Page 16 - Marine Magnetometer Processing

Figure 10: The effect of sample rate on the ability to record magnetic anomalies But the rate at which measurements are made is not the only factor to consider, we also have to consider the speed at which the magnetometer moves across the seabed. A magnetometer towed behind a boat moves through the water making measurements as it goes. If it is moving at 2 metres per second and makes a measurement every second then each measurement will be 2m apart, this is known as the sample interval. If the update rate is increased to 10Hz then each measurement will be 200mm apart. If the update rate is kept at once per second but the boat speed is increased to 4 ms-1 then the measurements will be 4m apart. As a general rule you need a fast update rate and slow vessel speed to be able to detect smaller objects. Noise Noise limits how well we can make any measurement and it is important to understand how it affects measurements made by a marine magnetometer. Any instrument producing measurements will actually report a mixture of valid measurement (called signal) and unwanted measurement (called noise). It can be difficult to separate the signal we want, the measurement of the Earth’s magnetic field, from the noise that we do not want, as sometimes it is hard to tell which is which. A typical source of unwanted noise is the noise made by the instrument itself, called instrument noise or self noise. The effect of underlying magnetic rocks and diurnal variation also change the magnetic field being measured and can be thought of as a source of noise. Marine Magnetometer Processing \[15\] © 3H Consulting Ltd