ADVANCED INVERSION OF AIRBORNE TRANSIENT ELECTROMAGNETIC DATA (SkyTEM) USING GEOLOGICAL PRIOR INFORMATION
In a study area covering about 100 km2 in the western part of Denmark a unique geophysical dataset consisting of 600 line km of airborne transient electromagnetic data (SkyTEM) and 77 km of high-resolution seismic lines has been collected. A new investigation well with appertaining logs has also been completed.
The preliminary SkyTEM results showed a distinct difference at great depths between the eastern and western parts of the area. The data from the eastern part consistently indicated the existence of a good conductor, whereas the western data showed a more variable resistivity pattern, with only sporadic evidence of a good conductor at depth. Based on the borehole and the seismics, the results were interpreted to indicate a geological surface with a dip towards the west, exceeding the SkyTEM exploration depth in the western-most part.
In order to improve the SkyTEM results, existing geological knowledge was used as a-priori information in the SkyTEM inversions. Thus, the seismic data was used to provide depth information of the Paleogene clay that constitutes the good conductor in this area. Based on logging results and geoelectrical measurements of the borehole samples, the resistivity of the Paleogene clay was estimated to 2.5 ohm-m. The seismic reflection that represents the top of the Paleogene clay was identified by correlating to the exploration well, and was subsequently interpreted on all seismic lines. Finally, the gridded surface of Top Paleogene was utilized in the SkyTEM inversion by using 2.5 ohm-m as an input value for the section below the Top Paleogene grid.
Finally, four different SkyTEM inversions were used in this study. In order to recognize the influence of the added a-priori, both the ‘few-layer’ models (5-6 layers) and the ‘smooth models’ (30 layers), were produced in versions with and without a-priori information. Inversions with both many and few layers were made in order to utilize the ability of the ‘smooth model’ to represent geology with a high-degree of structural heterogeneity, while still obtaining the more precise depth information provided by the few-layer models.
When evaluating the different models, it was obvious that the use of geological information from other data types led to significantly better SkyTEM inversions, hereby making the SkyTEM results more usable in the further work.