Often times when using ultrasound you have to understand the depth of a muscle when determining treatment parameters. Usually you ask whether a muscle is superficial or deep. In some cases are there thicker muscles such as the Gluteus Max that requires deeper depth even though they are more superficial? (Especially considering the fat layer that has to be accounted for). Do you have a good source to simplify each muscles depth or a way that you remember it yourself?

When setting treatment parameters, the depth of US penetration is determined by the frequency selected. Frequency refers to the number of vibrations per second. 

The most common frequency for therapeutic ultrasound is one million vibrations per second. This is usually stated as 1 megahertz (1 MHz).

Depth of penetration is different whether the underlying tissue is muscle, fat, tendon or bone. On the average, 1 MHz ultrasound penetrates up to about 1.5 inches in muscle; and to about 6.0 inches in fat. 3 MHz penetrates only about 1/3 as far as 1 MHz. 

This makes it good to use on very shallow tissue areas such as hands. This range serves only as a guide as each individual muscle may vary in depth/size depending on the morphology of the patient being treated.

Reference 1