Change logo per section / url stem?

Sharing solution – works for me:

function is_tree($pid) {      // $pid = The ID of the page we're looking for pages underneath
	global $post;         // load details about this page
	if(is_page()&&($post->post_parent==$pid||is_page($pid))) 
               return true;   // we're at the page or at a sub page
	else 
               return false;  // we're elsewhere
};

add_filter('avf_logo','av_change_logo');
function av_change_logo($logo)
{
    if(is_tree(20) )
    {
    $logo = "http://www.mysite.dev/wp-content/uploads/logo1.png";
    }
    elseif(is_tree(30) )
    {
    $logo = "http://www.mysite.dev/wp-content/uploads/logo2.png";
    }
    elseif(is_tree(40) )
    {
    $logo = "http://www.mysite.dev/wp-content/uploads/logo3.png";
    }
    return $logo;
}