In seismic signal processing, spectrum analysis is one of the commonly used signal analysis techniques. As a drawing technique tutorial, this article provides a visual display of seismic data, single-trace recorded waveforms, and their spectrum.
In fact, seismic waveform displays are usually drawn by filling the area of a half-polar closed curve, which can be achieved using the <span>fill</span> function in MATLAB. The spectrum analysis curve can assist in shadow filling to enhance the visual effect.
1. Drawing Example
Given seismic data with a sampling rate of 2ms and a duration of 500ms, totaling 301 traces, we will plot a 2-D pseudo-color seismic profile, single-trace seismic record, and spectrum graph.
The implementation code is as follows:
%=========================================================================
% DEMO: Seismic signal analysis
%
% Parameter description:
% nsamps: sample numbers
% ntrace: trace numbers
% dt: sampling time
% fs: sampling frequency
% nfft: FFT points
% df: frequency space
% Author: Peng Zhenming
% School of Information and Communication Engineering,
% University of Electronic Science and Technology of China
% http://idiplab.uestc.cn/
% Date: 2015.12.26,
%
% Revised: 2025.08.27
%=========================================================================
clc, clear, close all;
%=========================================================================
dt = 2.0; % Sampling time: Millisecond
%=========================================================================
% Read the seismic data from text file
y = importdata('seismic_nsamp251_tr301_2ms.txt');
%==========================================================================
[nsamps, ntrace] = size(y);
y = y/max(y(:));
imagesc(imadjust(y)); % imagesc(y(1:nsamps,1:ntrace));
colormap(diyCM(18,256)) % diyCM Colors
% colormap(jet);
colorbar
t = dt*(0:nsamps-1); % Millisecond
tick = floor(linspace(0, nsamps-1, 6)); % Number of marked points
yticks(tick); % Number of marked points
yticklabels(string(dt*tick)); % Label text
% Figure size and position
set(gcf, 'Unit', 'Centimeters', 'Position', [12, 8, 24, 12])
set(gcf, 'Color', 'White'); % White Background
xlabel('Trace no'); ylabel('Time/ms');
set(gca, 'XAxisLocation', 'top'); % x-axis ticks on top
xlim([0 max(ntrace)]), ylim([0 max(nsamps)])
%==========================================================================
traceid = 1; % Select trace number
% traceid = round(traceid);
traceid(traceid < 1) = 1;
traceid(traceid > ntrace) = ntrace;
s = y(:, traceid); % Get a seismic trace
s1 = s; s1(s1 <= 0) = 0; % Remain positive
s2 = s; s2(s2 > 0) = 0; % Remain negative
figure,
set(gcf, 'Unit', 'Centimeters', 'Position', [20, 8, 4, 12])
area(t, s1, 'FaceColor', 'b', 'EdgeColor', 'b', 'FaceAlpha', 0.6); hold on
% area(t, s2, 'FaceColor', 'b', 'EdgeColor', 'k', 'FaceAlpha', 0.6);
% plot(t, s2, 'k', 'LineWidth', 1.2);
plot(t, s, 'b')
% seisViewer(s, 0.1, 'k', 'k')
set(gca, 'ydir', 'reverse')
xlabel('Time/ms');
ylabel('Trace no');
% set(gca, 'XAxisLocation', 'top'); % x-axis ticks on top
set(gca, 'YAxisLocation', 'right', 'YTickLabel', ''); % y-axis ticks on right
grid on
view(-90, -90)
%==========================================================================
nfft = nsamps; % FFT using data points
fs = 1/dt;
df = 1000*fs/(nfft-1); % Frequency spacing/Precision
f = df*(0:(nfft-1)/2); % Physical frequency/Unilateral spectrum
%==========================================================================
Fs = fft(s, nfft);
amp = abs(Fs(1:floor(nfft/2)+1))/max(abs(Fs(:))); % Normalization
figure,
% Figure size and position
set(gcf, 'Unit', 'Centimeters', 'Position', [24, 8, 24, 12])
set(gcf, 'Color', 'White'); % White Background
% Set parameters
color = [0, 0.4470, 0.7410]; % [0.85, 0.33, 0.10]
alpha = 0.65;
lineWidth = 0.80;
patchshade(f, amp, color, alpha, lineWidth); % Patches for the shaded region
% area(f, amp, 'FaceColor', 'g'), hold on
% plot(f, amp', 'LineWidth', 1.2);
xlabel('Frequency/Hz'); ylabel('Amplitude(Normalized)');
xlim([0 max(f)]);
grid on
% Figure decoration
defaultAxes
2. Shadow Filling
The following code provides a curve plotting function for gradient shadow filling, <span>patchshade</span>, and other calling functions can be downloaded from the attachment.
function patchshade(x, y, color_value, alpha_value, line_width)
%-----------------------------------------------------------
% Create patches for the shaded region
narginchk(0, 5)
% ----------------
ax = gca;
if nargin < 2
error('Please input the x and y vectors!');
end
if nargin < 3 || isempty(color_value)
color_value = ax.ColorOrder(ax.ColorOrderIndex, :); % Default color
end
if nargin < 4 || isempty(alpha_value)
alpha_value = 0.15; % Default alpha
end
if nargin < 5 || isempty(alpha_value)
line_width = 2.0; % Default Width
end
y_min = min(y);
y_max = max(y);
V_alpha = alpha_value * ((y - y_min) / y_max);
F = [1 2 3 4];
hold on
for k = 1:numel(x)-1
V = [ x(k) y_min; x(k) y(k); x(k+1) y(k+1); x(k+1) y_min ];
A = [0, V_alpha(k), V_alpha(k+1), 0]';
patch('Faces', F, 'Vertices', V, 'FaceColor', color_value,
'EdgeColor', 'none',
'FaceVertexAlphaData', A, 'FaceAlpha', 'interp',
'AlphaDataMapping', 'none');
end
hold on
plot(x, y, 'LineWidth', line_width, 'Color', color_value);
end
The above is a general drawing function for shadow-filled curves, which can be directly called for other purposes of curve display.
Loading seismic (text format) data, the program runs with the following results.
Figure 1: Pseudo-color seismic profile
Figure 2: Single-trace record
Figure 3: Seismic record spectrum
Parameters can be adjusted to change waveform color, aspect ratio, display gain, axis properties, etc.
Test data download: seismic_251_301_2ms.txt Link: https://pan.baidu.com/s/1JfKqIzG8c4s00yMwBq9reg?pwd=idip
This article has no commercial purpose, only for teaching and technical sharing. Some reference codes are provided with sources. If there is any infringement, it will be deleted immediately.
“The most needed thing in learning is not a tragic will, but a thirst for the infinite unknown.”
Text and images: Peng Zhenming
Editor: Liu SiminPreviousIssuesReview
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