Configure mod_security for detection only

Do you want to use ModSecurity in a transparent mode? Meaning that no actions will be performed? You can use the SecRuleEngine command. The syntax is:

SecRuleEngine On|Off|DetectionOnly

The possible values are:
On: process rules
Off: do not process rules
DetectionOnly: process rules but never executes any disruptive actions (block, deny, drop, allow, proxy and redirect)

ModSecurity Wiki

For the transparent mode, where only no disruptive actions will be applied, use:

SecRuleEngine DetectionOnly

Put this directive at the top of the ModSecurity configuration file.

If you want to have a log, add:

SecAuditLog <path to the log file>

SecAuditLog /tmp/modsec_audit.log

Don’t forget to restart your web server.

References:
Modsecurity wiki