Signal processing method of raw data obtained by using chirp sub-bottom profiler and readable storage medium storing the method

1. A method of signal processing of raw data obtained using a chirp sub-bottom profiler, the method comprising:

(a) transmitting a chirplet (minimum frequency: initial first frequency, maximum frequency: initial second frequency, pulse length: initial pulse length) to survey target strata using a chirp sub-bottom profiler and acquiring raw data reflected and received from a target object;

(b) correcting the raw data;

(c) generating a comparison chirplet using a sub-bottom reflection signal of the raw data;

(d) cross-correlating the corrected raw data with the comparison chirplet to generate a first Klauder wavelet section;

(e) auto-correlating the comparison chirplet to generate a second Klauder wavelet; and

(f) deriving a physical property of the survey target strata by deconvoluting the first Klauder wavelet section and the second Klauder wavelet,

wherein the generating a comparison chirplet comprises:

setting a first frequency band for selecting a minimum frequency, a second frequency band for selecting a maximum frequency, and a pulse length range for selecting a pulse length, which are variables necessary for generating a comparison chirplet;

generating a plurality of first to third artificial chirplets by using one of the first frequency band, the second frequency band, and the pulse length range as a variable, cross-correlating at least one trace of the corrected raw data and the plurality of first to third artificial chirplets, and deriving and selecting a value corresponding to a maximum value of an envelope obtained by Hilbert transform as a comparison first frequency, a comparison second frequency, and a comparison pulse length; and

generating a comparison chirplet having the comparison first frequency, the comparison second frequency, the comparison pulse length as a minimum frequency, a maximum frequency, a pulse length.

2. The method of claim 1 , wherein the generating of the comparison chirplet uses a sub-bottom reflection signal in a trace area where anomaly does not exist.

3. The method of claim 1 , wherein the generating of the comparison chirplet comprises:

(1) setting a first frequency band for selecting a minimum frequency, a second frequency band for selecting a maximum frequency, and a pulse length range for selecting a pulse length, which are variables necessary for generating a comparison chirplet;

(2) setting one selected from the first frequency band as a minimum frequency, generating, by each minimum frequency, a plurality of different first artificial chirplets (maximum frequency: initial second frequency, pulse length: initial pulse length), cross-correlating at least one trace of the raw data with the plurality of first artificial chirps, and selecting an average value of a frequency corresponding to a maximum value of an envelope obtained by the Hilbert transform as a comparison first frequency;

(3) setting one selected from the second frequency band as a maximum frequency, generating, by each maximum frequency, a plurality of different second artificial chirplets (minimum frequency: comparison first frequency, pulse length: initial pulse length), cross-correlating at least one trace of the raw data with the plurality of second artificial chirps, and selecting an average value of a frequency corresponding to a maximum value of an envelope obtained by the Hilbert transform as a comparison second frequency;

(4) setting one selected from the pulse length range as a pulse length, generating, by each pulse length, a plurality of different third artificial chirplets (minimum frequency: comparison first frequency, maximum frequency: comparison second frequency), cross-correlating at least one trace of the raw data with the plurality of third artificial chirps, and selecting an average of a pulse length corresponding to a maximum value of an envelope obtained by the Hilbert transform as a comparison pulse length; and

(5) generating a comparison chirplet having the comparison first frequency, the comparison second frequency, and the comparison pulse length as a minimum frequency, a maximum frequency, and a pulse length, respectively.

4. The method of claim 1 , wherein the generating of the comparison chirplet comprises:

(1) setting a first frequency band for selecting a maximum frequency, a second frequency band for selecting a minimum frequency, and a pulse length range for selecting a pulse length, which are variables necessary for generating a comparison chirplet;

(2) setting one selected from the first frequency band as a maximum frequency, generating, by each maximum frequency, a plurality of different first artificial chirplets (minimum frequency: initial first frequency, pulse length: initial pulse length), cross-correlating at least one trace of the raw data with the plurality of first artificial chirps, and selecting an average value of a frequency corresponding to a maximum value of an envelope obtained by the Hilbert transform as a comparison first frequency;

(3) setting one selected from the second frequency band as a minimum frequency, generating, by each minimum frequency, a plurality of different second artificial chirplets (maximum frequency: comparison first frequency, pulse length: initial pulse length), cross-correlating at least one trace of the raw data with the plurality of second artificial chirps, and selecting an average value of a frequency corresponding to a maximum value of an envelope obtained by the Hilbert transform as a comparison second frequency;

(4) setting one selected from the pulse length range as a pulse length, generating, by each pulse length, a plurality of different third artificial chirplets (maximum frequency: comparison first frequency, minimum frequency: comparison second frequency), cross-correlating at least one trace of the raw data with the plurality of third artificial chirps, and selecting an average of a pulse length corresponding to a maximum value of an envelope obtained by the Hilbert transform as a comparison pulse length; and

(5) generating a comparison chirplet having the comparison first frequency, the comparison second frequency, and the comparison pulse length as a maximum frequency, a minimum frequency, and a pulse length, respectively.

5. The method of claim 1 , wherein the physical property is a reflection coefficient obtained through Sparse-Spike Deconvolution.

6. The method of claim 5 , further comprising inversing an impedance of each sub-bottom from the reflection coefficient and the equation;

Z n+1 =Z n *((1+ R n )/(1 −R n ))

(where Z 1 is an impedance of the seawater layer, Z n is an impedance of a sub-bottom located at the nth position with respect to the sub-bottom, and R n is a reflection coefficient of a sub-bottom located at the nth position with respect to the sub-bottom).

7. The method of claim 1 , wherein the correcting of the raw data is performed including at least one of Highpass filtering, amplitude normalization, and automatic gain control (AGC).

8. The method of claim 1 , further comprising correcting an influence of waviness with respect to the first Klauder wavelet section,

wherein the correcting of the influence of the waviness comprises:

extracting an arbitrary trace having no anomaly and cross-correlating with another trace to generate an initial peaking line;

calculating a moving average value for the initial peaking line to generate a latter peaking line; and

comparing the initial peaking line with the latter peaking line to move the initial peaking line based on the latter peaking line.

9. The method of claim 8 , further comprising performing muting to correct a value of an upper zone of the latter peaking line to zero, wherein the muting is performed before step (f) and after the correcting of the influence of the waviness.

10. The method of claim 9 , further comprising, when a generation variable of the comparison chirplet has a minimum frequency as a comparison first frequency, a maximum frequency as a second comparison frequency, and a pulse length of a comparison pulse, setting the comparison first frequency as f 1 , the initial first frequency as f 2 , the initial second frequency as f 3 , and the comparison second frequency as f 4 , and performing band pass filtering to block signals of a frequency band of f 1 or less and f 4 or more and passing signals of a frequency band between f 2 and f 3 ,

wherein the band pass filtering is performed before step (f) and after the muting.

11. A non-transitory computer readable recording medium for recording a program for performing the method according to claim 1 so that the program is readable by a computer to perform signal processing of raw data acquired using a chirp sub-bottom profiler.