Hi! Thank you for reaching out to us. Our magnetometer can reach 32 ft underground. It can be purchased here: www.fgsensors.com/product-page/full-magnetometer-kit
Hi! Thank you for your feedback. Currently you can find the explanation on how to calibrate the device in our manual: www.fgsensors.com/_files/ugd/1593cf_eca6f44df50247199dc3d66e81d40d7b.pdf In the future we also have plans on creating a full video tutorial, stay tuned 😀