FCONT

Description

Computes upward/downward continuation of a potential field. The filter is applied in the frequency domain. A 2-D Fourier transformation is first applied to the image. After filtering, the image is transformed back to the spatial domain.

Parameters

Name	Type	Length	Value range
Input: Input potential field channels *	Raster port	1 - 1
Output: Output filtered image channel *	Raster port	1 - 1
Height (Z level) *	Float	1 - 1
Report	String	0 - 192	See parameter description

* Required parameter

Parameter descriptions

Input: Input potential field channels

Specifies the input channels containing the potential fields to use in the transformation.

Output: Output filtered image channel

Specifies the output channel to receive the filtered image data.

Height (Z level)

Specifies the height at which to stop the transformation. Positive and negative values for this parameter correspond to upward and downward continuation, respectively. Values must be provided in the same units as the pixel size (usually meters).

Report

Specifies where to direct the generated report.

Available options are:

• LOG: generates a report on the terminal (default)
• <modulename.RPT>: appends a report to the module's RPT file
• DISK: generates a report on file "IMPRPT.LST"

Details

FCONT computes upward/downward continuation of a potential field in the input file. The filter is applied in the frequency domain. A 2-D Fourier transformation is first applied to the image. After filtering, the image is transformed back to the spatial domain.

This function is most efficient if the input window has dimensions of a power of 2; otherwise, FCONT must pad extra rows and columns to force the image dimensions a power of 2.

Algorithm

The input image is first transformed to the frequency domain using a 2-D Fast Fourier transformation (FFT). The dimensions of the transformed image are a power of 2 and are at least as large as the input image dimensions. After applying the filter, the frequency image is transformed back to the spatial domain and truncated to the input image size.

The upward/downward continuation filter has the following form:

        exp(-2 PI sqrt(u*u + v*v) ZL)

where u, v are the frequency components. The resolution of u, v are given as:

        Delta_u = 1/(SizeU * Delta_x)
        Delta_v = 1/(SizeV * Delta_y)

Due to a limitation in the theory, the computed continuation may differ from the actual field measurement at the desired level by a factor which is close to 1. This factor is a constant for the entire potential field at the given level. But if multiple sources with different depths exist, the factor varies.