Livrables
Two methods for integrated of MT(EM) data handling were developed as part of the project. The first was a development of the commercially available ARKFIELD interpretation suite whilst the second was based on a bespoke software program specifically developed for the SIMBA project.
A method for the allowing for the simultaneous display of potential field and seismic data, together with seismic interpretations and gravity/magnetic models already existed within the ARKFIELD software. The ARKFIELD software was modified such that processed MT data could be also be displayed. A workflow was specifically developed such that the additional information from the MT data could be utilised to enhance the modelling of the basalt and sub-basalt structures. The workflow developed was tested using the 2D synthetic data and on the Isle of Skye demonstration site data.
As part of the investigation into the use of a joint interpretation/inversion scheme for seismic/non-seismic integration, developed within Result 4, a software program incorporating a 2D gravity response and an approximated 2D MT forward response was produced. The program was developed using the 2D synthetic data (WP1) and utilised for the processing of data collected from the main demonstration site (WP3 and WP5).
The reprocessed data from the demonstration site show how careful attention to processing in particular the novel combination of deterministic deconvolution combined with robust multiple suppression yields an image from beneath basalt layers.
The final sections are reinterpreted in conjunction with other geophysical and geological information including gravity, magnetics and magneto-telluric data. Joint inversion and interpretation strategies have be developed and the relative contribution to the final interpretation of each type of data is presented.
The result will enable geoscientists to optimise data acquisition, processing, inversion and interpretation strategies for other areas with severe basalt layers worldwide. The exact processing and joint inversion strategies have been elaborated and validated using both 2- and 3-D synthetic data (see main project results 1 & 5 this document) as well as by using the real data obtained from the demonstration site.
A methodology to increase the value of gravity, magnetic and electromagnetic data as an additional constraint to seismic data in complex reservoir situations has been developed. It has been based on the link between the physical parameters, density (gravity data), seismic velocity (seismic data) and electrical resistivity (MT data). Since, no numerical relationship has been established, an empirical relationship was investigated. This relationship was based on the analysis of well log data taken from the Rock Property Database (WP1).
The methodology exploits the fact that MT measurements in basalt environments can be shown to be sensitive to the over lying and sub basalt sediment layers and are relatively insensitive to changes in the basalt layer whilst the gravity response can be shown to be relatively insensitive to the upper and sub basalt sediment layers but is sensitive to changes in the basalt layer.
A joint approach using a quasi 2D joint inversion scheme greatly reduced the level of uncertainty in the inversion of the complete dataset. The subsequent inversion code (developed as part of Result 2) incorporates a 2D gravity response and an approximated 2D MT forward response consisting of a succession of 1D layered strips underneath each station. The method was tested using the 2D synthetic dataset compiled as part of WP1.
The database contains 13784 entries of data derived from flood basalts and associated rocks. The majority of the data was obtained from the Ocean Drilling Project. The data contains wireline derived physical rock property data including density, velocity, resistivity, and gamma ray measurements. In addition to the wireline-derived data additional parameters measured on ODP cores (e.g. magnetic susceptibility and geochemical information) were added to the database. In order to make comparisons between different areas and allow sampling of the data a number of identifiers have been added to the database where relevant. These include for example lithology, flow type and whether the flows were subaerial or submarine.
The aim of this database is to allow users to sample rock property data from the database in order to populate their models with realistic and relevant values. Individual values, ranges or distributions can be obtained easily from the database and statistic analysis performed on the data.
The data base has been made into a browsable format for the SIMBA web site by ARK. The database will be placed on the SIMBA website for people to download and use.
Five new marine MT instruments were designed and built, along with a matched land instrument for use at a reference station. The instruments are autonomous, microprocessor-controlled units, the core of each being a 3-component magnetometer, a 2-component electrometer, and data acquisition and control circuitry. This assembly has been designed to be cheap, dismountable, light-weight and modular. The system has been successfully been used at sea (see part 7). The instruments are available for viewing at Brest facilities.
A complete suite of synthetic geophysical data from a 3-D model based on geological information. These data provide a reference dataset for the development and quantitative evaluation of inversion strategies for imaging and interpretation of the Earth's subsurface in challenging area with significant basalt layers.
The 3-D model is based on real seismic data with the basalt layers being parameterised from mapping of exposed basalt sequences and rock physics from borehole logs drilled into basalt layers.
The synthetic data consist of seismic reflection data in shot gathers, the form in which it is acquired in the field, modelled using the elastic 'phase-screen' modelling approach, this method has a higher frequency content than equivalent finite difference codes used for the 2-D data and can e computed very efficiently. However, the synthetics are limited to pre-critical arrivals. Two versions of the data will be available both with and without the free surface multiple. A 3-D free-air gravity anomaly and a 3-D magnetics profile along with a number of synthetic magneto-telluric stations.
The datasets may be used individually or in combinations to test which of the original model's physical properties can be recovered by particular inversion algorithms and their accuracy and sensitivity.
Both the data and the original model will be publicly available.
This result is a case study on the amount and structure of basalt cover in northern Skye, NW Scotland. The investigation started from geological sections, which were mapped during the project, and interpretations of existing seismic and bore hole data. Additional gravity and MT data were acquired within the project and these were also processed and interpreted.
The aim is to look at the effectiveness of each technique at identifying the base-basalt layer in the subsurface on Skye. This is quantified by comparison of the result from each technique with the others and with known base basalt contacts from Bore hole and rock outcrops. The main result is a 3-D geological earth model of the structure of the basalt cover in northern Skye. This is relevant as it tests the ability of different geological and geophysical techniques to predict the same geological structure.
The case study has been written up for the final EU report for work package 1; the report will be available through the SIMBA website, and the model and data are also potentially available on request. Additionally 2 scientific papers relevant to the study have been published/submitted:
Richard T. Single & Dougal A. Jerram, 2004, The 3-D facies architecture of flood basalt provinces and their internal heterogeneity: examples from the Palaeogene Skye Lava Field. Journal of Geological Society 161, 911-926.
Hautot, S., Single, R.T., Watson, J., Harrop, N., Jerram, D. A., Tatris, P. & Whaler, K., 2005, 3D magnetotelluric inversion and model validation with gravity data for the investigation of large igneous provinces. Submitted to Geophysical Journal International (may 2005).
Five new marine MT sensors were designed and built and tested at sea along a 2-D 17 sites profile. While the data from three sites have had to be temporarily put aside because of noise due to instrument motion, those from the other stations have allowed calculation of impedances for periods down to below 10s, which is exceptional for deep marine acquisition.
In this WP a 2D inverse modelling package was developed with specific features to allow for the marine context, that is, to permit current flow in the conductive water layer above/containing the receivers, and for the strong variations in bathymetry along the survey line. The inversion software was also modified to incorporate prior constraints derived from the seismic interpretation, and further constraints from the gravity data were incorporated. The results at this stage offer some new insight into the large-scale features of the profile. earing in mind that this was the first acquisition with the new instruments, and in an unusually deep marine setting, these results seem extremely satisfactory.
The marine MT data will be available at the SIMBA home page.
A complete suite of synthetic geophysical data from a suite of 2-D models based on geological and geophysical information. These data provide reference datasets for the development and quantitative evaluation of inversion strategies for imaging and interpretation of the Earth's subsurface in challenging area with basalt layers.
The 2-D models are based on real seismic data, with the basalt layers being parameterised from mapping of exposed basalt sequences and rock physics from borehole logs drilled into basalt layers.
The synthetic data consist of seismic reflection data in shot gathers, the form in which it is acquired in the field, modelled using an accurate elastic finite difference scheme. These data are available with and without free surface multiple contamination. 2-D free-air gravity anomaly, 2-D magnetics profile along with a number of synthetic magneto-telluric stations and controlled source EM have been generated using the models.
The datasets may be used individually or in combinations to test which of the original model's physical properties can be recovered by particular inversion algorithms and their accuracy and sensitivity.
Both the data and the original model will be publicly available.
A complex 3D model was built incorporating all the information available from the interpretation of seismic data, borehole log data, gravity data and geological observation. The aim was to create a realistic heterogenous geological 3D model that would help in providing an overall assessment of the sub-basalt imaging problem through synthetic wave simulations.
Accurate 3-D synthetic seismic and non-seismic (gravity, magnetic and EM) data have been produced based on the model, and these data are to be used in developing strategies for data integration into a common methodology to overcome the sub-basalt imaging problem.
The complex and detailed model was designed based on information extracted from real data, to replicate the velocity distribution, the highly scattering internal basalt structure and the irregular flow interfaces. It was created obtaining and adapting the geological interfaces from the interpretation of seismic data and filling the layers between them statistically with information from borehole log data. The statistical information on both interfaces and layer velocity structure is needed to overcome the lack of a deterministic knowledge of small-scale heterogeneity.
Both the model and the data are available to the public.
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