Service: Map the thickness of basement-covering material
DEPTH-to-source maps can help steer mineral and petroleum explorers toward their preferred exploration depth, and toward basement morphology of exploration significance. Our case study, involving southeastern Australia's Bass Basin offshore region, shows how depth-to-source estimation can augment exploration efforts.
An area's topography or bathymetry may look relatively subdued.
But does this simply reflect a flat crystalline basement, or does the basement contain features such as horst and graben blocks, incised channels, and igneous complexes?
At first, this question may seem academic. However, depending on the answer, an exploration project may turn out to be subeconomic.
For instance, some basement-related risk factors that explorers are exposed to include:
- the possibility that ore zones will prove to be excessively deep, or the possibility that a series of dry petroleum wells will be drilled in areas of excessively shallow basement — a sink on company cash flow,
- the opportunity cost of not having employed the above cash on something fruitful, either nearby in the same exploration tenement, or somewhere else entirely, and
- if the exploration tenement now possesses no redeeming features, it represents an unsellable blight on the balance sheet where previously the explorer had hoped to see a productive asset.
Depth-to-source: What is it good for?
Depth-to-source maps are a quantitative version of the well known and long-accepted geophysical tenet that smooth, broad features in magnetic data tend to involve source bodies cloaked by relatively thick piles of magnetically quiet material. A similar principle applies to gravity data, in some circumstances.  
A recently published paper describes the technique's ubiquitousness and a common industry application:
"The most universal application of gravity and magnetic data has been to determine the depth to the top of the geologic sources that produce observed anomalies. For hydrocarbon exploration, this is usually the equivalent for determining the maximum thickness of the sedimentary section or the location of igneous intrusive[s] in the section. ... Often, a well-defined boundary between zones with different degrees of magnetic relief can indicate the presence of [a] major basement fault. Therefore, calculations of the depth to the basement rock are very important [for outlining], quantitatively, the thickness variation of the sedimentary cover and to delineate the structural relief of the basement rocks and its effect on the overlying [clastic] deposits." 
Depth-to-source mapping may therefore help to even the basement-controlled exploration odds, by allowing explorers make preliminary decisions about whether a target location might be beneath cover too thick to be economically viable, or so thinly covered that the likelihood of orebody or reservoir preservation is probably vanishingly small. Conversely, promising areas may be definable based on estimated cover thicknesses shown in depth-to-source maps, in conjunction with supporting geoscientific evidence. 
And if mineralized zones or reservoir locations seem to lack strong associations with specific lithological units, or specific geological contact types, or specific geological structures, depth-to-source mapping may reveal a strong association with specific features or transitions in basement morphology and cover-sequence thickness.
Depth-to-source techniques may also help to establish and refine geological models  , and assess whether exploration areas contain any previously unknown structures or features. 
Clastic rock successions
Decades of onshore and offshore geological mapping by the geoscience community has shown that, generally, sedimentary successions vary laterally to a significant degree in overall thickness. They heavily cloak some basement sites, while other basement sites possess only a thinly draped veneer of cover. 
Basement morphology is influenced by processes such as faulting, tilting and rotation of blocks, and intrusion of magmatic material. Basement rocks are also shaped by variations in the intensity and ease of weathering and erosion, and by the sediment transport distances prevailing in the area.
The industry generally holds depth-to-source in high regard. For instance, one group of authors said:
"For resource exploration purposes, one of the most useful inferences that may be derived from analysis of potential field data is the depth [to] crystalline basement beneath [sedimentary and regolith] cover." 
Sediment-derived rocks tend to have only very weak magnetic susceptibilities. Therefore, the depths estimated for magnetic sources usually correspond to the depth to crystalline basement. In other words, a depth-to-source map indicates the thickness of non-magnetic cover material. 
An exception to this rule of thumb is where a relatively magnetic magmatic body, such as a sill or a dike, has invaded relatively porous or weak horizons in overlying sedimentary successions or has erupted onto the surface that existed at the time before being covered. However, a method of coping with this scenario appears in the published literature. 
Another exception is where the upper basement is composed of crystalline rocks that contain relatively little magnetite, and so are relatively magnetically quiet (such as lower-grade metamorphic rocks that were derived from clastic rocks). If a strongly magnetic lithology exists deeper in the basement, then it is the depth to this deeper source that is being determined.
Depth-to-source estimation methods
Many methods exist for estimating the depth to potential field sources. Some methods are applied to profile data, while others are applied to gridded data. Over the years, some methods have been improved,  and some methods now have a range of ways in which they can be performed.
Depth-to-source estimation methods include:
- Peter's half-slope method 
- Vertical integral 
- Power density spectral method   
- Analytic signal (total gradient)  
- Local wavenumber  
- Euler deconvolution       
- Werner deconvolution 
- Multi-deconvolution 
- Wavelets 
- Prism modeling  
- Similarity transforms  
- Tilt-depth method  and reciprocal tilt-depth method 
- Methods applicable to gradient tensor data 
- Combinations of methods     
Generally, there is no single overall 'right' method.  However, in practice, some methods may be better suited to a particular case study than other methods. And, all else being equal, the different depth-to-source estimations methods involve varying degrees of accuracy and ease of use.
Each method has its strengths and weaknesses. For instance, a drawback to early forms of the Euler deconvolution method was having to establish the parameter known as the structural index of the magnetic source.  The method also was considered unwieldy in the absence of clustering analysis, because of the spray of depth estimates produced for a given source body. 
As with any signal analysis technique, having data control points (when available) helps to establish confidence that depth-to-source results are firmly grounded in reality.
Uses for depth-to-source maps
Petroleum explorers have been heavy users of depth-to-source mapping to: help gauge overall basinal extent, glean clues about the structural development of basins, ascertain the existence of troughs and sub-basins within the wider basin, and help reduce exploration risk by assessing features relevant to petroleum maturation, migration and accumulation.
Further examples of depth-to-source map uses include:
- Locating previously unrecognized faults, or basement relief highs and lows, or the inflection zones bounding those highs and lows, which may occur where pronounced gradients, discontinuties and other exaggerated features are situated in depth-to-source map contours. 
- Locating reactivated basement faults, which may occur where major discontinuities in the depth-to-source map coincide with major discontinuties in the magnetic data.
- Constraining forward models of magnetic bodies and directing how their anomalies are best interpreted.  For example, areas with only relatively thin cover and possessing strong, noisy, multiple-lobed magnetic signatures are typical of relatively recent basaltic flows and sills. 
Depth-to-source mapping can help evaluate basins when it comes to subsurface geological storage of carbon dioxide.  Not only does knowledge of cover thickness and basement relief matter in these evaluations, but also knowledge of the whereabouts of any relatively recent magmatic activity may be crucial to fully understanding a basin's carbon dioxide storage potential. 
Researchers of Antarctica's geology and geodynamic evolution have been using depth-to-source estimation to help clarify the structure and origin of large portions of the continent.  
Geothermal energy explorers use depth-to-basement estimation as part of their activities. 
And minerals explorers have been using depth-to-source estimation for a long time. The technique, if implemented in a valid manner using proper testing, can help characterize areas where drillhole and seismic data are sparse.        
Here, we present a case study in the southeastern offshore region of Bass Strait that shows how depth-to-source mapping can benefit explorers.
Case study: Bass Strait, Australia
We applied depth-to-source estimation to magnetic data in the Bass Strait region, which separates Victoria and Tasmania.
The area covered in this case study is located east of King Island, west of Flinders Island and south of Wilsons Promontory.
The aim of this case-study excercise is to rapidly produce depth-to-source results from a large dataset, to enable a first-pass assessment of the regional variation in non-magnetic cover thickness.
For anyone new to Bass Strait, or a bit rusty, what follows is a primer on the area.
Various hypotheses have been proffered to explain how specific interrelationships among the Australian mainland, Antarctic continent and Tasmania evolved following supercontinent break-up.  Ideas also abound in the literature about how the present-day Bass Strait's physiographic features formed. 
The Bass Basin, now inundated due to sea-level transgression, is thought to have begun developing as a terrestrial intracratonic rift basin in the late Jurassic to early Cretaceous, and the gas-prone petroleum system the basin hosts is considered to be related to coal-bearing sequences.  
The Bass Basin's maximum Cretaceous to Tertiary sediment thickness was gravity modelled to be about 5.5 kilometers.  Most of the source rocks for liquid hydrocarbon were deposited from the late part of the early Eocene to the middle Eocene (54 million years ago to 44 million years ago), a period that coincided with a waning in the basin's subsidence and in its sediment supply. 
Noteworthy geological events experienced in the Bass Basin region include:
- The initiation of intracratonic rifting produced half-graben faults that generally strike west to northwest. 
- An extra degree of complexity was added to the Bass Basin's geodynamic behavior, because through much of its history the region was sandwiched between the Southern Rift System (which opened the Southern Ocean) and the rift system of the Tasman Basin (which opened the Tasman Sea). This scenario meant the Bass Basin was subjected to a string of extension and compression episodes triggered within and between tectonic plates.  The Bass Basin's evolution was also affected by long-lived stresses coming from the north-south-striking Tasman Fracture Zone that developed west of Tasmania.  
- NW-SE-striking faults predominated in the Cretaceous, then NNW-SSE-striking faults occurred in the Paleocene and Eocene, with NNE-SSW-striking faulting developing in the late Eocene. 
- Pulses of strike-slip reactivation starting in the late Eocene to late Miocene coincided with changes in the dynamics of the Australian Plate boundaries that established NW-SE-oriented to N-S-oriented compression across southeastern Australia.   Inversion of Bass Basin structures took place , to varying degrees depending on the locality.
- Widespread basaltic volcanism occurred in northwest Tasmania and the Bass Basin in the Cainozoic. 
- "Repeated fluctuations in sea level in the Quaternary (the last transgression commencing 17,000 years before present) caused the area to alternate between being land (the Bassian Plain), a lake, a marine embayment, and sea several times. Bass Strait today is mostly between 30 meters and 90 meters deep." 
- The main present-day physiographic domains in and around the Bass Basin include the King Island High, the Bassian Rise, the Durroon Sub-basin, and the Cape Wickham Sub-basin.  
Petroleum exploration in the area has revealed the Bass Basin's complex geological history  , and the region's resulting subsurface complexity. Industry observers have made note of this, saying, for example:
"The limited success in the Bass Basin contrasts to the adjacent Gippsland Basin, which has been Australia's premier petroleum province for the last 40 years." And: "An improved understanding of the structural development of the Bass Basin should aid future exploration decisions regarding source, reservoir, seal and trap formation." 
"An effective petroleum system is clearly present in the Bass Basin; however, the failure to discover more accumulations suggests that the system may be complex, with several risk factors present. The complexity of the petroleum system is probably related to the terrestrial nature and distribution of the primary source rocks, which are mainly coals and interbedded shales. ... An improved understanding of the regional controls on the complex facies relationships within the terrestrial Cretaceous to early Paleogene succession is required to assess these risk factors and to further understand the petroleum potential of the Bass Basin." 
Bass Strait depth-to-source map
We used an upward continuation-based method of depth-to-source estimation  on gridded aeromagnetic data obtained from Mineral Resources Tasmania (Figure 1) to produce the Bass Basin depth-to-source map shown in this case study (Figure 3).
To help the reader examine the correspondence between some features in the depth-to-source map and the magnetic data, we have included a map of the area's magnetic structure (Figure 2). Structures show where significant discontinuities occur in the observed data.
FIGURE 1: Reduced-to-the-pole (RTP) magnetic data for the Bass Basin case study area. Also shown are coastal boundaries (black lines).
FIGURE 2: Structural network (black lines) extracted from the Bass Basin case study area's magnetic data using Fathom Geophysics' computer-vision routines. Magnetic structures shown are those with wavelengths of between about 4,000 meters and about 12,000 meters. The structural network is in vectorized form, superimposed on the area's RTP magnetic data.
FIGURE 3: Results of automated depth-to-source calculations in the Bass Basin region, based on the area's magnetic data, using Fathom Geophysics' computer-vision routines, which are described in further detail in this article's text. To view a larger version, click on either the image or this text link.
This depth-to-source method used here involves looking the reciprocal of the geophysical measure known as tilt angle. Estimating the distance to a given magnetic source at depth with this method is similar to estimating the distance to a tree by measuring how much its apparent size changes when you take a step towards it: a distant tree would only grow in apparent size by a miniscule amount; while the apparent size of a closer tree would increase more rapidly. The method is quite robust to signal noise, but can be prone to problems in the presence of geologically-induced noise (e.g., basalt typically produces a 'noisy' magnetic signature).
In other work, we've implemented a depth-to-source estimation method that is based on how the analytic signal decays horizontally. 
We also produced a structure map of the depth-to-source results (Figure 4). These structures separate relatively thickly covered domains from relatively thinly covered domains.
FIGURE 4: Structural network (black lines) extracted from the Bass Basin case study area's depth-to-source map using Fathom Geophysics' computer-vision routines. Source-depth structures shown are those with wavelengths of between about 4,000 meters and about 12,000 meters. The structural network is in vectorized form, superimposed on the area's depth-to-source map.
To help keep the presentation of this case study's images simple and easily viewable, we've not included any depth legends with our depth-to-source maps, nor have we put any depth labels on our depth-to-source contours (Figure 5).
FIGURE 5: Results of automated depth-to-source calculations in the Bass Basin region, based on the area's magnetic data, using Fathom Geophysics' computer-vision routines, which are described in further detail in this article's text (background colors). Superimposed over the depth-to-source map are depth-to-source contours (black lines). To view a larger version, click on either the image or this text link.
To help orient the reader familiar with petroleum wells drilled in the region, we've included a contoured depth-to-source map (Figure 6) that also shows the approximate locations of a selection of wells and the approximate location of a seismic line appearing in the literature (see reference ).
FIGURE 6: The same depth-to-source map and depth-to-source contours for the Bass Basin region, this time with the approximate location of some well-known petroleum wells (yellow circles) and seismic line (thick black line).
And we made use of the depth-to-source results, by manually producing what we're informally calling a 'basinal onlap boundaries' map (Figure 7). We also produced a first-pass manually interpreted geology map (Figure 8) based on the area's magnetic data, gravity data (obtained from Geoscience Australia) and our depth-to-source map.
FIGURE 7: Manually-produced basinal-onlap boundaries for the Bass Basin area (dark gray lines), based on inspection of the area's depth-to-source results and magnetic data. The mid-gray shaded areas depict regions containing relatively thick cover, according to depth-to-source results.
FIGURE 8: First-pass manually interpreted geology map for the Bass Basin region, based on the area's magnetic data, gravity data (shown in later figures), and depth-to-source results. Gray features depict interpreted intrusions/extrusions, while green, orange, pink, olive, purple and red areas depict the interpreted extents of various distinct features seen across the datasets used. Blue areas depict the interpreted extent of undifferentiated regions. Note that darker shades of a given color indicate deeper cover thickness.
The basinal onlap boundaries map gives a regional, binary view of the thickly covered areas (gray-shaded areas on the map) versus the thinly covered areas (unshaded areas). We have superimposed these boundaries on our manually interpreted geology map (Figure 9) to get an overall feel for which geological features appear partially or completely covered by relatively thick cover.
FIGURE 9: First-pass manually interpreted geology map for the Bass Basin region, with the manually-produced basinal-onlap boundaries shown over the top (dark gray lines).
We like the combination of the manually interpreted geology map and depth-to-source contours (Figure 10). This kind of combination may be handy, for example, as an input when selecting localities to target for further reconnaissance work.
FIGURE 10: First-pass manually interpreted geology map for the Bass Basin region, with the depth-to-source contours (narrow lines) and the approximate location of petroleum wells and seismic line (yellow circles and thick straight line) shown over the top. To view a larger version, click on either the image or this text link.
We can also see how depth-to-source results, derived in this case solely from magnetic data, compare with the area's gravity data (Figures 11 and 12).
FIGURE 11: Gravity data for the Bass Basin case study area (background colors), shown with depth-to-source contours shown over the top (gray lines), to permit a comparison.
FIGURE 12: Again, gravity data for the Bass Basin case study area (background colors), shown with depth-to-source contours shown over the top (gray lines), to permit a comparison, this time with the approximate location of petroleum wells and seismic line (yellow circles and thick straight line).
What depth-to-source mapping reveals
In this case study of the Bass Basin region, depth-to-source mapping demonstrates:
- the complexity and compartmentalization of the area, such as its multiple depocenters, as discussed in the literature  ,
- general agreement with interpreted profiles constructed from seismic data that appear in the literature (such as Figures 3a, 8, 11 and 15 of reference ; and Figure 6 of ),
- the predominance of shallow igneous activity throughout much of the case study region   , which in many cases appears associated with lobe-shaped magnetic features (similar to those shown in Figure 10 of reference ) that are situated in thinly covered basement areas of our depth-to-source map,
- general agreement with basement relief mapping that was based on interpretation of 16 open-file 2D seismic surveys, as presented in the University of Adelaide 2010 PhD thesis by Natt Arian (Figure 13) , and
- the White Ibis and Yolla gas fields appear situated on or near pronounced transitions that involve going from thickly to thinly covered basement.
FIGURE 13: Depth-to-source contours (shown with the approximate location of petroleum wells and seismic line) over the top of basement relief mapping that was based on interpretation of 16 open-file 2D seismic surveys, as presented in the University of Adelaide 2010 PhD thesis by Natt Arian [Reference 38]. To view a larger version, click on either the image or this text link.
In addition, the depth-to-source map reveals the presence of an enigmatic corridor of deeply covered basement, flanked on both sides by thinly covered basement that possesses the strong magnetic signature of volcanics.
Igneous activity is seen in all extensional sedimentary basins situated at continental margins and may not always diminish a basin's petroleum prospectivity.  
Thin-section studies of volcanic rock in the Bass Basin has suggested that, in places, magmatic material horizontally infiltrated horizons of high-porosity lacustrine deltaic sandstones, with the implication that "... the sills could act as effective [petroleum] seals". Meantime, those sandstones may have good petroleum reservoir attributes wherever they remained unintruded. 
The depth-to-source mapped corridor we are describing extends roughly northward from the Bass Basin toward Victoria's Cape Paterson, and may be related to physiographic features of the fault-controlled Koorkah paleo-lake. The locality is demarcated with question marks in Figure 13 of reference .
Bass Basin researchers have noted evidence suggesting that "... while lakes occupied the deeper areas of the basin, oil-prone coals were forming on the fringing floodplains and peat mires". 
Perhaps the fringes of this thickly covered corridor once hosted vegetated floodplains — areas that may now be capped by sealing volcanic sills.
 See, for example, discussion on page 17 of: C. D'Ercole, F. Irimies, A.M. Lockwood and R.M. Hocking (2005) "Geology, geophysics, and hydrocarbon potential of the Waigen area, Officer Basin, Western Australia", Western Australia Geological Survey, Report 100, 51 pages.
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