WE CLAIM:

1. A data-processing method for reducing masking effects of shallow resistivity structures on an electromagnetic survey of a subsurface region, comprising:

(a) selecting a first survey data set generated at a first source frequency, said first source frequency having been selected to penetrate only said shallow resistivity structures;

(b) selecting a second survey data set corresponding to a second source frequency lower than said first source frequency, thereby revealing deeper lying resistivity structures of the subsurface region as well as said shallow resistivity structures;

(c) calculating the shallow resistivity structure by solving electromagnetic field equations using the survey data set generated at the first source frequency; and

(d) using the calculated shallow resistivity structure and the electromagnetic data from the second survey to distinguish shallow response from deeper response.

2. The method of claim 1, wherein the shallow resistivity structure is predicted by:

(a) assuming an initial shallow resistivity structure;

(b) calculating a theoretical electromagnetic response for the survey at said first source frequency using electromagnetic field equations and the assumed shallow resistivity structure;

(c) comparing the calculated response to said first survey data set; and (d) adjusting the assumed shallow resistivity structure and repeating steps (b) - (d) as necessary until the calculated response agrees with said first survey data set within a pre-selected tolerance.

3. The method of claim 1, wherein the shallow response is distinguished from the deeper response by:

(a) calculating a theoretical electromagnetic response for the survey at said second source frequency using electromagnetic field equations and the calculated shallow resistivity structure; and

(b) comparing the calculated electromagnetic response to the second survey data set to remove contributions to the second survey data set caused by the shallow resistivity structure.

4. The method of claim 2, wherein the initial shallow resistivity structure is obtained from seismic data interpretation by associating said resistivity structure with geologic structure identified from seismic reflection, refraction, or transmission data, and then estimating resistivity values in the seismically identified structure by correlating to seismic impedance.

5. The method of claim 1, wherein the shallow resistivity structure is predicted by inverting electromagnetic wave equations at said first source frequency to solve for resistivity structure corresponding to acquisition parameters and the electromagnetic data set from said first survey.

6. The method of claim 1, further comprising calculating a depth-dependent resistivity structure for said subsurface region from said calculated shallow resistivity structure and an estimated deeper resistivity structure; said depth-dependent resistivity structure containing at least two depth zones corresponding to skin depth of said first and second source frequencies.

7. The method of claim 6, wherein said depth-dependent resistivity structure calculation comprises: (a) taking the calculated shallow resistivity structure and supplementing this resistivity model with the estimated deeper resistivity structure to produce an assumed resistivity model covering shallow and deeper regions;

(b) calculating a theoretical electromagnetic response for the survey at said second source frequency using the electromagnetic field equations and the assumed resistivity model;

(c) comparing the calculated response to said second survey data set; and

(d) adjusting the assumed resistivity model and repeating steps (b) - (d) as necessary until the calculated response agrees with said second survey data set within a pre-selected tolerance.

8. The method of claim 6, wherein the estimated deeper resistivity structure is obtained from seismic data interpretation by associating said resistivity structure with geologic structure identified from seismic reflection, refraction, or transmission data, and then estimating resistivity values in the seismically identified structure by correlating to seismic impedance.

9. The method of claim 6, wherein the depth-dependent resistivity structure is calculated by inverting electromagnetic wave equations at said second source frequency to solve for resistivity structure corresponding to acquisition parameters and the electromagnetic data set from said second survey.

10. The method of claim 6, further comprising selecting a third survey data set corresponding to a third source frequency and adding a third depth zone to said calculated depth-dependent resistivity structure, thereby improving depth resolution of sub-surface resistivity structures.

11. The method of claim 10, further comprising repeating the described process to add at least one additional depth zone.

12. The method of claim 3, wherein the shallow resistivity structure contributions are removed from the second survey data set by using the calculated shallow electromagnetic response to scale the second survey data set, position by position.

13. The method of claim 12, wherein the scaled second survey data set is plotted vs. lateral position on a map using a color scale to represent the scaled data.