A COMPARISON OF DEPTH SECTIONS DERIVED FROM THE HONEYSUCKLE CREEK AEM SURVEY AND THE 2006 VICTORIAN SEISMIC TRANSECTS 3 & 4

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 26/2/2008.

i

Contents 1. Introduction 1 1.1 Study area 2 2. AEM data 2 2.1 AEM inversions 3 2.2 Interpreting basement elevation from the AEM inversion results 3 3. Seismic processing 5 3.1 Processing seismic transect 3 5 3.2 Processing seismic transect 4 5 4. Comparing the AEM and seismic results 7 References 10

List of Figures Figure 1 Location of the 2006 Victorian Seismic Transects 3 and 4 in the Honeysuckle

Creek AEM survey area. The image shows elevation (histogram equalised colour stretch with a gradient enhancement), which was derived from the Honeysuckle Creek MAGSPEC survey. 1

Figure 2 Scatter plot showing the correlation between the bore interpreted basement depths against the AEM interpreted basement depths when a threshold conductivity of 0.140 S/m is chosen. 4

Figure 3 AEM conductivity-depth sections calculated along seismic transects 3 and 4. Also shown are bedrock surfaces that were derived from the AEM inversion results and the seismic processing. 6

Figure 4 The image shows an estimate of the basement topography (histogram equalised colour stretch with a gradient enhancement) derived from the AEM inversion. Depressions showing the Goulburn River Palaeovalley and the Violet Town Sump are clearly visible. 7

Figure 5 A 3-dimensional comparison of the AEM inversion data (conductivity sections) and the seismic basement surface (white lines) using GOCAD. See Figure 2 for more detail on each transect. 9

1

1. Introduction This report presents a comparison of depth-to-basement estimates derived from AEM and seismic data, for the Honeysuckle Creek area, near Shepparton, Victoria (Figure 1). The study used time domain airborne electromagnetic (AEM) data recorded with the TEMPEST system during a salinity mapping study in 2002, and the Victorian Seismic Transects 3 and 4, which were part of a deep seismic reflection study conducted by the pmd*CRC and Geoscience Australia in 2006. Permission to use the seismic data prior to its public release was given by Dr Russell Korsch (pmd*CRC and Geoscience Australia). While the data have not yet been publicly released, they are no longer confidential in nature. This report is an update of work previously presented in Lorimer and Fisher (2007) and Fisher and Brodie (2007).

Figure 1 Location of the 2006 Victorian Seismic Transects 3 and 4 in the Honeysuckle Creek AEM survey area. The image shows elevation (histogram equalised colour stretch with a gradient enhancement), which was derived from the Honeysuckle Creek MAGSPEC survey.

2

1.1 Study area The Honeysuckle Creek AEM survey is located to the southeast of Shepparton in northern Victoria. It encompasses the flat expanse of the floodplains of the Goulburn River and associated tributaries (Riverine Plain); the gentle rises of the Caniambo Hills; and the northern edge of the Strathbogie Ranges (Figure 1). Victorian Seismic Transect 3 traverses eastwards across the AEM survey area, rising gently across the Goulburn River floodplain and onto the Violet Town Plain. Transect 4 traverses northwards from the Violet Town Plain, intersecting transect 3 before crossing over the Caniambo Hills and onto the Riverine Plain. The basement for most of the survey area is described on the 1:250,000 geological maps as undifferentiated Silurian – Devonian deformed sedimentary rocks. These sediments are variously known as the Jordan River and Walhalla Groups (Maher et al., 1997), or the Murrindindi Supergroup, which occurs as a thick unbroken sequence across the Melbourne Structural Zone of the Lachlan Fold Belt (Vandenberg et al., 2000). They are thin-bedded fine-grained sandstones and siltstones, with some mudstone and rare pebble conglomerate, which were deposited in a deep marine environment (Maher et al., 1997). The sediments were folded and faulted in the Middle Devonian Tabberabberan Deformation (Maher et al., 1997). The basement rocks are also present as outcrop throughout the Caniambo Hills. Dent et al. (2002) suggest that they have experienced extensive periods of deep weathering, giving rise to thick saprolites of 40 - 70 m. The weathering has resulted in bleached and ferruginous zones, which appear to be the source of magnetic gravels present in buried palaeo-drainage and visible in airborne magnetic data (English et al., 2004). For most of the survey area the basement rocks are covered by Cainozoic sediments of the Murray Basin. Previous studies have shown that the elevation of the basement surface beneath the Murray Basin sediments should show greater relief than the present ground surface. This basement topography surface should include a palaeovalley beneath the Goulburn River (Brown and Stephenson, 1991; Brodie et al., 1996), and a local hydrogeological depression or sump beneath the Violet Town Plain (Brodie et al., 1996; Dent, 2003). The connectivity between the palaeovalley and the sump is not known, and quantifying the nature of the palaeo-topography has been hampered by a lack of deep drill holes. The AEM and seismic data offer an alternative way of imaging the basement surface.

2. AEM data The TEMPEST AEM data were acquired and processed by Fugro Airborne Surveys during 2001. Geoscience Australia has carried out geophysical inversions on the processed data by methods developed at Geoscience Australia (Lane et al., 2004). This resulted in the production of a 3-dimensional distribution of conductivity across the survey area. The method is described briefly below, and will also be the subject of a separate more detailed report (Brodie and Fisher, 2008). Further analysis was also conducted, in order to generate a depth-to-basement surface from the inversion results. This required a new method to be developed, which was based on the known relationship between conductivity and geology that had been observed in boreholes. This method is also described briefly below.

3

2.1 AEM inversions 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 reference model, whose conductivity varied both spatially and with depth, was used to constrain the layer conductivities. The reference model conductivity values were chosen via statistical analysis of downhole conductivity logs and grids of total conductance generated from prior inversions of the data. Maximum smoothness (minimum roughness) constraints were imposed on the vertical conductivity profile at each inverted sample. 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. Each AEM sample was inverted independently 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 Figure 2.

2.2 Interpreting basement elevation from the AEM inversion results Since the inversion model was a 25 layer model with smoothness constraints imposed, each of the layers are not intended to represent specific lithologic or stratigraphic units. Therefore the regolith-basement interface cannot be directly extracted from the inversion results without some interpretation. In this study we developed an automated technique for interpreting the depth to basement based on the supposition that the regolith-basement interface occurs at a depth below which conductivity is less than a given threshold. The method operated on a flight line by flight line basis on the point located inversion data. At each sample the greatest depth at which the conductivity fell below the threshold conductivity was identified via linear interpolation of the logarithm of the layer conductivities at their midpoint depths. These depths were converted to elevations (i.e. heights above the Australian Height Datum) and were along-line filtered with an adaptive low pass filter to minimise the effects of artefacts in the conductivity sections. The adaptive filter worked by respectively vanishing any elevations that differed by more than; (a) 20 m from the local median over a 100 point (~1250 m) window; (b) 10 m from the local median over a 50 point (~625 m) window; and (c) 5 m from the local median over a 25 point (~312 m) window. The vanished elevations were then in filled by linear interpolation and smoothed with a 50 point (~625 m) moving average filter. The resulting basement elevations were converted to depths. The depths were micro-levelled using parameters to remove elongate artefacts in the flight line direction that were narrower than 1,500 m and longer than 5,000 m (Minty, 1991). Grids

4

of the basement depth and elevation were generated at 40 m cell size using minimum curvature gridding. The procedure was repeated several times choosing various threshold conductivity values. For each choice of threshold conductivity, the gridded basement depths were compared to the depth to the base of sediment and/or base of saprolite that had been interpreted from available borehole stratigraphic logs. It was found that a threshold conductivity of 0.140 S/m produced the best fit between the bore interpreted basement depth and the AEM interpreted basement depth (Figure 2). This value of has been used to produce the AEM interpreted basement depth referred to in this study, which is shown in Figures 3 and 4. In the legend key of the Figure 2 the prefix ‘sedbase’ refers to bores in which the base of sediments has been identified and ‘sapbase’ refers to bore in which the base of saprolite has been identified. The suffixes ‘a’, ‘b’, ‘c’ and ‘’d’ respectively refer to high, medium, low, and very low confidences in the borehole interpretations.

Figure 2 Scatter plot showing the correlation between the bore interpreted basement depths

against the AEM interpreted basement depths when a threshold conductivity of 0.140 S/m is chosen.

5

3. Seismic processing The seismic data were processed by Dr Derecke Palmer (Honorary Visiting Fellow, School of Biological, Earth and Environmental Sciences, UNSW) while under contract to the Seismic Acquisition and Processing (SAP) Project in Geoscience Australia. The method that was used is known as the generalised reciprocal method (GRM) and the refraction convolution section (RCS) (Palmer, 1980; 1981; 2001a, b). The results illustrate that the depth-to-basement estimates derived from the seismic data show features that are consistent with our current geological understanding of the area, as well as being consistent with the AEM inversion results (Figure 3). It should be noted that the seismic data were acquired with parameters designed for kilometre scale deep earth imaging and not for the accurate determination of regolith thickness. In particular, the geometry of the survey creates difficulties when attempting to gain information about the near surface. For example, three vibroseis trucks were used, which have a combined footprint of 30 m. Each truck produced three sweeps, moving forward 15 m between sweeps, giving a total source footprint of 60 m. Furthermore, to attenuate ground roll, the receivers comprised arrays with a footprint of 40 m. Problems arise during the data processing when the 60 m long sources and 40 m long receivers are assumed to be points. Such an approximation has little effect on deep crustal scale analysis; however, it can create significant problems in any analysis of the near surface where the regolith thickness may be 100 m or less.

3.1 Processing seismic transect 3 A depth-to-bedrock section was derived from the first-break-picks of the 2006 Victorian Seismic Transect 3 by first generating a GRM time model of the regolith-bedrock interface, and then checking the model against the RCS. The time model was then converted to a depth model through assigning velocities to the two layers (regolith layer and bedrock layer). The seismic velocity of the bedrock was reasonably well determined from the basement refractor as being 5000 m/s. The seismic velocity of the regolith layer could not be well determined; however, based on previous experience in similar geological situations an estimate of 1200 m/s was used. The processing resulted in an estimate of the depth-to-bedrock at each station along the transect, which was then plotted against the AEM inversion conductivity-depth section in Figure 3.

3.2 Processing seismic transect 4 The same method was also used for transect 4, however rather than a two layer model, a three layer model was used. The GRM time model included the water table interface as well as the regolith-bedrock interface. The seismic velocity of the unsaturated regolith layer was assumed to be 800 m/s, while the seismic velocity of the saturated regolith layer was assumed to be 1800 m/s. These values were estimated based on previous experience in similar geological situations. The estimate of the depth-to-bedrock at each station along the transect was then plotted against the AEM inversion conductivity-depth section in Figure 3.

6

Figure 3 AEM conductivity-depth sections calculated along seismic transects 3 and 4. Also shown are bedrock surfaces that were derived from the AEM inversion results and the seismic processing.

7

4. Comparing the AEM and seismic results In order to simplify the following discussion the depth-to-basement estimates derived from the AEM and seismic data are described as the “AEM surface” and the “seismic surface”. Significantly, these two interpreted surfaces show the same trends (Figure 3), which are discussed in the following paragraphs. The AEM and seismic surfaces in transect 3 both reveal three main features of the basement topography: a gentle and intermittent eastwards rise from the western edge of the AEM survey near the Goulburn River palaeovalley to a peak or saddle 7 km to the east; a 10 km wide depression from the 7 km mark, which corresponds to the Violet Town Sump; and a flatter and shallower 1 km wide section at the eastern edge of the transect (Figure 3). A more subtle rise can also be seen in the middle of the Violet Town Sump. This rise can be seen in plan view, where it clearly corresponds to small spurs projecting into the southern edge of the sump (Figure 4).

Figure 4 The image shows an estimate of the basement topography (histogram equalised colour stretch with a gradient enhancement) derived from the AEM inversion. Depressions showing the Goulburn River Palaeovalley and the Violet Town Sump are clearly visible.

8

The bedrock surfaces in transect 4 also show three main features: the Violet Town Sump is visible in the southern section; the bedrock rise visible in the central section underlies the Caniambo Hills; and a gentle and intermittent decline is present in the northern section that corresponds to a gradual thickening of the Murray Basin. The seismic surface in transect 4 shows more variability than the AEM surface. It also contains a significant narrow depression that is not present in the AEM surface. This depression, approximately 1 km wide and 70 m deep, is located near the northern end of the AEM survey area and may be the result of localised deeper weathering. It is thought that this depression is not a palaeovalley, as the presence of a palaeovalley would most likely also show up in the AEM data. The seismic transects intersect in the south eastern section of the Violet Town Sump, which can be seen in the plan view image of the AEM surface (Figure 4). The seismic surfaces generated from the two transects correspond well at this intersection point (Figure 5). The seismic surface from transect 3 is approximately 15 m beneath that generated from transect 4, which is close given the high frequency variation in the surfaces. Interestingly, the seismic surface is consistently deeper than the AEM surface, and corresponds to depths that have a lower conductivity (~0.030 S/m) than the threshold conductivity (0.140 S/m ) used to generated the AEM interpreted basement surface (Figures 3 and 5). This difference in depth could be due to the following two main reasons:

The two surfaces were created from datasets that were imaging different geophysical properties, which may relate to different positions in the weathering profile. The AEM surface is located at depths that correspond to the conductivity measured in boreholes that intersected basement. The transition from regolith to basement observed in these boreholes may in fact be the transition from saprolite to saprock. The seismic surface is located at depths where there is a significant change in seismic velocity, which is likely to occur between porous saturated saprock and non-porous un-saturated fresh bedrock. This means that the AEM surface may represent the saprolite-saprock interface and the seismic surface may represent the lower interface between saprock and basement.

The seismic velocities assigned to the regolith layers may not be valid, and if different

seismic velocities were used it would shift the seismic surface up or down. It must be noted that while the production of the AEM surface was constrained by borehole data the seismic surface had no such constraints.

9

Figure 5 A 3-dimensional comparison of the AEM inversion data (conductivity sections) and the seismic basement surface (white lines) using GOCAD. See Figure 2 for more detail on each transect.

10

References Brodie, RC and Fisher, A (2008) Inversion of TEMPEST AEM survey data, Honeysuckle

Creek, Victoria: Unpublished report produced by Geoscience Australia for the Bureau of Rural Sciences.

Brodie, RS, Overton W and Manson V (1996) GIS Database of Murray Basin Hydrogeology.

Canberra. Australian Geological Survey Organisation, Canberra. Brown, CM and Stephenson, AE (1991) Geology of the Murray Basin, southeastern Australia.

Bureau of Mineral Resources, Australia, Bulletin 235. Dent, D, Munday, TJ, Brodie, RC and Lawrie, KC (2002) Implications for salinity and land

management – Honeysuckle Creek, Victoria: a preliminary interpretation of high-resolution airborne geophysical data. In: Phillips, GN and Ely, K (eds.) Victoria undercover: Benalla 2002 Conference Proceedings and Field Guide. Collaborative Geoscience in Northern Victoria. CSIRO, Melbourne, Australia, p223-233.

Dent, DL (2003) MDBC Airborne Geophysics Project: Final Report. Consultancy D 2018.

Bureau of Rural Sciences, Canberra. English, P, Richardson, P and Glover, M (2004) Interpreting Airborne Electromagnetic Data

As An Adjunct To Hydrogeological Investigations: Honeysuckle Creek Catchment, Victoria. In: Roach, IC (ed.) Regolith 2004. CRC LEME, p76-80.

Fisher, A and Brodie, R (2007) A comparison of depth sections derived from AEM and

seismic data, Honeysuckle Creek, Victoria: and update on Victorian Seismic Transect 3. Unpublished report produced by Geoscience Australia for the Bureau of Rural Sciences.

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.

Lorimer, T and Fisher, A (2007) A preliminary report on the comparison of depth sections

derived from AEM and seismic data, Honeysuckle Creek, Victoria. Unpublished report produced by Geoscience Australia for the Bureau of Rural Sciences.

Maher, S, Vandenberg, AHM, McDonald, PA and Sapurmas, P (1997) The geology and

prospectivity of the Wangaratta 1:250 000 map sheet area. Victorian Initiative for Minerals and Petroleum Report, 46. Victorian Department of Natural Resources and Environment.

Minty, BRS (1991) Simple microlevelling for aeromagnetic data: Exploration Geophysics, v.

22, p. 591–592. Palmer, D (1980) The generalized reciprocal method of seismic refraction interpretation:

Society of Exploration Geophysicists, p104.

11

Palmer, D (1981) An introduction to the generalized reciprocal method of seismic refraction interpretation: Geophysics, 46(11), 1508-1518.

Palmer, D (2001a) A new direction for shallow refraction seismology: integrating amplitudes

and traveltimes with the refraction convolution section. Geophysical Prospecting 49, 657-673.

Palmer, D (2001b) Imaging refractors with the convolution section. Geophysics 66, 1583-

1589. Vandenberg, AHM, Willman, CE, Maher, S, Simons, BA, Cayley, RA, Taylor, DH, Morand,

VJ, Moore, DH and Radojkovic (2000) The Tasman Fold Belt System in Victoria. Geological Survey of Victoria Special Publication, 462p.