In drupal 8 for logging we use logger service, to log array data in some readable format, you should use little hack like in drupal 7 adding html code and print_r to array.
\Drupal::logger('some_channel_name')->warning('<pre><code>' . print_r($responseObj, TRUE) . '</code></pre>');
Channel name is type of log message, warning is severity method you call and $responseObj is array you need to put to log file to have some formating, if you try to output array directly you will get an exception, so best way is to format it like above.