Method for obtaining resistivity from controlled source electromagnetic data

A method for emphasizing deeper resistive anomalies over masking effects of shallow resistive anomalies arising in measured electromagnetic field data from a controlled-source electromagnetic survey of a subsurface region, said method comprising:

• (a) decomposing the measured data into components in the frequency domain;
• (b) selecting one or more data components corresponding to frequencies higher than a minimum frequency, said minimum frequency being chosen to provide a pre-selected degree of shallow penetration of said subsurface region;
• (c) generating an initial resistivity data volume for the subsurface region through one dimensional (1-D) 1D inversion of the selected high frequency components of the measured electromagnetic field data, wherein said 1D refers to the subsurface region's vertical dimension;
• (d) solving Maxwell's electromagnetic field equations in three dimensions, assuming the initial resistivity data volume and a selected low frequency from the survey source's frequency spectrum, for selected source/receiver locations from the survey, said low frequency being selected for its ability to penetrate to a pre-selected depth in the subsurface region thereby producing calculated electromagnetic field values;
• (e) dividing a measured electromagnetic field value from the frequency-domain data component associated with the selected low frequency by a calculated electromagnetic field value, for different source-receiver pairs along a plurality of source towlines, yielding a set of ratios;
• (f) assigning an (x,y,z) position to each ratio, in consideration of the locations of the source and receiver pair associated with each ratio, thereby creating a ratio data volume;
• and
• (g) defining resistive objects in the ratio data volume by selecting cells in the volume having scaled values above a pre-selected threshold.

A method for producing hydrocarbons from a subsurface region, comprising:

• (a) obtaining measured electromagnetic field data from a controlled-source electromagnetic survey of the subsurface region;
• (b) generating an initial resistivity data volume for the subsurface region through 1D inversion of the measured electromagnetic field data, wherein said 1D refers to the subsurface region's vertical dimension;
• (c) solving Maxwell's electromagnetic field equations in three dimensions, assuming the initial resistivity data volume and a selected frequency from the survey source's frequency spectrum, for selected source/receiver locations from the survey, thereby producing calculated electromagnetic field values corresponding to measured survey data;
• (d) decomposing the measured data to the frequency domain and selecting the component of the data corresponding to the selected frequency;
• (e) dividing a measured field value from the selected data component by a calculated field value, for different source-receiver pairs along a plurality of source towlines, yielding a set of ratios;
• (f) assigning an (x,y,z) position to each ratio, in consideration of the locations of the source and receiver pair associated with each ratio, thereby creating a ratio data volume;
• (g) multiplying the initial resistivity data volume by a function of the ratio data volume, location-by-location, to generate a 3D resistivity model corresponding to the selected frequency;
• and
• (h) producing hydrocarbons from hydrocarbon-bearing formations represented by resistive anomalies discovered by inspection of the 3D resistivity model of the previous step.