function [spec]=powspec(seis,dt);

% [SPEC]=POWSPEC(SEIS,DT) calculates and plots the power spectrum of
% seismogram(s) in SEIS. DT is the sample interval and PSPEC is the
% power spectrum. If SEIS is a single seismogram, then a log-log plot
% of the power spectrum is displayed. If SEIS is a matrix, then the 
% spectrum is calculated for each line of SEIS, but no plot is
% produced.

fftseis=fft(seis')';

spec=(real(fftseis).^2+imag(fftseis).^2).^.5;

if min(size(spec))==1
  figure(11)
  lspec=length(spec);
  lhspec=round(lspec/2);
  frequ=1/dt;
  nyfrequ=frequ/2;
  minfrequ=1/(lspec*dt);
  loglog(minfrequ:nyfrequ/lhspec:nyfrequ,spec(1:lhspec))
end