Fourier Transform of Seismic Waves

I have data that is in general units called Digital Units vs Milliseconds, which I need to convert to ground acceleration and apply filtering. However, the first step is to convert the data from time-domain to freqency-domain. I have used the Excel file "TestFile.xlsx" which I have attached, to generete the original plot, and compute the single-sided spectrum P1 based on Ps and even-valued signal length (L) as recommneded by the MATLAB Help. However, I get a plot that does not make much sense to me, and I am not sure if I need to fix something.

This is the code that I did:

close all

clear

clc

filename = 'TestFile.xlsx';

% Load excel table

M = readtable(filename);

%disp(T);

% Load column as vectors

ms = table2array(M(:,1)); % Time Vector

lpx = table2array(M(:,2)); % Signal + Noise

T = 604; % Sampling Period

Fs = 1/T; % Sampling frequency

L = 4211;

%t = (0:L-1)*T

figure

plot(ms,lpx)

title('Original signal before filtering')

xlabel('t millisecond')

ylabel('Digital Units')

% Computing Fourier transform of signal

Y = fft(lpx);

%Compute the single-sided spectrum P1 based on P2 and the even-valued

%signal length L

P2 = abs(Y/L);

P1 = P2(1:L/2+1);

P1(2:end-1) = 2*P1(2:end-1);

figure

f = Fs*(0:(L/2))/L;

plot(f,P1)

title('Single-Sided Amplitude Spectrum of X(t)')

xlabel('f(Hz)')

ylabel('|P1(f)|')

Answer 1

[Jonas]

subtract the mean before fft to improve visibility of frequencies or use dB view by calculating 20*log10(P1)

explanation: the DC part (the mean part) of the signal is much higher compared to the amplitude of the oscillation. The Mean part is the amplitude value at f==0 Hz in the spectrum