Cyclic noise removal in borehole imaging
View Patent ↗A method for removing cyclic noise from a borehole image includes transforming the image into the frequency domain using a two-dimensional (2-D) transform (e.g., using a discrete cosine transform). The cyclic noise components (peaks) are removed from the transformed image which is then inverse transformed back into the spatial domain using an inverse 2-D transform to obtain a corrected image. An automated method enables the cyclic peaks to be identified and removed from the borehole image via downhole processing.
1. A computer implemented method for removing cyclic noise from a borehole image, the method comprising:
(a) acquiring at least one bottom hole assembly spacing;
(b) computing at least one frequency from the bottom hole assembly spacing acquired in (a);
(c) creating a two-dimensional filter using the at least one frequency computed in (b);
(d) acquiring a logging while drilling borehole image;
(e) transforming the borehole image acquired in (d) into a frequency domain using a two-dimensional transform to obtain a transformed image;
(f) filtering the transformed image obtained in (e) using the filter created in (c) to obtain a filtered transformed image; and
(g) inverse transforming the filtered transformed image obtained in (f) using a two-dimensional inverse transform to obtain a corrected image.
2. The method of claim 1 , wherein the at least one bottom hole assembly spacing comprises at least one of a bit to stabilizer spacing, a bit to trimmer spacing, a bit to blade spacing, and a bit to bend spacing.
3. The method of claim 1 , wherein the frequency is computed in (b) by taking an inverse of the at least one spacing acquired in (a).
4. The method of claim 1 , wherein the filter created in (c) comprises a two-dimensional matrix of filter coefficients, a predetermined first band of frequencies about the at least one frequency computed in (b) being assigned a value of zero, a second band of frequencies about the first band being assigned transition values between 0 and 1, and all other frequencies being assigned a value of one.
5. The method of claim 4 , wherein the transformed image is multiplied by the filter in (f).
6. The method of claim 1 , wherein the two-dimensional transform is a Fourier Transform, a cosine transform, a sine transform, a polynomial transform, a Laplace transform, a Hartley transform, or a wavelet transform.
7. The method of claim 1 , wherein the logging while drilling image comprises a two dimensional image of a logging sensor measurement at discrete azimuth angles and measured depths in a borehole, the logging sensor measurement being acquired using a micro-resistivity sensor, an ultrasonic sensor, a density sensor, a gamma-ray sensor, or a physical caliper sensor.
8. The method of claim 1 , wherein (d), (e), (f), and (g) are performed downhole during a logging operation.