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Seismic sub-basalt imaging is challenging and often leaves remaining uncertainties about the sub-basalt structures. This paper considers how the marine controlled source electromagnetic (CSEM) and the magnetotelluric (MT) method can reduce the interpretation uncertainty by providing structural and quantitative resistivity information about the thickness and extent of the basalt sequence and the sediments below. In order to demonstrate the capability of these technologies, we will present two recent studies: (1) a synthetic inversion test from a realistic model based on well-log and seismic data and (2) inversion of field data acquired over the Faroe-Shetland Escarpment. We will show that this information can also be used to improve the velocity models for seismic depth imaging. Introduction Basalts are extrusive igneous rocks that can be found worldwide. Often the compact basalts are interlaid with volcanoclastics and sediments. Such sequences are formed by a series of eruptive events. Sub- and intra-basalt sediments can be hydrocarbon bearing, which makes sub-basalt exploration very interesting to the oil and gas industry. Using seismic, one can usually establish the top of the basalt accurately. But due to the heterogeneities in the basalt layer in combination with high seismic impedance contrasts it can be challenging to accurately image the base of the basalt and underlying sediments (Fliedner and White, 2001; Fruehn et al., 2001). Conversely, several modelling and inversion studies have shown that EM methods can play a significant role in mapping both the thickness of the basalt layer and the sediments below (Jegen et al, 2002; MacGregor, 2003; Hoversten et al.,2013; Heincke et al., 2014). The CSEM and MT methods utilize electromagnetic (EM) fields to measure resistivity variations in the sub-surface. Compact crystalline rocks such as basalt have virtually no porosity and are very resistive in contrast to sediments with comparably large pore spaces filled with more conductive pore fluids. That makes EM methods well suited for imaging of the thickness of the basalt layer and the structures underneath. In this paper, we discuss two studies, (1) a synthetic data study based on a realistic model (Herredsvela et al., 2012) and (2) a field data example from the Faroe-Shetland Basin. Synthetic Basalt Model Figure 1 shows the model which was used for the synthetic modelling and inversion study. The water depth in the model varies from 300 to 350 m. The model consists of a fine layered basalt sequence with a varying thickness from 200 meters to almost 2 kilometers and a detailed sediment structure underneath the basalt. The internal structure of the basalt sequence is based on the observations from well logs and seismic. It consists of many thin layers with varying resistivities which are sensed as one thick anisotropic layer due to low resolution of the EM methods. The information used to construct this model makes it analogue to a realistic scenario.
Seismic sub-basalt imaging is challenging and often leaves remaining uncertainties about the sub-basalt structures. This paper considers how the marine controlled source electromagnetic (CSEM) and the magnetotelluric (MT) method can reduce the interpretation uncertainty by providing structural and quantitative resistivity information about the thickness and extent of the basalt sequence and the sediments below. In order to demonstrate the capability of these technologies, we will present two recent studies: (1) a synthetic inversion test from a realistic model based on well-log and seismic data and (2) inversion of field data acquired over the Faroe-Shetland Escarpment. We will show that this information can also be used to improve the velocity models for seismic depth imaging. Introduction Basalts are extrusive igneous rocks that can be found worldwide. Often the compact basalts are interlaid with volcanoclastics and sediments. Such sequences are formed by a series of eruptive events. Sub- and intra-basalt sediments can be hydrocarbon bearing, which makes sub-basalt exploration very interesting to the oil and gas industry. Using seismic, one can usually establish the top of the basalt accurately. But due to the heterogeneities in the basalt layer in combination with high seismic impedance contrasts it can be challenging to accurately image the base of the basalt and underlying sediments (Fliedner and White, 2001; Fruehn et al., 2001). Conversely, several modelling and inversion studies have shown that EM methods can play a significant role in mapping both the thickness of the basalt layer and the sediments below (Jegen et al, 2002; MacGregor, 2003; Hoversten et al.,2013; Heincke et al., 2014). The CSEM and MT methods utilize electromagnetic (EM) fields to measure resistivity variations in the sub-surface. Compact crystalline rocks such as basalt have virtually no porosity and are very resistive in contrast to sediments with comparably large pore spaces filled with more conductive pore fluids. That makes EM methods well suited for imaging of the thickness of the basalt layer and the structures underneath. In this paper, we discuss two studies, (1) a synthetic data study based on a realistic model (Herredsvela et al., 2012) and (2) a field data example from the Faroe-Shetland Basin. Synthetic Basalt Model Figure 1 shows the model which was used for the synthetic modelling and inversion study. The water depth in the model varies from 300 to 350 m. The model consists of a fine layered basalt sequence with a varying thickness from 200 meters to almost 2 kilometers and a detailed sediment structure underneath the basalt. The internal structure of the basalt sequence is based on the observations from well logs and seismic. It consists of many thin layers with varying resistivities which are sensed as one thick anisotropic layer due to low resolution of the EM methods. The information used to construct this model makes it analogue to a realistic scenario.
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