Simultaneous modelling of the phase and amplitude components of downhole magnetometric resistivity...

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Journal of Applied Geophysics 54 (2003) 1–14

Simultaneous modelling of the phase and amplitude components of

downhole magnetometric resistivity data

Matthew B.J. Purss*, James P. Cull1, Michael W. Asten2

School of Geosciences, Monash University, PO Box 28 E, Clayton, Victoria 3800, Australia

Received 2 October 2002; accepted 19 May 2003

Abstract

The downhole magnetometric resistivity (DHMMR) technique is an effective method for follow-up exploration of massive

and disseminated sulphide deposits. The method comprises the ‘‘in-hole’’ measurement of low-amplitude, low-frequency

magnetic fields associated with galvanic current flow between two current electrodes. This paper presents methods for the

simultaneous modelling of both the magnetometric resistivity (MMR) amplitude and phase (or magnetic-induced polarisation

[MIP]) response for DHMMR data. Analyses of the MIP response and the interactions between conductive and polarisable

bodies and their host are calculated using variations of the Cole–Cole model for complex impedance. The inphase and quadrature

components provide symmetric signatures at the target depth, but some results are counter-intuitive when expressed as the MIP

(phase) response. These methods are used to provide an interpretation of DHMMR data obtained from the Flying Doctor Prospect

near Broken Hill, New South Wales.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Magnetometric resistivity (MMR); Downhole magnetometric resistivity (DHMMR); Magnetic-induced polarisation (MIP); Forward

modelling; Cole–Cole

1. Introduction particular, DHMMR surveys can be highly effective

Downhole magnetometric resistivity (DHMMR)

techniques can provide attractive options assisting

with the detection and definition of major mineral

deposits (Acosta and Worthington, 1983; Asten,

1988; Bishop et al., 1997; Nabighian et al., 1984). In

0926-9851/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0926-9851(03)00044-2

* Corresponding author. Tel.: +61-3-9905-3097; fax: +61-3-

9905-4903.

E-mail addresses: [email protected] (M.B.J. Purss),

[email protected] (J.P. Cull),

[email protected] (M.W. Asten).1 Tel.: +61-3-9905-4898; fax: +61-3-9905-4903.2 Tel.: +61-3-9905-1639; fax: +61-3-9905-4903.

for mapping bodies with either a very poor or a very

strong inductive response. This is because poor con-

ductors produced little or no electromagnetic response,

and very good conductors become inductively saturat-

ed and hence are difficult to detect using normal TEM

methods. However, both poor conductors and very

good conductors will exhibit a magnetic response to

galvanic coupling. In this regard, the DHMMR meth-

od complements the more common electromagnetic

methods (e.g. the downhole time-domain electromag-

netic method [DHTEM]) that would normally be

considered the method of choice in any search for

massive sulphides (e.g. Anderson and Logan, 1992;

Brescianini et al., 1992; Cull, 1993; Cull et al., 1998).

M.B.J. Purss et al. / Journal of Applied Geophysics 54 (2003) 1–142

The surface magnetometric resistivity (MMR)

method (Edwards, 1974; Edwards and Howell, 1976;

Edwards et al., 1978; Gomez Trevino and Edwards,

1979) consists of a current dipole similar to that used

in the DHMMR method (i.e. two electrodes connected

by wire to an IP transmitter), although the connecting

wire is not displaced by 400 m (see Fig. 5). Data is

acquired in traverse lines perpendicular to the strike of

the current dipole. In common with the surface

magnetometric resistivity methods (Edwards, 1974;

Edwards and Howell, 1976; Edwards et al., 1978;

Gomez Trevino and Edwards, 1979), the DHMMR

method is normally assumed to rely on variations in

resistivity that generate current channelling effects in

the survey region. Anomalous current densities asso-

ciated with a low frequency transmitter signal (1–5

Hz) are readily detected in terms of the magnitude of a

low-amplitude local magnetic field (f 100 pT).

However, Purss et al. (2001) have emphasised the

complex nature of the ground response demonstrating

the role of the complementary phase shift (MIP) data

that can also be obtained from standard DHMMR

surveys.

Phase shift data have been previously obtained

using the MMR survey configuration for surface

surveys and the data have been reported in terms of

the magnetic-induced polarisation (MIP) response

(e.g. Seigel, 1974). Practical surveys employing the

surface MMR and MIP methods have been demon-

strated by Howland-Rose (1980a,b) and Silic (1980).

Previous authors have also presented the MIP compo-

nent of DHMMR data (Asten, 1991, 2001; Bishop et

al., 1997; Elders and Asten, submitted for publication).

However, apart from Purss et al. (2001), numerical

models assisting with the interpretation of downhole

MIP surveys have been neglected and there has been

no recognition of a broader role for MIP in DHMMR

data.

Methods for the simultaneous modelling and anal-

ysis of both amplitude (MMR) and phase (MIP)

response in DHMMR data are presented in this

paper. The complex components of a target response

are analysed in relation to the primary field associ-

ated with the survey system and the host response

using a proprietary software (MARCO, an imple-

mentation of the algorithm described by Xiong and

Tripp, 1995). These methods are then used to pro-

vide an interpretation of DHMMR data obtained

from the Flying Doctor Prospect near Broken Hill,

New South Wales.

2. DHMMR parameters

The Cole–Cole relaxation model has been previ-

ously developed as a means of discriminating IP

responses from a variety of different targets in terms

of a limited number of parameters (Pelton et al., 1978).

However, IP data may also be viewed as a special case

of the complex resistivity method where only one or

two frequencies are employed (Zonge and Wynn,

1975). Similarly, the MIP component of a DHMMR

response can be viewed as the magnetic expression of

variations in induced polarisation, resulting from gal-

vanic current flow in the earth, measured at a single

frequency (typically 1 or 4 Hz). Consequently, the

same interpretive techniques employed for the model-

ling of an IP response from a complex resistivity

survey may be employed: namely, the application of

the Cole–Cole relaxation model to the integral equa-

tion for solving the electromagnetic field response

from three-dimensional conductive and polarisable

bodies (Xiong and Tripp, 1995).

The simple Cole–Cole relaxation model is defined

by the transfer function:

ZðxÞ ¼ R0 1� m 1� 1

1þ ðixsÞc� ��

ð1Þ

where R0 is the low frequency resistance, x is the

angular frequency, m is the intrinsic chargeability as

defined by Seigel (1959), s is the time constant (in

seconds) and c is the frequency dependence. This

expression governs the calculation of individual cur-

rent elements (and consequently the magnetic field) in

the MARCO program.

It is then a simple matter to define the MMR

amplitude response from the complex DHMMR field

according to the expression:

AmplitudeMMR¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiððReðBMMRÞÞ2þðImðBMMRÞÞ2Þ

r

ð2Þwhere BMMR is the total magnetic field calculated over

all current elements.

M.B.J. Purss et al. / Journal of Applied Geophysics 54 (2003) 1–14 3

Similarly, the phase (MIP) component of the com-

plex DHMMR field is defined by the expression:

/MIP ¼ tan�1 ImðBMMRÞReðBMMRÞ

� �ð3Þ

where /MIP is the phase response of the complex total

magnetic field (BMMR).

Fig. 1. Attenuation of horizontal current density as a function of depth i

showing the flow of current from the current source to the current sink and

current density ( Jy) versus depth down a drillhole located in the vertical p

density ( J0) at depth Z= 0. After Telford et al. (1990, p. 526, Fig. 8.6).

While the IP response of a polarisable body is the

phase component of the complex electric field (Zonge

et al., 1972, p. 630), the MIP response is the phase

component of the complex magnetic field (the DHM

MR response in this instance). This can lead to some

unusual occurrences where the MIP anomaly will be

suppressed in one location but enhanced in another as

a result of the magnetic interactions between bodies.

n a uniform host. (a) Block diagram of a typical DHMMR survey

the horizontal current density Jy. (b) The horizontal component of the

lane midway between electrodes normalized by the surface current

Fig. 2. Modelled MMR (amplitude), MIP (phase) and quadrature (imaginary component of the complex magnetic field) component responses for

various conductive and polarisable bodies about a 45j inclined drillhole located in the mid-plane between two current electrodes (similar to that

shown in Fig. 1). Models 1 and 2 show the response from a body below and above the drillhole, respectively. Models 3 and 4 show the response of

a horizontal and vertical plate intersecting the drillhole at the mid-point. Models 5–9 show the response from plate models at different orientations

to the drillhole. Models 10–16 show the response of a single prismatic body at increasing depth but the same distance from the drillhole. A

nonpolarisable uniform half-space was chosen as the background earth model with a resistivity of 500 V m. The strike length of the bodies is

1000 m (perpendicular to the page) and the Cole–Cole parameters specified were R0 = 50 V m, c= 0.25, m = 0.5 and s= 0.01.



This is a result of the magnetic field being more

complex in nature than the electric field. And conse-

quently, there will be many sign reversal occurrences

that are observable in MIP (either surface or down-

hole) that are not present in surface electrical IP (EIP)

surveys.

Fig. 3. Attenuation of the peak phase and amplitude anomaly

from a conductive and polarisable body with increasing distance

from a vertical drillhole. A nonpolarisable uniform half-space

was chosen as the background earth model with a resistivity of

500 V m. The anomalous body was defined to have dimensions

of 10 m in width, 10 m in vertical extent and 1000 m in strike

length (perpendicular to the page). The Cole–Cole parameters

specified for the body were R0 = 5 V m, c= 0.25, m = 0.7 and

s= 0.01.

3. Characteristic axial component phase (MIP)

and amplitude (MMR) responses from the

DHMMR method

Asten (1988) has presented a simple 2D formula-

tion of the DHMMR field computation along the axis

of a borehole where the current electrodes are suffi-

ciently far apart as to produce current flow normal to

the axis of the borehole. It was shown that conductive

bodies of infinite strike length produce diagnostic

DHMMR amplitude signatures along the axis of a

borehole (Asten, 1988, p. 14, Fig. 3). The MARCO

program can be used for targets of this type by

assigning three-dimensional conductive and polaris-

able bodies with extended strike length. The results

provide a similar set of diagnostic MIP and MMR

component signatures for the DHMMR anomaly along

the axis of a drillhole. Fig. 2 shows the modelled

amplitude (MMR), phase (MIP) and quadrature (imag-

inary component of the complex magnetic field) com-

ponent responses for various conductive and polar-

isable bodies about a 45j inclined drillhole.

The sign of the MIP component anomaly is similar

to that of the MMR component, i.e. a body below the

hole will produce a negative phase anomaly, and vice

versa. However, the magnitude of the MIP anomaly

response increases with depth. This characteristic of

the MIP component at first appears to be counter-

intuitive and may be the cause for some concern,

particularly when conventional theory from the surface

IP method tells us that when the target body becomes

deeper, its anomalous response decreases as the re-

sponse from the host material becomes dominant.

However, in the DHMMR method, unlike the surface

MMR method, the receivers are located at increasing

depths in the ground and thus the host response

observed at each receiver decreases with depth. One

must keep in mind that the measured magnetic field is

the sum of the host response and the anomalous target

response.

The rate of attenuation of the horizontal current

density, normalized by the surface current density in a

uniform half-space ( Jy/J0), was indicated in Fig. 1 as a

function of the downhole depth. This attenuation

results in a reduced MIP response from the host as a

function of depth. The anomalous response of the

target body, however, remains relatively unchanged

due to the effects of current channelling into the target

body. This is demonstrated in Fig. 2 where the quad-

rature component of the DHMMR field (which exhib-

its the response resulting from current channelling in


the target body) shows a more consistent response with

depth, similar to that of the MMR component. It is

clear from Figs. 1 and 2 that the strong disparities in

the magnitude of the MIP response are largely an

artifact related to the uneven distribution of current

in the host and the target body. In Eq. (3), the

magnitude of the quadrature (or imaginary) component

is normally low but constant (f 1–2 pT/A), and any

variation between models will be amplified through

the progressive reduction of the inphase (or real)

component.

While the magnitude of the quadrature component

behaves better than the MIP (phase) component,

modelling of the quadrature component in isolation

is not an ideal solution. In practice, phase data can

be obtained directly in the field providing the oper-

ator with an intuitive feel for any response. As a

result, realistic models are required to assist with

control of data quality prior to any final interpretation

based on partial reductions of the complex magnetic

field.

Fig. 4. Geologic section of the Flying Doctor Prospect. The ore body seque

chalcopyrite and pyrrhotite (of which the main Flying Doctor ore body and

The ore body sequence is bounded to the northwest by the ‘‘Western Shea

The ‘‘Sundown Group’’, overlying the ore body sequence consists of met

It should also be noted that the background trend

due to geometric relationships between the receiver

location and the current dipole have been removed

from the data presented in Fig. 2.

4. Resolving power of the MIP component of

DHMMR data

The ability to see ‘‘off-hole’’ anomalies in

DHMMR data is governed by the resolving power

of both the MMR and MIP components. Fig. 3 shows

the attenuation of the peak anomaly for both the MMR

and MIP component of the DHMMR response with

increasing distance from a vertical drillhole. The rate

of attenuation for the MIP component is comparable to

that of the MMR component. Both the MIP compo-

nent and the MMR component exhibit a rate of

attenuation of almost 1/r (Fig. 3). However, because

the magnitude of the MIP component is at least an

order of magnitude closer to the background noise

nce consists of semiconformal lenses of sphalerite with minor galena,

the lower extension are the most notable) dipping to the northwest.

r Zone’’ and to the southeast by the ‘‘Globe Vauxhall Shear Zone’’.

asediments.


level than that of the MMR component, the resolving

power of the MIP component is considerably less than

that of the MMR component. As a result, the observed

Fig. 5. Location diagram of the Flying Doctor Prospect with an idealised

connecting wire and positions of drillholes FD3418 and FD3071. Also sho

and surface IP survey (Fig. 6).

MIP anomaly from a body 150 m away from the

drillhole will be about 2.5 mrad, only marginally

above the background noise level, while the observed

DHMMR survey plan showing the electrode positions, transmitter,

wn is the transect A–AVused for both the geologic section (Fig. 4)


MMR anomaly will still be easily identifiable, being

about 25 pT/A. The background noise level is typi-

cally between 1 and 2 mrad for the MIP (phase)

component and about 5–10 pT/A for the MMR

(amplitude) component.

As discussed by Hallof (1974), for low frequency

IP measurements (0.05–1.25 Hz), the phase shift

caused by the IP effect is approximately constant over

a limited frequency range, whereas the phase shift

caused by inductive coupling varies as different

powers of frequency. While the inductive coupling

effect in the dipole–dipole survey method is mainly

the expression of the magnetic coupling between the

connecting wires of the current and potential electro-

des, the inductive coupling effect in the DHMMR

method is the expression of the magnetic coupling

between the wire connecting the current electrode and

the magnetic receiver (typically a downhole electro-

magnetic probe) through a given host.

Because of the distorting effects of inductive cou-

pling on the MIP component of DHMMR data, it is

desirable, and certainly general practice in collecting

field data, to remove any obvious artifacts. This is of

particular interest in the Australian environment where

there is typically a conductive layer over a more

resistive host. Various methods have been proposed

for the removal of the inductive coupling effect from

DHMMR survey data. Of these, the methods employ-

ing the three-point quadratic formulae are the most

commonly used (Coggon, 1984; Hallof, 1974). The

idea behind the three-point decoupling formulae is to

extrapolate the data back to a frequency of zero and

thus remove any inductive coupling effects from the

data. Most IP receiver systems employed in DHMMR

surveys (e.g. Zonge GDP-16 or SMARTem) apply a

decoupling formula to the data as it is being recorded

using higher harmonics of the transmitted frequency.

For consistent comparison between the model data

presented in this paper and field data, it is necessary to

compute the response from the models at three frequen-

cies (typically 1, 3 and 5 Hz) and apply a three-point

Fig. 6. Inverted resistivity data from a surface dipole–dipole resistivity surv

in 1986 (the Flying Doctor Exploration Lease is now controlled by Perilya

A–AVof Fig. 5. Panels (a) and (d) are the observed resistivity and charg

response from of the observed resistivity and chargeability, respectively

respectively. The location of drillholes FD3071 and FD3418 are shown in

decoupling formula to produce a zero frequency phase

model.

5. Case study: Flying Doctor Prospect (Broken Hill,

NSW)

The Flying Doctor Prospect was chosen as a suit-

able case study for the application of simultaneous

MIP and MMR modelling of DHMMR data, because

there is reliable data for a number of drillholes in

which standard DHMMR surveys have been con-

ducted (Asten, 2000, 2001; Bishop et al., 1997; Elders

and Asten, submitted for publication). In addition, a

number of surface resistivity surveys have been con-

ducted (e.g. Webster, 1985), and the geology is well

known through drill sections that provide adequate

comparisons for modelled interpretations (Fig. 4). The

DHMMR data presented in this paper were acquired

using an 800-m current dipole centred about the drill-

holes and striking in a northeast–southwest orientation

(along strike of the Flying Doctor Deposit) and with

the connecting wire displaced 400 m to the west of the

dipole (Fig. 5). Data for drillholes FD3071 and

FD3418 were collected in 1988 using an axial-com-

ponent Sirotem slimline DHS-1S probe and the

SMARTem digital receiver system. Additional data

were collected for FD3418 in 2001 using the three-

component VECTEM probe and the Zonge GDP-16

receiver system. Data were not collected during the

repeat survey for drillhole FD3071 due to a blockage

in the hole. Both surveys were conducted at a frequen-

cy of 1 Hz and with a transmitter current of 11.5 and 10

A, respectively. The processed MIP and MMR com-

ponents of the DHMMR field data collected in 2001

are offset by about 15 mrad (for the MIP component)

and 100 pT/A (for the MMR component), respectively,

from those collected in 1988. This may be due to either

a better ground coupling of the current electrodes in

the 2001 survey or an improved resolution in the

VECTEM probe, particularly in the MIP component.

ey conducted by RioTinto over the Flying Doctor Exploration Lease

Broken Hill). The resistivity profile was conducted along the section

eability, respectively. Panels (b) and (e) are the inverted modelled

. Panels (c) and (f) are the inverted resistivity and chargeability,

panels (c) and (f).

Fig. 7. Modelled axial MIP and MMR components of DHMMR data acquired over the Flying Doctor Prospect. (a) Cross-sectional view of the

model showing the model parameters for each body. The dashed lines indicate the outline of the major geologic units shown in Fig. 4. (b)

Comparison of modelled MIP and MMR components for drillhole FD3071 with DHMMR data collected in 1998. (c) Comparison of modelled

MIP and MMR components for drillhole FD3418 with DHMMR data collected in 1998 and 2001.



In both surveys, the same data processing procedures

were followed.

5.1. The Flying Doctor exploration target

The Flying Doctor Prospect is a small extension of

Broken Hill style mineralization comprising sphaler-

ite with minor galena, chalcopyrite and pyrrhotite

hosted by metasediments and occurring as semi-

conformable lenses that plunge to the north to a depth

of 300–400 m (Bishop et al., 1997). Bishop et al.

(1997) found the mineralization to be only weakly

conductive; however, due to the scarcity of graphite

in the survey area, the observed response has been

largely attributed to the sulphides. Asten (2000)

presented a model for the MMR component of the

DHMMR field for drillhole FD3418 by employing

tubes of electric current in free space, but made no

comment (other than the geometric placement of

modelled bodies) on the comparison of this model

with known geological or geophysical information for

the Flying Doctor ore body. Elders and Asten (sub-

mitted for publication) also presented data from drill-

hole FD3418 and concluded that the negative

anomaly in both the MIP and MMR components of

the DHMMR field at 380 m downhole are likely to be

representative of mineralization. Asten (2001) pre-

sented both MMR and MIP components of the

DHMMR field for FD3071 as compared to DHTEM

data collected in the same drillhole.

5.2. Specification of model parameters

The MIP and MMR model results presented were

produced using the MARCO program and compared

to both surface dipole–dipole resistivity data and

known geology. The initial model geometry was

derived from that presented by Asten (2000) and

discretised into a block model suitable for input into

the MARCO program. All bodies in the model are

centred about local grid line 20,400 mN (see Fig. 5).

An attempt was then made to derive suitable conduc-

tivity parameters from the resistivity pseudosection of

Flying Doctor presented in Webster (1985, p. 321, Fig.

3). However, due to problems in locating this pseudo-

section with respect to drillholes FD3418 and FD3071,

it was necessary to use surface resistivity data collected

by RioTinto over the Flying Doctor exploration lease

(now controlled by Perilya Broken Hill). Inversion of

this data provided a model that showed good correla-

tion with the observed data in both apparent resistivity

and chargeability (Fig. 6). The inverted resistivity from

this model yielded a background resistivity of about

1000 V m. The anomalous zones identified after

inversion coincided with the main Flying Doctor ore

body and had a resistivity of between 30 and 50 V m.

A 10-m layer of conductive overburden with a resis-

tivity of 30 V m was included in the model.

The background earth model was chosen to be a

nonpolarisable two-layer earth consisting of a 10-m

layer of conductive overburden, with a resistivity of 30

Vm, overlaying a resistive half-space, with a resistivity

of 1000 V m. The target bodies were assigned Cole–

Cole parameters suitable for massive sulphide deposits

(Pelton et al., 1978): R0 = 10–50 V m; c = 0.25; m =

0.5–0.9; and s = 0.001–1.0 s (Pelton et al., 1978) (see

Fig. 7).

6. Results

The final model was computed to simultaneously

model both the MIP and MMR components for drill-

holes FD3418 and FD3071. Fig. 7 shows the final

model that achieved a best match for both holes.

Bodies 1 and 2 represent the main Flying Doctor

ore zone. Body 3 represents a deeper extension of the

main ore zone. Body 4 represents a broader less

conductive and polarisable zone above the deeper

extension of the Flying Doctor ore zone (body 3)

but still contained in the main ore body sequence (see

Fig. 4). Body 5 was included to improve the ampli-

tude component match for FD3418. It represents a

relatively conductive but poorly polarisable zone

lying between FD3418 and FD3071. There appears

to be no known geologic feature corresponding to this

body that is consistent with either a zone of mineral-

ization or a shear zone present in the drill logs of

FD3414; however, this body is located in the Sun-

down Group (Fig. 4), which has been shown to exhibit

localized conductive zones corresponding to deeper

weathering penetrations of the conductive overburden

(Carrol, 2002, personal communication). Bodies 6–

8 represent a broad conductive and polarisable zone

indicating a region of massive and disseminated

sulphides associated with the Globe Vauxhall Shear


Zone (see Fig. 4). These bodies also coincide with a

change in dip of the mineralization from westward

dipping to the west of the shear zone to eastward

dipping to the east of the shear zone. Drill logs for

FD3071 note the occurrence of veins (up to 30 cm in

width) of massive pyrite with minor sphalerite, galena

and chalcopyrite mineralization at about 168–172 m

downhole, which broadly correspond with the place-

ment of bodies 6–8. Body 9 represents a localized

conductive and polarisable zone lying immediately

below the conductive overburden, which may corre-

spond to a deeper weathering horizon.

As shown in Fig. 7, the final model provides a good

match with both the known geology (Fig. 4) and

surface dipole–dipole resistivity inversion data (Fig.

6). This exercise shows that it is possible to simulta-

neously model the MIP and MMR components of the

DHMMR field using the Cole–Cole relaxation model

in conjunction with the integral solution for three-

dimensional conductive and polarisable bodies. No

follow-up drilling program has been conducted to

determine if any new ore zones have been identified

at the Flying Doctor Prospect as a result of models

presented in this paper. However, the simultaneous

modelling of both the MMR (amplitude) and MIP

(phase) components of the DHMMR field provides a

more comprehensive interpretation of the conductive

and polarisable zones in the area of interest than

previous modelling methods that considered the

MMR (amplitude) component alone (Asten, 1988,

1991, 2000; Bishop et al., 1997).

7. Conclusions

The application of standard IP interpretive tech-

niques to the MIP component of DHMMR data has

shown promising results that may lead to a new

understanding of the magnetic field response from

both resistivity and induced polarisation heterogene-

ities in the presence of galvanic current flow in the

earth. Theoretical models presented in this paper

demonstrate some of the fundamental properties of

the MIP component of the DHMMR field. These

are

(1) As with the axial MMR (or amplitude) component

of the DHMMR field, the axial MIP (or phase)

component exhibits a negative anomaly for a body

below the drillhole and a positive anomaly for a

body above the drillhole.

(2) The general shape of the axial MIP anomaly is

comparable to that of the axial MMR anomaly,

particularly the asymmetric anomalies resulting

from plate models at oblique angles to the

drillhole.

(3) Unlike the axial MMR component, the magnitude

of the axial MIP component increases with depth.

This is due to the current channelling ability of a

conductive and polarisable body in a more

resistive host and the decrease in the host response

with depth.

(4) There is a comparable attenuation of both the axial

MMR component and axial MIP component

response with increasing distance from the drill-

hole. This attenuation is close to 1/r. However, the

axial MIP component is an order of magnitude

closer to the background noise level than does the

axial MMR component, resulting in a significantly

smaller search radius for the axial MIP component.

Although the models presented in this paper

were successfully computed using a pseudo-three-

dimensional algorithm for dispersive current flow in

the earth, the development of a true three-dimen-

sional computation algorithm will be of great ben-

efit to the numerical interpretation of current flow in

a complex earth. With the employment of three-

component probes to the DHMMR survey method

becoming routine, there is an increasing interest to

better understand the benefits of the additional

information provided by sensors orthogonal to the

axial sensor of the probe. Further work is needed to

characterise and understand both the MMR and MIP

components of the DHMMR field in a three-dimen-

sional setting, and the geometrical relationships

between the current dipole, the receiver and the

target body.

Acknowledgements

A Strategic Partnership for Industry Research and

Technology (SPIRT) grant from the ARC and mining

companies BHP MINERALS, MIM EXPLORA-

TION, NORTH and Pasminco Exploration supported


M. Asten. DHMMR data were acquired at Broken

Hill in 1988 as part of the SPIRT project with support

from Pasminco. John Theodoridis was involved in the

acquisition of these data. DHMMR data were

acquired at Broken Hill in 2001 as part of a Monash

University PhD thesis being studied by Matthew

Purss with the assistance of Perilya Broken Hill.

RioTinto acquired the surface resistivity data over the

Flying Doctor Exploration Lease, which is now under

the control of Perilya Broken Hill. The program

MARCO was used with permission of the Coopera-

tive Research Centre for Australian Mineral Explora-

tion Technologies (CRCAMET). The assistance of

Noel Carroll, Duncan Massie and George Jung is

acknowledged.

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Simultaneous modelling of the phase and amplitude components of downhole magnetometric resistivity...

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