Just what the title states ...
A geophone gives an analog output. It must be digitized before it may be posted on USB. Is mere digitization sufficient? What else must happen after digitization before the signal may be put on USB?
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No, you cannot just "put data on USB".
USB is Universal Serial Bus. A bus (in a computer) is a control and data pathway, in this case to connect periperal devices (aka adapters) to the computer. USB connects periperal devices just like PCI accepts adapter cards/boards. The USB Host expects to communicate with a USB device using the USB protocol. USB is simply the method for a peripheral to connect with the computer; USB (the bus itself) is not an end-user device or connection.
Most likely you are confusing "USB" with a serial (RS-232) communications USB adapter. What you think is just a USB port is really a USB device for a serial communication port. Windows will report such a USB device as a COM port; the fact that the connection is through USB is hidden unless you hunt for more info, such as looking at the Device Manager.
To answer the obvious follow-up question: "No, you cannot just put data on a serial communication link" if you expect to read it reliably on the receiving end.
You may have to normalize the signal to some standard. You also have to decide the range of values that you wish to digitize. To recreate the data from the data source will you use 8, 16 or 32 bit digitized data. Also with what skew does the source signal change so how many points will you read in a second to again recreate the source data.
So Yes conversion alone is sufficient, but what specifics of conversion are going to be important. You will have to experiment with.
You must ensure that digitization frequency (sampling frequency) is 2x larger than maximum frequency (that is considered to be non-noise) in input signal.
Otherwise you'll get a plain garbage.
This is called Nyquist sampling theorem.