A COMPARISON OF CONDUCTIVITY SECTIONS DERIVED FROM AEM DATA AND REFRACTOR DEPTHS DERIVED FROM SEISMIC

DATA, NARROMINE, CENTRAL WEST NSW

Adrian Fisher and Ross Brodie

This report was produced by Geoscience Australia (GA) for the Bureau of Rural Sciences (BRS) project: Salinity mapping – Geophysics. The report was submitted to BRS on 23/5/2008.

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Executive Summary This report presents a comparison of conductivity sections derived from airborne electromagnetic (AEM) data and refractor depths derived from seismic data in the Narromine area of central west NSW. The study used time domain AEM data recorded with the TEMPEST system by Fugro Airborne Surveys, and data from the Narromine Seismic Transects (N1, N2 and N3) recorded by the Australian National Seismic Imaging Resource (ANSIR). The AEM data was used to conduct geophysical inversions that generated 2-dimensional conductivity distributions along depth-sections following each seismic transect. The seismic data was used to generate two-layer refraction models along each transect, which produced regolith thickness and bedrock velocity. In general, the basement depths estimated from the refraction models show good correspondence to features in the conductivity sections generated by inverting the AEM data. Results from both AEM and seismic data do not show the location of the palaeovalley that borehole interpretation has shown exists in the area. The results do show a narrow ridge of Hervey Group Sandstone as a steep sided, probably fault bounded unit trending north-northwest across the area. This ridge outcrops in the south and continues under cover to the north; however the AEM inversion results show it is broken by a substantial gap that may represent a gorge in the westwards flowing buried palaeovalley. The results also reveal a shallow, low velocity refractor to the west of the buried Hervey Group ridge, where the conductivity section shows a buried layer of relatively high conductivity. These features may correspond to buried Mesozoic rocks, such as weathered Drildool beds, abutting the western side of the ridge.

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Introduction This report presents a comparison of conductivity sections derived from AEM data and refractor depths derived from seismic data in the Narromine area of central west NSW (Figure 1). The study used time domain airborne electromagnetic (AEM) data recorded with the TEMPEST system during a survey flown from December 2006 to April 2007 in a salinity mapping study of the Lower Macquarie region. The Narromine Seismic Transects (N1, N2 and N3) were recorded as part of a shallow seismic survey conducted in 2002 for the Australian Nuclear Science and Technology Organisation (ANSTO) by the Australian National Seismic Imaging Resource (ANSIR), Geoscience Australia. The seismic transects were acquired to examine palaeovalley aquifers to the west of Narromine, and to compare seismic results to ANSTO’s electro-kinetic sounding (EKS) method. The seismic processing was conducted by Leonie Jones of the Seismic Acquisition and Processing (SAP) Team, Geoscience Australia. The survey specifications were recorded in the ANSIR report (Bokor, 2003), while initial results were presented by Jones et al. (2004).

Figure 1 Location of the Narromine seismic transects (black lines) and AEM flight lines

(red lines) over surface geology from Whitaker et al. (2006).

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Seismic processing Refraction models were generated from the first-break-picks of each seismic transect, using the REFSOL module of the DISCO/FOCUS seismic processing software package. Two-layer refraction models were used, where the velocity of the upper layer was fixed at 700 m/s, and the thickness of the upper layer (regolith thickness) and the velocity of the lower layer (bedrock velocity) were allowed to vary. The regolith velocity was estimated from the gradient of the time-distance plot of the first-break data. If this had been set at a greater value, the regolith-basement interface would be lower. A faster regolith velocity may even be expected, especially where the regolith is known to be saturated. The variation in calculated regolith thickness was outputted and plotted over conductivity-depth sections (Figures 2, 3 and 4). The variation in the calculated bedrock velocity was outputted and graphed by SAP, and printed hard copies were made available to the authors. Plots of both calculated regolith thickness and calculated bedrock velocity were presented by Jones et al. (2004). In transect N2 the calculated bedrock velocity was relatively constant and the change in velocity between the layers appears to correspond with the interface between regolith and basement. The change in calculated velocity between layers in transect N3 also appears to correspond to the regolith-basement interface, however, the western 1.5 km of transect N3 has a distinctly lower calculated velocity. This may indicate that the bedrock in this region has different physical properties that correspond to a lower velocity. The geology map and cross section of Sherwin (1997) interpret this area as likely to have Girilambone Group volcanics as bedrock rather than Hervey Group or Cotton formation, which underlies the rest of the transect. It is unlikely that the Girilambone Group volcanics would have a significantly lower velocity than the other Lachlan Fold Belt units. A better explanation of the lower velocity is that Mesozoic rocks may be present between the Cainozoic cover and the Palaeozoic bedrock, as speculated by Sherwin (1997). Alternatively, the basement rock in this area may have a thicker saprolite/saprock. The calculated velocity of the lower layer in transect N1 appears to correspond to an intermediate refractor within the regolith rather than basement. It is assumed that the basement rock beneath transect N1 is too deep to generate a refraction model of the regolith-basement interface. Despite this problem, the calculated thickness of the layer above the first refractor was still plotted against the conductivity-depth section in Figure 2. The regolith thickness revealed in transects N2 and N3 ranges considerably, from 0 to 70 m depth (Figures 3 and 4). The thin regolith at the southern end of transect N2 corresponds to an outcropping bedrock ridge of Hervey Group. This ridge also appears to cut across the central section of transect N3 where the regolith thickness decreases to around 10 m. AEM inversions The TEMPEST AEM data were acquired and processed by Fugro Airborne Surveys. As part of the survey, Fugro also acquired and processed three lines of data (81090, 82090 and 83090), which followed the Narromine seismic lines as closely as was possible. Geoscience

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Australia carried out geophysical inversions along these three lines using methods developed in-house (Lane et al., 2004). These inversions generated 2-dimensional conductivity distributions along depth-sections following each seismic transect. The method is described briefly below. The inversion of the full dataset and the production of 3-dimensional conductivity distributions for the Lower Macquarie survey will be the subject of a separate more detailed report (Brodie and Fisher, 2008). Two conductivity-depth slices for the part of the survey area near the seismic lines were produced from the inversion of the full dataset (Figure 5). The conceptual model used in the AEM inversion was a 25 layer model with the first layer having a thickness of 2 m and each layer getting progressively thicker by 10% until the 24th layer had thickness 17.90 m and the 25 layer had infinite thickness. The layer thicknesses were kept fixed and the inversion solved for the conductivity of each layer. A different homogenous halfspace reference model was used for each inversion location to constrain the layer conductivities. The halfspace reference model conductivity value for each measurement location was chosen to be the conductivity estimate generated from prior halfspace inversions of the data. Maximum smoothness (minimum roughness) constraints were imposed on the vertical conductivity profile at each inversion location. The starting model in the inversion was identical to the reference model. The inversion also solved for three parameters of the TEMPEST AEM system geometry that are not measured during the data acquisition. These were the in-line horizontal-and vertical separations between the transmitter and the receiver coils and the angular pitch of the receiver coils. Measured parameters of the system geometry, (transmitter height, pitch and roll), were taken to be the measured values and were not solved for. Other unmeasured geometry parameters (transmitter-receiver transverse separation, receiver roll and yaw), were assumed to be zero as there is insufficient information to solve for these parameters. Recordings at each AEM observation point were inverted independently of recordings at other observation points, and the resulting vertical conductivity profiles were stitched together to form conductivity sections. A five point (~62m) along line median filter was applied to each of the layer conductivities prior to stitching them into the sections shown in Figures 2, 3 and 4. Comparison of AEM and seismic depth sections Figure 2 shows that the depth of the intermediate refractor from seismic transect N1 (~25 m) approximately follows the transition from shallow conductive to deeper resistive materials (~0.05 S/m). This velocity and conductivity transition may correspond to a sedimentological boundary within the Cainozoic palaeovalley fill. Boreholes in the area reveal a significant thickness (~100 m) of palaeovalley sediments beneath the seismic line N1, which is not revealed in the conductivity section. The section does show a region where the surface layer has a lower conductivity (3.5 - 9 km along Figure 2), which roughly corresponds to the width of the palaeovalley as inferred by the boreholes. This region of lower surface conductivity over the palaeovalley sediments may be due to fresh water flushing of salts over many years. A more local example of this flushing effect can be observed at 6.5 km in Figure 2 where it is assumed that freshwater leakage from the Backwater Cowal has caused a shallow resistive feature.

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Figure 2 Comparison of AEM inversion results and seismic refraction results for AEM

flightline 81090 and seismic transect N1, using two different colour stretches for the conductivity data. The black line represents the depth of the first refractor, which in this case is an intermediate regolith refractor and not basement.

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Figure 3 Comparison of AEM inversion results and seismic refraction results for AEM

flightline 82090 and seismic transect N2, using two different colour stretches for the conductivity data. The black line represents the depth of the first refractor, which appears to correspond to the base of the Cainozoic.

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Figure 4 Comparison of AEM inversion results and seismic refraction results for AEM

flightline 83090 and seismic transect N3, using two different colour stretches for the conductivity data. The black line represents the depth of the first refractor, which appears to correspond to the base of the Cainozoic. However, the western (left) 1.5 km has a lower bedrock velocity than the rest of the transect.

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The depth to basement from seismic transect N2 shows good correspondence with the conductivity section from flightline 82090 at ~0.05 S/m. The southern end of the seismic transect traverses an outcrop of Hervey Group Sandstone (Figure 1 and 3) that also features as a prominent resistive ridge in the conductivity section. There is also a good contrast between conductive Cainozoic cover and underlying resistive bedrock, with the AEM and seismic results both showing a steep increase in thickness of the cover sediments north of the Hervey Group ridge. It is likely that the basement rock changes to the north of the ridge, which appears to be fault bounded, and it is possible that Mesozoic rocks are present between the Cainozoic and basement. Neither the seismic or AEM data are able to determine whether Mesozoic rocks are present. The depth to basement from seismic transect N3 and the conductivity section from flightline 83090 at around 0.05 S/m show the same general trends along the central and eastern portions of the transect (e.g. between 4.5 and 7 km in Figure 4). The main features are a steep sided subsurface resistive bedrock ridge (4.5-5.5 km) and a gradually increasing sedimentary cover thickness to the east (5.5-7 km). The location and characteristics of the ridge are consistent with it being a northwards subsurface continuation of the outcropping Hervey Group ridge visible in transect N2 (Figure 3). An examination of conductivity-depth slices for this area (eg Figure 5) shows however that the resistive ridge is not continuous between the two transects, but appears to have a gap, that is filled with more conductive material. The gap is around 1.5 km wide at around 50 m depth (Figure 5) and may represent a sediment filled palaeovalley that once eroded through the ridge. The western end (3-4.5 km) of the depth to basement surface from seismic transect N3 shows poor correspondence to the conductivity section. This area of the conductivity section shows a conductive layer at around 75-100 m depth, which abuts the Hervey Group ridge to the east. The seismic depth to basement surface sits around 20-30 m above the top of this conductive layer. As described previously, the refraction model along this section of the seismic line attributed a lower velocity to the basement, which may correspond to the presence of Mesozoic sedimentary rocks (such as the Drildool Beds) above the Palaeozoic basement. Given that the Mesozoic rocks are relatively conductive, the presence of the conductive layer at 75-100 m depth appears to support this theory. An alternative explanation may be that weathering has affected the basement rock far more in this area, and the conductive zone is a thick saprolite or saprock. Conclusions In general, the basement depths estimated from the refraction models show good correspondence to features in the conductivity sections generated by inverting the AEM data. They both clearly show the narrow ridge of Hervey Group as a steep sided, probably fault bounded unit. They show that while this ridge continues to the north under cover, it is broken by a substantial gap that may represent a gorge in the westwards flowing buried palaeovalley. While the location of the palaeovalley is not clearly shown in any of the data, it may be causing some subtle features in the conductivity sections. For example, the less conductive near-surface sediments in Figure 2 appear to overly the palaeovalley as revealed in boreholes. Comparison of the results also reveals a shallower, low velocity refractor to the west of the buried Hervey Group ridge, where the conductivity section shows a buried layer of relatively high conductivity. These features may correspond to buried Mesozoic rocks (perhaps weathered Drildool beds) abutting the western side of the ridge, or they may be caused by different saprolite/saprock characteristics of the basement.

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Figure 5 Conductivity-depth slices for 0-10 m depth (A) and 40-60 m depth (B). The

images are coloured using a logarithmic scale from 0.01-1 S/m (shown in Figures 2, 3 and 4). They have also been enhanced using a north-east sun angle.

(A) Conductivity at 0-10 m depth.

(B) Conductivity at 40-60 m depth.

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References Bokor, MA (2003) ANSTO 2002 seismic survey: operations report for ANSIR.

Geoscience Australia Record 2003/024. Brodie, R and Fisher, A (2008) Inversion of TEMPEST AEM survey data, Lower

Macquarie, New South Wales. Unpublished report by Geoscience Australia for the Bureau of Rural Sciences.

Jones, L, Waring, C, Hankin, S, Johnstone, D and Fomin, T (2004) Comparison of

shallow seismic and seismoelectric techniques for groundwater surveys: a case study at Narromine NSW. Presentation at the 17th ASEG-PESA Geophysical Conference and Exhibition, 2004, Sydney, NSW.

Lane, R, Brodie, R and Fitzpatrick, A (2004) Constrained inversion of AEM data

from the Lower Balonne area, Southern Queensland, Australia: CRC LEME Open File Report 163.

Sherwin, L (1997) Narromine 1:250 000 Geological Sheet, SI/55-3 Second Edition.

Geological Survey of New South Wales, Sydney. Whitaker, AJ, Raymond, OL, Liu, S, Champion, DC, Stewart, AJ, Retter, AJ,

Percival, DS, Connolly, DP, Phillips, D, and Hanna, AL (2006) Surface geology of Australia 1:1,000,000 scale, eastern States. Regional GIS dataset, Geoscience Australia, Canberra.