The three components of the geomagnetic field were measured at sea by STCM (Shipboard Three Component Magnetometer) with a relative accuracy of 50 ± 25 nT. The STCM was controlled by a microcomputer, and the three geomagnetic components were calculated by reducing the magnetic field due to the ship’s remanent and induced magnetic moments, the only assumption being that the magnetic field is linearly related to the magnetic moments. Although STCM has no particular magnetic compensation device, it can be set at any place on board of any kind of ship. In the KH82-5 cruise, the three component measurements were conducted along latitude 42 °N north of the Mendocino fracture zone where there are well‐defined magnetic anomaly lineations trending north and south. STCM measured the north component anomalies with very small amplitudes compared to the east and downward component anomalies, which shows that lineations trend north and south. The three measured component anomalies are almost identical to the anomalies calculated from the well‐established time scale; thus, STCM measures the three components correctly. The vector geomagnetic measurements are more useful for analysis of marine geomagnetic anomalies than are the total‐intensity measurements. The DELPWAKASHIO 84/1 cruises provide good examples for examination of the linearity of magnetic anomalies on a single track by using the Fourier transform of the horizontal and vertical component anomalies.

S U M M A R YWe present a new method which provides precise determination of the position and strike of magnetic boundaries using vector data of the geomagnetic anomaly field. Assuming a 2-D magnetic structure, the intensity of spatial differential vectors (ISDV) exhibits a peak at the boundary of magnetic structures. The strike of the magnetic boundary is obtained using the fact that the magnetic field is zero in the direction parallel to the boundary. We have also applied this method to 3-D magnetic structures. Our method requires only the relative changes of the three components, which are more accurately measured than their actual magnitudes. W e have applied this method to data from the southern Juan de Fuca Ridge obtained on the R/V 'Hakuho-maru' expedition in 1988 (KH-88-3). The results are summarized in magnetic boundary strike diagrams (MBSD); these are maps that show the position, strike and character of the magnetic boundaries. MBSD provides valuable information for understanding the magnetic structure of oceanic crust, such as magnetic lineations, pseudo-faults, fracture zones and the central anomaly magnetic high (CAMH). Therefore, relatively few vector magnetic profiles provide information similar to extensive total intensity geomagnetic surveys.Key words: geomagnetic anomaly field vector, magnetic boundary position, magnetic boundary strike diagram (MBSD), southern Juan de Fuca Ridge.

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