anomalous FK response

[luigiV]

Hi Marc,
I made the FK of an L shaped array (24 channels, 2m geophones spacing, sampling rate 500Hz, 21 'of recording). In the attached .page a dispersive trend is evident (not of the fundamental mode) which however is not the object of my question, but rather the "antidispersive" one much more evident in the FK vertical Capon than in the conventional vertical one.
Observing the azimuth graphs, it can be seen that in the frequency interval of the "antidispersive" trend the directivity of the source is much more defined in that of Capon than in that of Conventional one. What can its interpretation be?

Thank you

Luigi