code: def ricker(f, length, dt):
      
      t = np.linspace(-int(length/2), int((length-dt)/2), int(length/dt))
      y = (1. - 2.*(np.pi**2)*(f**2)*(t**2))*np.exp(-(np.pi**2)*(f**2)*(t**2))
      return t, y
docs: A Ricker wavelet.
  
      Args:
          f (float): frequency in Haz, e.g. 25 Hz.
          length (float): Length in s, e.g. 0.128.
          dt (float): sample interval in s, e.g. 0.001.
  
      Returns:
          tuple. time basis, amplitude values.