How to create index on json array in postgres

Hi,

     I have a json field called 'elements' in my table demo which
contains an array 'data' containing key value pairs. the 'data' array
has the below structure. the data array may have multiple json entries.I
am using postgres version 9.5

{ "data": [{ "ownr": "1", "siUsr": [2], "sigStat": "APPR", "modifiedOn":
1494229698039, "isDel": "false", "parentId": "nil", "disName":
"exmp.json", "uniqueId": "d88cb52", "usrType": "owner", "usrId": "1",
"createdOn": 1494229698039, "obType": "file" }] }

In my query I have multiple filters based on obj(Eg : obj->>usrId,
obj->>siUsr etc) where obj corresponds to
json_array_elements(demo.elements->'data').How do I create btree indices
on filters like obj->>userId ,obj->>sigUsr? Please revert.

Regards

Surya