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	Caption	Length	Value range
FILI *	String	Input file name	1 - 192
FILO *	String	Output file name	1 - 192
DBIC *	Integer	Input potential field channels	1 - 1
DBOC *	Integer	Output filtered image channel	1 - 1
DBIW	Integer	Raster input window	0 - 4	Xoffset, Yoffset, Xsize, Ysize
DBOW	Integer	Raster output window	0 - 4	Xoffset, Yoffset, Xsize, Ysize
ZL *	Float	Height (Z level)	1 - 1
FILB	String	Buffer file name	0 - 192
REPORT	String	Report mode	0 - 192	Quick links
MONITOR	String	Monitor mode	0 - 3	ON, OFF Default: ON

* Required parameter

Parameter descriptions

FILI

Specifies the name of the PCIDSK file from which image data is read.

FILO

Specifies the name of the PCIDSK file receive the transformed image data.

The output file can be the same as the input file. The specified output file must exist running FCONT.

DBIC

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

DBOC

Specifies the output channel to receive the filtered image data.

DBIW

Specifies the raster window (Xoffset, Yoffset, Xsize, Ysize) read from the input image. If this parameter is not specified, the entire image is processed by default.

Xoffset, Yoffset define the upper-left starting pixel coordinates of the window. Xsize is the number of pixels that define the window width. Ysize is the number of lines that define the window height.

DBOW

Specifies the raster window (Xoffset, Yoffset, Xsize, Ysize) to be output. If this parameter is not specified, the entire layer is output by default.

Xoffset, Yoffset define the upper-left starting pixel coordinates of the window. Xsize is the number of pixels that define the window width. Ysize is the number of lines that define the window height.

ZL

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).

FILB

Optionally specifies the name of the temporary buffer file.

Because this function uses large amounts of RAM, a temporary buffer file is automatically created for large images. This parameter records the name of the buffer file created by FCONT. Typically, no user interaction is required for this parameter.

REPORT

Specifies where to direct the generated report.

Available options are:

• TERM: generates a report on the terminal (default)
• DISK: generates a report on file "IMPRPT.LST"
• OFF: turns off report generation
• <filename>: appends a report to the specified file

MONITOR

The program progress can be monitored by printing the percentage of processing completed. A system parameter, MONITOR, controls this activity.

Available options are:

• ON: Turns the monitor ON (default)
• OFF: Turns the monitor OFF (recommended if running in batch or background mode)

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.

Because this function does not require the entire image to be loaded into memory, there is no limit on the size of input image. If necessary, the function automatically creates a buffer file, if FILB is not specified. The buffer file name is recorded in FILB for efficiently reusing the buffer in another run.

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.

Example

In the following example, channel 1 in the file 'irvine.pix' contains the measurement at ground level. FCONT computes an upward continuation of 100 meters. The output image stored to channel 8 will be the measurement at 100 meters above the ground level.

EASI>fili	=	"irvine.pix"	! input file
EASI>filo	=	"irvine.pix"	! output file
EASI>dbic	=	1	! input channel
EASI>dboc	=	8	! output channel
EASI>dbiw	=		! process entire image
EASI>dbow	=	
EASI>zl	=	100	! height of 100m
EASI>filb	=		! no buffer file specified

EASI>run FCONT

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.