A Monte Carlo approach to constraining uncertainties in modelled downhole gravity gradiometry applications
Samuel J. Matthews 1 2 3 Craig O’Neill 1 Mark A. Lackie 11 Australian Research Council Centre of Excellence for Core to Crust Fluid Systems/GEMOC, Macquarie University, Sydney, NSW 2109, Australia.
2 Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), National Farmers’ Federation House, Canberra, ACT 2600, Australia.
3 Corresponding author. Email: sam.matthews@mq.edu.au
Exploration Geophysics 48(3) 305-315 https://doi.org/10.1071/EG15039
Submitted: 6 May 2015 Accepted: 11 April 2016 Published: 2 June 2016
Abstract
Gravity gradiometry has a long legacy, with airborne/marine applications as well as surface applications receiving renewed recent interest. Recent instrumental advances has led to the emergence of downhole gravity gradiometry applications that have the potential for greater resolving power than borehole gravity alone. This has promise in both the petroleum and geosequestration industries; however, the effect of inherent uncertainties in the ability of downhole gravity gradiometry to resolve a subsurface signal is unknown. Here, we utilise the open source modelling package, Fatiando a Terra, to model both the gravity and gravity gradiometry responses of a subsurface body. We use a Monte Carlo approach to vary the geological structure and reference densities of the model within preset distributions. We then perform 100 000 simulations to constrain the mean response of the buried body as well as uncertainties in these results. We varied our modelled borehole to be either centred on the anomaly, adjacent to the anomaly (in the x-direction), and 2500 m distant to the anomaly (also in the x-direction). We demonstrate that gravity gradiometry is able to resolve a reservoir-scale modelled subsurface density variation up to 2500 m away, and that certain gravity gradient components (Gzz, Gxz, and Gxx) are particularly sensitive to this variation in gravity/gradiometry above the level of uncertainty in the model. The responses provided by downhole gravity gradiometry modelling clearly demonstrate a technique that can be utilised in determining a buried density contrast, which will be of particular use in the emerging industry of CO2 geosequestration. The results also provide a strong benchmark for the development of newly emerging prototype downhole gravity gradiometers.
Key words: borehole, gradiometry, gravity, Monte Carlo.
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