneous regions where the divergence of the electric field is zero the Maxwell equations reduce to vector Helmholtz equations with decoupled components. These Helmholtz equations can be readily approximated by central differences, resulting in diagonally dominant matrices that can be used as preconditioners for the original curl curl equation of the staggered grid. The diagonals of the Helmholtz equation can also be used as a Jacobian preconditioner, which is superior to Jacobian preconditioning using the diagonal elements of the original curl curl equation. Numerical results have demonstrated the efficiency of these approaches.

The TEMPEST airborne electromagnetic system is designed to measure the information that is required to derive accurate, highresolution three-dimensional conductivity estimates of the subsurface.

TEMPEST is configured with a transmitter loop located on a fixed-wing aircraft and receiver coils located in a towed bird. The system measures the EM response of the ground over a widebandwidth (25 - 37.5 kHz). The transmitter waveform is a square wave with 50% duty cycle, i.e. equal on and off times, and variable switching ramp. Very low noise levels are achieved by recording the received signal at a high sampling rate, 75 kHz, then applying sophisticated signal processing techniques. The signal processing operates in the frequency domain to perform a full deconvolution of the measured response, removing the system transfer function characteristics and dynamically compensating for variations in the transmitted waveform. The broad bandwidth allows the variable primary field effects that result from changes in coupling between the receiver coils in the towed bird and the transmitter loop to be more accurately removed by reducing the uncertainty in the ground response. To assist interpretation, the deconvolved ground response signal is converted to a 100% duty cycle square-wave B-field response, allowing a single transient decay to be presented for the full 20 ms half cycle length available for a base frequency of 25 Hz. The system geometry can be accurately monitored. This involves measuring the orientation of both the transmitter loop located on the aircraft and the receiver coils located inside the towed bird. These orientation measurements are used to compensate the ground response data for the effects that result from variations in the system geometry as the aircraft flies along the survey line.

A 3D conductivity grid is produced by combining a series of 1D inversions. In addition to standard conductivity sections, the contents of these data volumes can be displayed as conductivity slices and iso-conductivity surfaces.

Data from Walford Creek, Queensland, define conductivity distributions in close agreement with ground EM and drilling information.

Electromagnetic radiation from distant, discrete, ground-to-cloud lightning return-strokes can be recorded as atmospherics (spherics), using both electric and magnetic field antennas. The Earth-ionosphere waveguide filters this radiation into the ULF-VLF radio bands from 3 Hz to 30 kHz with a notch centred at approximately 2 kHz. The relationship between the orthogonal horizontal electric and magnetic field signatures at the receiver is directly related to the surface impedance of the Earth at that point. Transforming the transient fields to the frequency domain allows the calculation of the surface impedance over the frequency range determined by the sample rate and record length. From this, resistivity versus depth models of the Earth can be determined. This paper demonstrates that a discrete spheric can be considered as a broadband vertically polarised plane-wave source suitable for magnetotelluric (MT) style geophysical surveying, and that the data collected could be used to deduce the near-surface resistivity profile. The results from measurements made with MIM Exploration’s proprietary distributed acquisition system (MIMDAS) are presented. Multi-station data were acquired in a telluric profiling mode (without a magnetic field reference) to demonstrate spatial coherence and correlation with conventional MIMDAS MT pseudosections and IP inversion models. The advantages of the technique include the broad band of measurement and rapid data-acquisition time for reliable statistics. Valid data were collected between 24 kHz and less than 200 Hz and while stacking improves data integrity, useful information may be extracted from less than 5 ms of a 6 s record, corresponding to a single strong spheric. This technique has potential value in near-surface geotechnical applications and as an aid in removing statics from conventional continuous-MT survey data.

An airborne electromagnetic survey using the helicopter-borne DIGHEM^V system was carried out over lower Port Jackson, Sydney Harbour. The study served to review the capacity of this system to provide accurate bathymetry data in shallow seawater down to 30 m, which is the maximum depth of the survey area. The DIGHEMV system operated at frequencies of 328 Hz, 7337 Hz and 55300 Hz in a horizontal coplanar transmitter/receiver coil configuration and 889 Hz and 5658 Hz in a vertical coaxial transmitter/receiver coil configuration. In order to obtain highquality data with optimised signal to noise ratio, rigorous calibration procedures were carried out to ensure data accuracy. These included low-altitude profiles over deep seawater of known electrical conductivity, thorough calibration of phase and gain over very resistive background at a remote site, and high-altitude zero level calibrations throughout the survey flights. Two-layer inversion of the data yielded a thickness of the upper layer (bathymetry) which is generally consistent with depth-sounding data. This study verified the use of airborne electromagnetics as a promising low-resolution bathymetric reconnaissance tool that can be used to rapidly survey shallow coastal waters.

Seafloor and land magnetotelluric (MT) data were collected in the SWAGGIE (Southern Waters of Australia Geoelectric and Geomagnetic Induction Experiment) project in April-May 1998, from 30 seafloor and 23 land sites. The principal objective of the experiment was to delineate the strike and depth of a zone of high electrical conductivity, known as the Eyre Peninsula Anomaly (EPA) in South Australia. Three linear arrays of marine magnetotelluric instruments were deployed across the continental shelf and slope to locate the offshore extension of the EPA on the continental margin, and to image the continental-oceanic lithosphere-asthenosphere transition. A land array of magnetometers was deployed at the same time to better resolve the EPA in the southern Eyre Peninsula. Robust remote-reference processing of time-series magnetic and electric data gives good MT and geomagnetic depth sounding responses in the bandwidth of 10¹ to 10⁵ s, corresponding to skin-depths of mid-crustal (10 km) to mantle transition (400 km) range. Initial processing of marine and land data clearly indicates that the EPA is continuous to the edge of the continental shelf, with a conductance greater than 15 000 S confined to a narrow, near-vertical zone. At sites distant from the EPA, one-dimensional MT inversions fit the data well and provide a background conductivity structure for two and three-dimensional forward and inverse modelling of the EPA.

A test drillhole magnetometric resistivity (DHMMR) survey has recently been carried out at a nickel prospect in Western Australia to compare the effectiveness of this relatively new method with that of drillhole electromagnetics (DHEM), which is routinely used for nickel sulphide exploration.

The DHMMR clearly defined a zone of shallower matrix sulphides which were only weakly detected by some of the earlier DHEM surveys (and then only in hindsight). In contrast, the DHMMR only weakly responded to the massive sulphides that had been well defined by the DHEM. Thus, these two methods can be considered, at least for this type of target, as complementary methods, with the DHMMR responding well to bodies with a relatively large cross-sectional area and DHEM to bodies of much larger long-sectional area. Although usually of lower grade, the matrix sulphides at this prospect, with their larger cross-sectional area and hence bigger tonnage, may make a more attractive economic proposition.

Cosmos is a typical Kambalda-style, komatiite-hosted, highly conductive and magnetic, massive pyrrhotite-pentlandite sulphide deposit. It is located approximately 40 km south of the Mt. Keith nickel mine in the northern Yilgarn Craton of Western Australia. It was discovered in mid-1997 using a combination of geological, geochemical and geophysical exploration techniques during routine evaluation of a prospective ultramafic belt. The discovery hole was the initial test of a short strike-length, late-time transient electromagnetic (TEM) anomaly in an area that had previously been partially drill tested by other companies. The original moving-loop TEM anomaly, recognised on two 150 m spaced lines, was defined in detail using a large, fixed transmitter loop survey. Modelling of the late-time fixed-loop data indicated that the source was a steeply east dipping conductor at about 50-75 m depth. Oregrade massive nickel sulphides were intersected in the predicted position in the first diamond hole drilled to test the conductor. The limited depth extent of the conductor suggested by the modelling was confirmed by subsequent drilling and downhole TEM surveys. Detailed ground and aeromagnetic data subsequently collected over the deposit defined the host ultramafic well, but did not clearly distinguish the magnetic massive sulphide zone. The significance of the Cosmos TEM surveys was their ability to quickly and accurately focus drilling on the small but high-grade massive sulphide lens within the much larger mineralised halo.

Geophysical techniques played a significant role in the discovery of the Emily Ann massive nickel-sulphide deposit and extensions to the Maggie Hays deposit, which are associated with komatiitic olivine cumulate ultramafic rocks, in an Archaean greenstone belt located about 500 km east of Perth, Western Australia.

Detailed aeromagnetic surveys were used to outline komatiitic rocks and structures. Physical property measurements on drill core showed the mineralisation to be highly conductive and magnetic. Trial induced polarisation, audio magnetotelluric and time-domain electromagnetic (TEM) surveys indicated that the latter had the most potential for detecting the nickel-sulphide mineralisation.

The Maggie Hays deposit comprises both disseminated and massive nickel-sulphides concentrated at the base of an ultramafic unit 200-500 m below the surface. A limited moving-loop TEM survey in 1992 located an anomaly immediately north of the main part of the deposit. Diamond drilling of this anomaly failed to discover extensions to the deposit or explain its source. In 1995, a fixed-loop TEM survey delineated an excellent response confirming the earlier moving-loop anomaly, which when drilled, resulted in the discovery of the Maggie Hays North zone 100 m below the surface.

A moving-loop TEM survey resulted in the discovery of the blind, high-grade Emily Ann nickel-sulphide deposit 3 km north of the Maggie Hays deposit, at a depth of 120 m. Downhole TEM surveys aided delineation diamond drilling of the deposits with the location of extensions of mineralisation.

High-powered, late-time moving-loop TEM, with fixed-loop TEM follow up, is currently being used routinely to explore for additional deposits. However, the highly conductive overburden response obscures the signal from bedrock conductors, which are often represented only as low-amplitude, late-time anomalies. Geophysical targeting is further complicated by the close proximity of highly conductive barren banded iron formation-hosted massive sulphides. Trial airborne EM surveys have detected Maggie Hays North, but not Emily Ann and probably not the Maggie Hays main zone.

Nickel-sulphide deposits lying at the base of the regolith usually give strong, readily interpreted, surface time-domain electromagnetic (TEM) anomalies. In the Leinster area of Western Australia much of the prospective stratigraphy has been covered by TEM surveys and the strong anomalies have been drill tested. Deep nickel sulphides may be economic if they occur close to existing mining and processing infrastructure, but targeting the subtle TEM responses of these bodies requires skilful application and interpretation of TEM surveys. Conductive regolith can mask the response of deep nickel-sulphide orebodies to surface TEM systems. IP effects and other geological noise originating in the regolith can make recognition of weak bedrock anomalies difficult. Taking a slingram or out-of-loop reading can minimise regolith effects, and give a better response from sub-vertical bedrock conductors than in-loop readings. Downhole electromagnetic (DHEM) surveys in reconnaissance drill-holes increase the effective penetration depth of TEM, and can assist surface TEM interpretation in a prospect area. Despite weak surface TEM anomalism, DHEM surveys of deep drill-holes can yield precise information about the position and conductivity of nickel sulphide systems.

The regolith in the Lawlers district, Western Australia, can be divided into a number of geologically identifiable layers. Saprolite commonly forms one of the thickest and least dense of these layers, and appears to be by far the most porous material encountered in the section. Porosity derived from many samples averaged 37.8%. The solid constituents of saprolite are not solely clay, saprolite rather is an extremely weathered rock type with a retained fabric that supports its structure and which behaves as a clayey sand. While the constituent clays (predominantly smectite over mafics and ultramafics, and kaolinite over felsics) are inherently conductive, the very high porosity suggests that the measured conductivity of saprolite in-situ should be dominated by the salinity of the local groundwater, even if contained fluids are only slightly saturated.

Previous work has suggested that the conductance and/or conductivity of the saprolite layer are different over different lithologies. However, physical property measurements indicate that the conductivity of saprolite cannot uniquely be used to determine parent lithology. These physical property studies indicated overlaps in electrical properties for metasedimentary, felsic, mafic and ultramafic saprolites and also overlying palaeochannel clays. However provided that groundwater conductivities are not too variable, diagnostic conductance differences may occur, particularly since the saprolite layer over felsic lithologies is usually reported to be thinner than that developed over mafics or ultramafics.

Below the saprolite layer, a saprock layer is defined that contains the transition from virtually unweathered bedrock to heavily weathered saprolite. This transitional layer retains much of the structural strength and density of bedrock, and thus has very similar seismic velocity to bedrock. Its conductivity however, is far greater than that of the bedrock: possibly reflecting the influence of connected conductive paths that mark the passage of fluids during the weathering process. Above the saprolite, regolith materials have much lower porosities and higher densities. They also tend to be lower in conductivity than the saprolite. The geological processes of deep argillisation and surface and mid-level ferruginisation, accompanied by silicification are responsible for the physical property variations described above.

Economic benefits can flow from geophysical logging at all stages of the mining cycle. The most commonly cited benefit is the substitution of delineation diamond drilling with cheaper percussion or reverse circulation drilling in cases where geophysical logs can substitute for core. This approach can deliver an attractive direct saving in drilling costs if drill metreage is unchanged, or a potentially greater indirect benefit from better resource delineation if more holes are drilled within the original drilling budget. Operational advantages of logging include data objectivity, speed of interpretation, and reduced core handling and analysis costs.

Substitution of diamond drilling with percussion drilling is not always feasible. However, the amount of core drilling which can be foregone in favour of more economical drilling plus logging expands enormously if grade can be reliably inferred from petrophysical logs. For some ore systems a close correlation can exist between a petrophysical parameter and grade. More commonly, individual petrophysical grade estimates are less reliable than assays, but the overall correlation with grade may still be adequate for discrimination between grade ranges, for example, ore versus waste, or low-grade versus high-grade. In order to exploit the potential for petrophysical grade estimation in these cases, efficient means must be found to infer grade ranges from borehole logs.

An automated interpretation tool, LogTrans, has been developed for a geophysical log analysis. LogTrans performs rapid analysis of multi-parameter logs and expedites presentation of interpreted results in a form meaningful to mining engineers and geologists. The LogTrans algorithm exploits the contrasts in petrophysical signatures between different classes of rock, distinguished by lithology, grade, mechanical properties, or a combination of characteristics. Interpretation entails two stages; the first, is a statistical characterisation, where geophysical logs and core-based geoscientific data from a set of control holes are combined to form an attribute lookup table. The second stage statistical discrimination, examines the measured physical properties at a particular depth and compares them to the lookup table for a rock class assignment.

In this paper we present two examples of petrophysical grade estimation, one from the Newlands Coal Mine and one from the Mount Isa underground copper operations.

The Xi-Cheng lead-zinc orefield lies in Gansu Province in north-central China. The orefield consists of several separate deposits and more than 200 orebodies in Middle Devonian schist and limestone. Examples include the Changba (Zn+Pb >3.8 Mt), Lijiagou (Zn+Pb >3.2 Mt), Huangchang (Zn+Pb ~1.0 Mt), Dengjiashan (Zn+Pb ~1.0 Mt) and Bijiashan (Zn+Pb >1.0 Mt) deposits. The deposits occur as stratiform and/or tabular forms with ores mainly comprising massive and banded pyrite, sphalerite and galena. The ore contains >8.5% of Zn+Pb, plus associated Ag, Cd, Ga, and Ge. Self-potential, ground magnetic, induced polarisation, resistivity and mise-a-la-masse surveys were carried out in the early stages of exploration in the late 1960s to early 1970s. This paper presents some examples of such surveys over the Changba, Lijiagou and Huangchang deposits. This early geophysical practice showed that: (1) the self-potential method was cheap and effective in rapid reconnaissance for mineralisation below thin cover, but the interpretation of the results was sometimes complicated by the presence of graphite in the host limestone; (2) the ground magnetics was able to define small but recognisable anomalies over shallow concealed mineralisation that contains a small amount of pyrrhotite and/or magnetite, as is often the case; (3) induced polarisation was very useful for mapping mineralisation hosted in schist, but was less useful in areas of graphite-bearing limestone; (4) attempts to use resistivity surveys for mapping mineralisation were largely unsuccessful; (5) mise-ala- masse surveys were not only useful for determining the horizontal extent of intersected mineralisation in drillholes, but were also able to indicate the presence of other mineralisation in the vicinity of the intersected mineralisation.

The ability to rapidly and accurately determine the vertical and spatial distribution of soil salinity at a farm or paddock scale is extremely important for land managers and researchers involved in managing land salination. Knowledge of current (shallow) soil salinity levels is required to make informed short-term management decisions as to what pasture, crop or tree species will be the most productive, or should be grown to minimise surface soil salination. Prognosis of future (deeper) salinity development is required for longer-term management. Hence, characteristics of regolith salt store and groundwater depth are essential information. The vertical distribution of salt throughout the regolith is also very important at the regional scale and for calibrating larger-scale airborne EM mapping systems.

Accurate site assessment using soil sampling and laboratory analysis is time consuming, expensive and can be highly variable because of large spatial variability compared with sample size and practical sampling density. Clearly, an accurate and efficient system of salinity mapping using portable ground-based geophysical instruments is required.

The Geonics EM38 and EM31 instruments have been routinely used to map soil salinity. Recently, automated ground-based data acquisition and real-time mapping systems have been developed. In the past, most use has been made of the instruments’ relative readings. However, continued calibration in variable terrain will allow for more practical and absolute use to be made of readings.

The Geonics EM34 and EM39 instruments have also been used to determine salinity profiles to depth; however, usually in experimental situations. They have the potential to replace deep drilling (EM34) and drill-sample assay (EM39), and have been used to evaluate airborne EM systems.

We have reviewed the use of the four Geonics instruments for salinity investigations and present over thirty calibrations with soil salinity from various terrains in southwestern Australia and compare them with calibrations from eastern Australia. We found that the instruments are reliable and the information they generate is reproducible. In all situations studied, salinity was the dominant contributor to EM response. However, soil type, moisture and temperature can have secondary contributions, which may be more important when EM is used to assess and predict plant response to salinity. We show that for practical purposes ground-based EM can be confidently used in salinity mapping.

Groundwater in the Clare Valley is drawn from fractured rock aquifers. As such aquifers underlie approximately 40% of Australia, an understanding of sustainability and distribution of groundwater resources is important for agricultural and domestic purposes. The primary goal of the work described here was to assess the value of geophysical methods in determining the capture zones of irrigation bores in the Clare Valley aquifers. Azimuthal resistivity surveys utilising DC resistivity and EM induction were performed to evaluate the presence of electrical (and thus hydraulic) anisotropy and heterogeneity. A mise-à-la-masse survey was also carried out at one borehole. Both symmetric and asymmetric azimuthal DC resistivity and EM induction data were acquired. The asymmetric-array results indicate the presence of heterogeneities. The major axes of the DC-resistivity ellipses are approximately parallel to the direction of vertical bedding planes for electrode separations between 20 and 40 m. However, EM-resistivity ellipses at most sites are nearly perpendicular, with resistivity minima corresponding to observed bedding planes.

Parna, an aeolian sediment of dominantly clay size, forms a significant component of the regolith developed in western New South Wales. These materials blanket parts of the contemporary landscape and, in places, they form surficial deposits in excess of 8 m thick. Recent studies have suggested that parna constitutes a significant store and source of soluble salts, hence its significance to land management. This paper describes the mineralogical and petrophysical characteristics of parna, making specific reference to materials adjacent to a recognised parna section at Marinna, near Junee, which is located in the Murrumbidgee catchment. Rising water tables and dryland salinity have been identified as major concerns to the overall health of this catchment. Therefore information on the distribution and movement of salt within the upper reaches of this catchment is important, as is the role of parna in controlling these dynamics. Results from multiparameter borehole geophysical studies indicate that parna is characterised by sub-horizontal variations in conductivity and natural gamma activity. These properties are attributed to contrasts in the porosity and permeability of these materials and to variations in their composition. Significant vertical conductivity contrasts have been noted between parna and underlying saprolite, with the results from EC1:5 measurements and downhole induction logs suggesting that conductivity is primarily controlled by textural variations rather than the soluble salt and moisture content of these materials. Ground electromagnetic surveys (EM31) suggest that areas covered by parna may exhibit marked lateral variations in average conductivity attributable to differences in the thickness and proportion of parna in these materials.

The Institute of Geological and Nuclear Sciences (GNS) recently re-processed a set of 1991 2-D seismic reflection data from the offshore Taranaki region for the Spectrum Exploration/ Fletcher Challenge Energy Taranaki Joint Venture. A more detailed stacking velocity analysis, incorporating a 40% interval velocity inversion in the Eocene units, resulted in a markedly improved seismic image of Eocene and deeper reflectors. A dense semblance analysis formed the initial part of the velocity analysis sequence. It was followed by interactive constant velocity stack analysis of the velocity inversion, where the weak intra-Eocene reflectors were masked in amplitude by multiples from an overlying strong limestone reflection. A third, automatic, velocity analysis was performed on the final stack using a combination of semblance and digitised horizon times, producing a horizon-based velocity model with data points every 5 traces (63 metres). The combination of these analyses can provide relatively accurate estimates of interval velocities needed in the calculation of overpressures in basin modelling studies.

The use of seismic refraction data to determine the long wavelength statics correction not adequately addressed by residual statics routines, is widespread. While the importance of accurate traveltimes is well known, the effects of the geophone array on the inversion process are not.

This study used two coincident sets of data recorded with two recording systems at Cobar in southeastern Australia. One system used a 60 m trace interval with end-to-end arrays consisting of 16 detectors, while the other recording system used single detectors, which were 10 m apart.

The temporal resolution achieved with the 60 m groups is poor, and does not accurately measure the thickness of weathering as determined with the data using the 10 m trace spacing. One factor limiting the achieved resolution is the finite length of the geophone array. With extended arrays, the location of the effective receiving point, depends upon the direction from which the seismic signal arrives. For signals traveling in the forward direction, the effective receiving point is the first detector in the array, while for signals traveling in the reverse direction, it is the last detector. For the data used in this study, the forward and reverse traveltimes which are referenced to the same station number, represents a lateral interval of the sub-weathering interface of approximately 100 m, when both the length of the geophone array and the offset distances are added. The traveltime differences of the refracted arrivals over this interval can be up to 20 milliseconds, which implies that an accuracy of only about 10 milliseconds is meaningful with model based inversion methods, unless the effect of the length of the array is accommodated.

An analysis using the generalized reciprocal method, which uses refraction migration in multiples of the array length, shows that a migration distance of one station interval essentially removes the effect of the finite array length, when end-to-end arrays are employed. However, the static then applies to each end of the array, and in the study area, there can be a variation of up to 7 milliseconds in the weathering correction from one end of the array to the other. Therefore, it is necessary to further increase the refraction migration by an additional array length, in order to determine the static in the central region of the geophone array, which is then taken as the static correction for the entire array. Although there is an improvement in the resolution achieved with the 60 m groups, it is still poor, when compared with that achieved using the 10 m single detectors.

Further analysis of the data will be carried out to confirm that the finite length of the 60 m geophone arrays is the significant factor limiting the resolution of the refraction data, rather than the variable signal-to-noise ratios of the data.

A fundamental issue which impacts on the inversion of refraction data using any method, is the large variations in the accuracies of the traveltime data. These variations are related to the wide range of signal-to-noise (S/N) ratios along the refraction spread, and in turn, are a result of the large amplitude variations caused by geometric spreading. For shallow refraction investigations, the geometrical spreading can be more rapid than the generally accepted inverse of the distance squared function.

At any particular location, a detector will be close to a source point, and the traveltime will be comparatively accurate, because the S/N ratio is high. However, for other shot records, generally in the reverse direction, the source to receiver distance will be much larger, and the accuracy will be greatly reduced. As a result, computations at each detector location are carried out with data which have large variations in accuracies. This adversely effects the quality of the processing using any inversion method.

Most approaches to the processing of seismic refraction data perceive the problem as achieving satisfactory, rather than uniform S/N ratios, and commonly, a simple gain function is applied to adjust amplitudes to a desired level. However, this does not address the issue of the large range in accuracies of the traveltime data due to the variations in S/N ratios.

This study demonstrates that, to a very good first approximation, the convolution of forward and reverse seismic traces compensates for the large variations in S/N ratios due to geometric spreading. This facilitates the use of signal enhancement techniques analogous to the common mipoint (CMP) stacking methods, which are an integral component in reflection seismology. Examples demonstrate the improvements in S/N ratios with stacking of convolved refraction data.

However, in order to achieve suitable data multiplicity or fold, as well as more efficient field methods, it is necessary to employ CMP acquisition techniques, instead of the standard static refraction spread with multiple source points which is the norm in most geotechnical and groundwater investigations. This will require substantial re-capitalization of most shallow refraction operations as very few currently employ sufficient numbers of recording channels or roll along capabilities.