Have you used the right density in your gravity terrain correction?
I once wasted two weeks doing field follow-up on a set of airborne gravity gradiometry targets. All turned out to be dusters.
I later learned that a mistake had snuck into the initial processing of the airborne gravity gradient data. The gravity terrain correction had not been done as well as it possibly could have been.
Needless to say, I have been slightly obsessive about gravity terrain corrections ever since.
Gravity Terrain Corrections
The gravity terrain correction attempts to remove the gravity effects of the surrounding hills and valleys from the field observations, so as to isolate the gravity signatures only associated with the subsurface geology.
The effects of nearby terrain can be one of the largest effects in a gravity gradient data set. If you want to see the gravity effects from the subsurface only, then it is important to remove the terrain effect as well as possible.
With dedicated Lidar surveying and regional digital elevation models readily available you can create a high-resolution terrain model that can be used to compute the gravity responses from the surrounding terrain and subtract this effect from the observed gravity gradient data.
Whilst it is numerically possible to compute the effects of the terrain with a spatially varying terrain density distribution, this is almost never done in standard gravity gradiometer data processing. Instead you generally assume a constant terrain density for the entire survey area.
Which Terrain Correction Density?
But what density value should be used in the computation of the terrain effect?
Some survey providers assume the standard density value of 2.67 g/cm3 [the average density of crystalline granitic surface rocks of the continents). A standard terrain correction density value at least allows us to merge different regional surveys. Alas, this density is not suitable in areas with recent sedimentary landforms where terrain densities rarely exceed 2.2 g/cm3.
If you know the density of the landforms in your exploration area from drilling or from surface sampling, then that is an excellent value to start with. However, if you do not have this information, then indirect methods, such as Nettleton’s method may be used.
Nettleton’s Method
A closely spaced gravity traverse is run over some topographic feature, such as a small hill or valley. When the profile of observed values is plotted, the gravitational effect of the feature itself is calculated at each observation point along the profile and removed from the observed value for that point. The calculation is repeated a number of times, different densities being assumed for each computation. The density value at which the hill is least conspicuous on the terrain corrected gravity profile is considered to be most nearly correct.
Nettleton’s method assumes that the correct terrain density is the density that produces a terrain corrected gravity data set with the least correlation to the topography. Note, that this approach is only correct when the subsurface geology is not correlated with topography. If the subsurface geology is correlated with the topography (such as a dense volcanic plug of some depth extent sticking out of the ground and creating a local hill) then Nettleton’s method will overestimate the actual density of the hill.
It is relatively straight forward to redo a terrain correction with another terrain density value, once an initial terrain correction has been computed. But what density value to use?
Some survey providers run Nettleton’s method on a few selected locations in the survey area. Most use visual inspection of the effect of various terrain densities over the entire survey area. I do not think that it is sufficient.
It is my opinion that we do not pay sufficient attention to the terrain correction density, nor do we analyse the spatial variations of terrain density. This can cause residual terrain effects to be left in a final gravity data set.
Could your gravity drill targets be residual terrain effects?
Otway Airborne Gravity Gradiometry Survey
Let me illustrate this with an example, taken from the airborne gravity gradiometer survey over the Otways in southwestern Victoria, Australia. The public domain survey was flown by the Geological Survey of Victoria in 2018-2019 as part of the Victorian Gas Program, using CGG’s FALCON airborne data acquisition system.
The example area is 23km by 23km subset of the survey, in mostly open agricultural (left) and undulating hilly terrain (right) reaching 175m above the nearby sea-level.
The surface geology of the area is the result of a marine regression and tectonic uplift with the Oligocene-Miocene (28.4My-11.1My) continental shelf sediments of the Gellibrand Marls (Nhg) exposed in the eastern part of the survey area, overlain by the Miocene (16.3My-5.4My) marine Port Campbell Limestone (Nhp) exposed in the northern and western part of the survey area, followed by the Pliocene (5.4My-1.8My) fluvial and alluvial out-wash fan sedimentary Hanson Plain Sand (Nbh), and finally capped by the recent Quaternary sheet flow basalts (Qno) in the north-west corner of the survey area.
Terrain effects
Prior to terrain correction the observed vertical gravity gradient GDD (left) is strongly affected by the topography – note how many of the gravity gradient lows appear to be associated with topographic valleys and the gravity highs with topographic hills. This is confirmed when comparing with the computed vertical gravity gradient response from the digital elevation model assuming a terrain density of 1.0 g/cm3 (right).
Nordic Geoscience Terrain Correction Workflow
Nordic Geoscience does not apply Nettleton’s method on a few selected locations only, and does not rely on visual inspection to determine the optimum terrain density. Instead Nordic Geoscience applies an automated and numerically objective version of Nettleton’s method for each data point over the entire survey area. This process yields an estimate of the optimum terrain correction density at each data point (left), along with a confidence measure in the computed terrain density value at that point (right). Only points with more than 25% confidence are shown.
Note the slightly higher optimum terrain density (~2.0 g/cm3) over areas mapped as Gellibrand Marls. Likewise note the slightly lower optimum terrain density (~1.8 g/cm3) over areas where the Port Campbell Limestone is outcropping.
These products are standard deliverables from Nordic Geoscience.
Does your gravity survey provider offer you such deliverables? I think they should.
Statistical analysis of the estimates of the optimum terrain correction density over the example area shows that the mean value, as well as the median value, is 1.92 g/cm3. This is the optimum terrain correction density for this survey area.
With the original survey data delivery, the survey provider did include a data stream with a terrain correction density of only 1.8 g/cm3. Such density is too low a value for the subset area discussed here, but may possibly be a reasonable compromise for the entire survey area.
Effects of Terrain Correction
The (left) figure shows the observed vertical gravity gradient data with no terrain correction. Note the strong correlation with topography.
The (centre) figure shows the vertical gravity gradient data with terrain correction using the NGS optimum terrain density of 1.92g/cm3. Note the near-absence of correlation with topography. This is the best terrain corrected data for this area.
The (right) figure shows the vertical gravity gradient data with terrain correction using the standard terrain density of 2.67g/cm3. Note the strong anti-correlation with topography (gravity highs over topography lows and vice versa) indicating artefacts from over-correction of the terrain effects. A terrain density of 2.67g/cm3 is clearly not correct in this area.
The (left) figure shows the vertical gravity gradient data with terrain correction using the NGS optimum terrain density of 1.92g/cm3. The (centre) figure shows the corresponding vertical gravity data. The gravity data shows a strong correlation with the mapped surface geology (right): The continental shelf sediments of the Gellibrand Marls (Nhg) are associated with a muted gravity high, the Port Campbell Limestones (Nhp) are associated with a subtle gravity low and the Quaternary sheet basalt flows (Qno) are associated with a strong gravity high. The gravity low in the central eastern part of the survey area is unexplained, and may be structurally controlled.
Summary:
- With sufficient care when removing terrain effects, the resulting airborne gravity gradiometer data can reveal so much more.
- Your survey provider should provide you with a map of the spatial distribution of optimum terrain correction densities along with a confidence measure.
- This approach is also applicable for ground gravity data.
You are welcome to contact Nordic Geoscience to discuss how you can extract maximum value from ground gravity and airborne gravity gradiometry data.
Thanks for reading, stay at home and stay safe!
Asbjorn Norlund Christensen is a consulting geophysicist at Nordic Geoscience, a geoscience consultancy with bespoke solutions in exploration geophysics and data science – www.nordicgeoscience.com
PS: You can download the entire Otways AGG survey here:
http://earthresources.efirst.com.au/product.asp?pID=1175&cID=68&c=27378