I'm not sure of any direct Snap methods to use mousewheel, but I guess you can just add a mousewheel listener...this example works in Chrome, you may need to tweak and add test case for different browsers.
var s = Snap(400, 620);
var c = s.circle(30,30,30);
if( (/Firefox/i.test(navigator.userAgent)) ) {
s.node.addEventListener("DOMMouseScroll", mouseWheelHandler, false);
} else {
s.node.addEventListener("mousewheel", mouseWheelHandler, false);
}
function mouseWheelHandler (ev) {
ev.preventDefault();
console.log( ev.target.localName );
}
Edit: Have updated to check for firefox as well.