Exploration Geophysics - Volume 18, Issue 3, 1987

Following the discovery of the Hellyer base metal deposit in Tasmania, test drillhole electromagnetic (DHEM) surveys were conducted in order to determine their effectiveness and to compare several field systems. The DHEM profiles reflect both the variation in size of the ore body along strike and the approximately equidimensional cross-section of the ore zone. At the small southern end of the deposit, results from a drillhole illustrate that the surface transmitter loop has generated a rapidly decaying local eddy current system as well as a more slowly decaying current system in the main part of the conductor to the north. In the central part of the deposit, DHEM results show clearly that in a thick conductor, currents are not restricted to flowing in a particular orientation. Comparison of all the currently available impulse response DHEM systems shows that all are very effective exploration tools if the field work is conducted properly.

Down-hole electromagnetic surveys have been used in the vicinity of the Renison mine to explore for further lodes of cassiterite-bearing pyrrhotite at depths approaching 1 km. Surveys down one hole have located a significant off-hole conductor. A simple current filament model does not adequately describe the behaviour of the response from this conductor. The effects of other conductors above and about the target body are suggested as possible causes for the complexity in the data. They may also be responsible for the first of two changes of sign with time.

Downhole TEM is being used extensively by CRA Exploration Pty Ltd (CRAE) in the Broken Hill district as a means of detecting massive base metal sulphides which may be close to but not intersected by drillholes. Coupling diagrams used in conjunction with drill sections help to plan loop layout and to develop starting models for quantitative forward modelling schemes. Nomograms for estimating time constants and for relating these to target sizes and tonnages assist interpretation, and help maintain an economic perspective. A field example from the NBHC mine at Broken Hill illustrates the significance of drillhole depth and the importance of establishing the relative polarities of transmitter and receiver in the field.

Several examples are presented of drillhole transient electromagnetic (DHEM) surveys from the Thalanga area of northern Queensland, where the mineralisation comprises stratiform bodies of zinc, lead and copper volcanogenic massive sulphides. The first example demonstrates that in favourable situations conductive massive sulphides can be detected for a distance of at least 150 m from the drill hole. In the second example discrete DHEM anomalies due to massive sulphides which change in sign from early to late delay times are interpreted to arise either from current channelling or the drive-delay effects of a conductive earth. The third example shows another case of sign reversals but the signs are opposite those expected and although several explanations are proposed, none are entirely satisfactory. The last example illustrates the effects of loop size on DHEM anomalies in an area of high surface conductivity and it is concluded that small loops using relatively high currents are preferable to large loops in maximising the response of off-hole massive sulphide conductors relative to the host rock.

Field examples of downhole SIROTEM surveys investigating massive sulphide targets in the Agnew Nickel Belt, WA are presented to highlight different applications of the technique. The examples show that the downhole EM (DHEM) method can successfully solve specific exploration problems posed by initial drilling of a prospect. Such problems include: the presence or absence of deep conductors in favourable geological positions; the location or confirmation of the source of a surface EM response; the resolution of the geometry and extent of a drilled conductive target; and the effect of conductive overburden on surface and downhole EM responses. Interpretation of the Agnew DHEM survey data, based on computer forward modelling aids, shows good agreement with drill indicated conductor positions. The results demonstrate that the DHEM method represents an efficient and cost-effective means for second phase exploration for massive sulphide targets.

The fixed receiver electromagnetic (FREM) method is the reverse of the fixed transmitter method. The receiver is fixed, the transmitter is moved, and measurements are plotted in profile or plan at the transmitter positions. In drill hole applications the method extends the way in which information can be gathered and provides improved definition of conductive mineralization. In order to predict and interpret anomaly patterns, it is convenient to invoke reciprocity and visualise the receiver as an electromagnetic source, and the transmitter as a signal measurer. Model and field results confirm that a finite conductor causes a dipolar type magnetic field anomaly, which readily gives information on the strike, dip and approximate location of the conductor. However, conductive overburden may also produce a significant down hole electromagnetic response, and a FREM anomaly that is approximately dipolar. It is therefore necessary to identify the overburden effects and perhaps attempt to remove them, before interpretation.

In addition to rules of thumb, a dipole source, and a rectangular current filament source, have been used successfully for interpreting conductor geometry from FREM data. The FREM approach is novel; it complements conventional drill hole electromagnetic survey measurements. A further appreciation of its advantages as well as drawbacks will come from experience.

A downhole EM (DHEM) case history is presented for a massive pyrite-pyrrhotite body discovered during base metal exploration within the Kanmantoo Trough, South Australia. Two transmitter loop locations were used to survey hole WCP2 which intersected two sulphidic intervals totalling a width of 35 m. Changes in the shape of the DHEM profiles with time indicate a complex evolution of the induced eddy current system after transmitter turnoff. At early times, the DHEM response is dominated by currents in the host rock. Superimposed on the host response is the conductor anomaly which changes in sign from early to late time. This sign change is interpreted to result from the use of dB/dt rather than B field measurements. With time, the system of eddy currents within the conductors first expands across the drill hole, then contracts to a late time position above the drillhole. This behaviour is related to the nature of diffusion of electromagnetic fields into a conducting earth and to the strong coupling between the upper parts of the conductor and the transmitter.

Coal seams adjacent to post-depositional igneous intrusives may show very high conductivities due to presence of graphite in the cindered (metamorphosed) coal. A downhole TEM survey through the Bulli coal seam, in the south Sydney basin shows the conductance of a 4 metre intersection of cindered coal to be about 50 S, indicating that the TEM method, both surface and downhole, has applications in the identification of areas of cindered coal. The anomaly due to the highly conductive cindered coal shows a sign-reversal at intermediate times which is attributable to the influence of overlying conductive sediments and which demonstrates the importance of considering the influence of conductive overburden or host rock before attempting interpretation of target geometry using free-space models.

The transient electromagnetic (TEM) responses of several layered earth models are examined for the configuration consisting of a vertical downhole receiver and a large fixed loop source, on or above the earth’s surface. It is shown that a conductive overburden will cause the magnetic field at depth to be greatly attenuated at early times, but considerably enhanced at late times. The use of a downhole receiver will improve the resolution of conductive units, but adds nothing to the already low probability of detecting resistive units using TEM methods. Compared with surface receivers, a downhole receiver will have a much improved probability of detecting whether or not a buried layer has an intrinsic IP response. Lastly, it is shown that the combination of an airborne transmitting loop and a downhole receiver would constitute an interesting and feasible exploration method.

Sign changes at a particular receiver location are commonly observed in downhole TEM data as a function of delay time. There are many causes but generally they can be classified as being due to either a change in the sense of coupling between the downhole probe and the analagous secondary field, or a reversal of the analagous secondary field itself. The most common coupling-related reversals are due to analagous current migration in finite plate-like, overburden and host conductors. On the other hand, analagous secondary field reversals are more frequently seen in multiple conductor cases. In particular the impulse response exhibits a sign reversal for the case of a conductor directly under a transmitter placed on an overburden. In this case the step response does not. For the case of the same conductor located at some horizontal distance away from the transmitter, the step response may exhibit a sign reversal while the impulse response may exhibit two. The overall response is further complicated by the fact that the superimposed overburden response changes sign as well. Sign reversals may also arise from non-uniform primary field coupling, current channeling and induced polarization effects and these can add further uncertainty to the interpretation process.

A circular current filament in free space constitutes a useful idealisation of late-time secondary current flow in a geological conductor. The mathematical simplicity of the current filament model permits efficient inversion of down-hole TEM (DHEM) logs via iterative adjustment of a starting guess. The application of current filament modelling in exploration is illustrated using data from a shallow hole intersecting a mineralised quartz vein. The DHEM log at 1.2 msec is closely replicated by combining the effects of two filaments centred up-dip, one encircling the hole and one away from the hole; these are interpreted to represent a single distributed current system pierced by the hole. Inversion of the 5.8 msec DHEM log suggests that this current system contracts and migrates up-dip. Inductive coupling between the vein and the regolith zone is hypothesised as the cause of the upward migration. The current system dominating the DHEM response is small in diameter and located near the hole; the down-hole survey therefore provides no indication of the better mineralisation later intersected down-dip.

The magnitude, as well as the direction of the primary field from a transmitter loop must be considered when estimating the position of maximum coupling between the transmitter and a conductive target. In the case of dipping conductors, maximum coupling is achieved by placing the transmitter loop on the hanging wall side of the target where maximum coupling occurs almost directly under the leading edge of the loop.

The transient magnetic fields in a borehole near a plate-like conductor can be simulated with a simple current loop in the plane of the conductor. The shape of the response with depth in the hole is related to the position and orientation of the current loop, so that characteristics of the response such as the zero-crossing separation can be used to predict the location of the current loop and therefore the location of the conductor in which the current circulates. Physical model results verify the application of the characteristics in this way.